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Diff amps spec at 500ohms plus, why?

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  Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-21 15:48 -0800
    Re: Diff amps spec at 500ohms plus, why? George Herold <gherold@teachspin.com> - 2017-12-21 18:53 -0800
      Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 09:31 -0800
    Re: Diff amps spec at 500ohms plus, why? Winfield Hill <hill@rowland.harvard.edu> - 2017-12-21 19:48 -0800
      Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 09:34 -0800
      Re: Diff amps spec at 500ohms plus, why? bloggs.fredbloggs.fred@gmail.com - 2017-12-24 09:37 -0800
    Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-21 19:55 -0800
    Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-21 19:59 -0800
      Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 09:39 -0800
    Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 12:07 -0800
      Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highland_snip_technology.com> - 2017-12-22 12:54 -0800
        Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 13:29 -0800
          Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highland_snip_technology.com> - 2017-12-22 14:06 -0800
            Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 14:19 -0800
              Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highland_snip_technology.com> - 2017-12-22 14:40 -0800
                Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 15:16 -0800
                  Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-22 16:30 -0800
                    Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-22 16:57 -0800
                      Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-22 20:33 -0800
                        Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-23 08:21 -0800
                          Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-23 10:20 -0800
                            Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-23 11:33 -0800
                  Re: Diff amps spec at 500ohms plus, why? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-12-23 13:56 -0500
                    Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-23 11:08 -0800
                      Re: Diff amps spec at 500ohms plus, why? Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-12-23 14:11 -0500
                        Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-23 11:42 -0800
    Re: Diff amps spec at 500ohms plus, why? LM <sala.nimi@mail.com> - 2017-12-23 21:36 +0200
      Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-23 12:18 -0800
    Diff amps spec at 500ohms plus, why? Klaus Kragelund <klauskvik@hotmail.com> - 2017-12-25 00:56 -0800
      Re: Diff amps spec at 500ohms plus, why? krw@notreal.com - 2017-12-25 08:51 -0500
        Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-25 07:26 -0800
          Re: Diff amps spec at 500ohms plus, why? krw@notreal.com - 2017-12-25 10:46 -0500
            Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-25 07:51 -0800
              Re: Diff amps spec at 500ohms plus, why? krw@notreal.com - 2017-12-25 12:53 -0500
                Re: Diff amps spec at 500ohms plus, why? John Larkin <jjlarkin@highlandtechnology.com> - 2017-12-25 09:58 -0800
                Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-25 11:39 -0800
                  Re: Diff amps spec at 500ohms plus, why? whit3rd <whit3rd@gmail.com> - 2017-12-25 17:16 -0800
                    Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-27 16:17 -0800
                      Re: Diff amps spec at 500ohms plus, why? Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-12-27 19:06 -0800
                        Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-28 00:47 -0800
                    Re: Diff amps spec at 500ohms plus, why? Lasse Langwadt Christensen <langwadt@fonz.dk> - 2017-12-27 16:38 -0800
          Re: Diff amps spec at 500ohms plus, why? George Herold <gherold@teachspin.com> - 2017-12-27 18:03 -0800
            Re: Diff amps spec at 500ohms plus, why? Joerg <news@analogconsultants.com> - 2017-12-28 00:50 -0800

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#488995 — Diff amps spec at 500ohms plus, why?

On a design I am working right now I need to drive differential cables 
at up to 100MHz and with gusto, several volts swing. I'd use coax 
instead but the customer is king. Looking at the various diff amps du 
jour I consistently find specs like this:

http://www.ti.com/lit/ds/symlink/ths4503.pdf

That example has all the performance data at an even higher load 
resistance of 800ohms. Considering that regular ribbon is around 100ohms 
diff and even grandpa's old TV cable was 300ohms, what good does that do?

I guess now I have to not just find a diff amp but also add an analog 
diff buffer IC (if they still exist) or roll my own transistor stuff. 
Which will double as a space heater where I really can't use heat. Grumble!

-- 
Regards, Joerg

http://www.analogconsultants.com/

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#489012

On Thursday, December 21, 2017 at 6:48:50 PM UTC-5, Joerg wrote:
> On a design I am working right now I need to drive differential cables 
> at up to 100MHz and with gusto, several volts swing. I'd use coax 
> instead but the customer is king. Looking at the various diff amps du 
> jour I consistently find specs like this:
> 
> http://www.ti.com/lit/ds/symlink/ths4503.pdf
That's a lot of spec sheet.. I didn't read it all.
Max current >100mA that should be enough current for ~100 ohms. 
Maybe some spec is better when driving 800 ohms. 
(less distortion? see figure 69.)

George H.  
 
> 
> That example has all the performance data at an even higher load 
> resistance of 800ohms. Considering that regular ribbon is around 100ohms 
> diff and even grandpa's old TV cable was 300ohms, what good does that do?
> 
> I guess now I have to not just find a diff amp but also add an analog 
> diff buffer IC (if they still exist) or roll my own transistor stuff. 
> Which will double as a space heater where I really can't use heat. Grumble!
> 
> -- 
> Regards, Joerg
> 
> http://www.analogconsultants.com/

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#489050

On 2017-12-21 18:53, George Herold wrote:
> On Thursday, December 21, 2017 at 6:48:50 PM UTC-5, Joerg wrote:
>> On a design I am working right now I need to drive differential cables
>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>> instead but the customer is king. Looking at the various diff amps du
>> jour I consistently find specs like this:
>>
>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
> That's a lot of spec sheet.. I didn't read it all.
> Max current >100mA that should be enough current for ~100 ohms.
> Maybe some spec is better when driving 800 ohms.
> (less distortion? see figure 69.)
>

That is exactly what has me concerned. Yes, it can drive it but then the 
distortion goes from not that good to horrid. Possibly I can parallel 
two or three but that eats real estate.


> George H.
>
>>
>> That example has all the performance data at an even higher load
>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>
>> I guess now I have to not just find a diff amp but also add an analog
>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>> Which will double as a space heater where I really can't use heat. Grumble!
>>
>> --
>> Regards, Joerg
>>
>> http://www.analogconsultants.com/
>


-- 
Regards, Joerg

http://www.analogconsultants.com/

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#489017

Joerg wrote...
>
>On a design I am working right now I need to drive differential cables 
>at up to 100MHz and with gusto, several volts swing. I'd use coax 
>instead but the customer is king. Looking at the various diff amps du 
>jour I consistently find specs like this:
>
>http://www.ti.com/lit/ds/symlink/ths4503.pdf
>
>That example has all the performance data at an even higher load 
>resistance of 800ohms. Considering that regular ribbon is around 100ohms 
>diff and even grandpa's old TV cable was 300ohms, what good does that do?
>
>I guess now I have to not just find a diff amp but also add an analog 
>diff buffer IC (if they still exist) or roll my own transistor stuff. 
>Which will double as a space heater where I really can't use heat. Grumble!

 In Aoe III, Table 5.10, page 375, covers full differential
 amplifiers, with diff IN+OUT, listing about 45 parts.  Many
 are intended to drive ADCs, etc., rather than low-Z cables. 

 Two differential outputs at +2 and -2 volts, driving a back-
 terminated 100-ohm differential load, will experience a
 load current of 4V/200 ohms = 20mA, which isn't too bad.

 2 volts amplitude at 100mHz requires an wA = 1260 V/us
 slew rate. That's within the range of the THS4503 specs.
 I tabulated the Vpp-out max and the slew rates, and many
 parts are dramatically faster then the THS4503.   I'm
 sorry I didn't tabulate the maximum output currents.


-- 
 Thanks,
    - Win

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#489051

On 2017-12-21 19:48, Winfield Hill wrote:
> Joerg wrote...
>>
>> On a design I am working right now I need to drive differential cables
>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>> instead but the customer is king. Looking at the various diff amps du
>> jour I consistently find specs like this:
>>
>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>
>> That example has all the performance data at an even higher load
>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>
>> I guess now I have to not just find a diff amp but also add an analog
>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>> Which will double as a space heater where I really can't use heat. Grumble!
>
>   In Aoe III, Table 5.10, page 375, covers full differential
>   amplifiers, with diff IN+OUT, listing about 45 parts.  Many
>   are intended to drive ADCs, etc., rather than low-Z cables.
>
>   Two differential outputs at +2 and -2 volts, driving a back-
>   terminated 100-ohm differential load, will experience a
>   load current of 4V/200 ohms = 20mA, which isn't too bad.
>
>   2 volts amplitude at 100mHz requires an wA = 1260 V/us
>   slew rate. That's within the range of the THS4503 specs.
>   I tabulated the Vpp-out max and the slew rates, and many
>   parts are dramatically faster then the THS4503.   I'm
>   sorry I didn't tabulate the maximum output currents.
>

Most have drivers that are in the single-digit ohms and can easily drive 
100ohms. However, the distortion performance drops dramatically under 
such load and in many of my cases that really matters. Sometime they end 
the distortion plots in the datasheet below 200ohm, probably because 
that wouldn't look good.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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#489242

On Thursday, December 21, 2017 at 10:49:12 PM UTC-5, Winfield Hill wrote:
> Joerg wrote...
> >
> >On a design I am working right now I need to drive differential cables 
> >at up to 100MHz and with gusto, several volts swing. I'd use coax 
> >instead but the customer is king. Looking at the various diff amps du 
> >jour I consistently find specs like this:
> >
> >http://www.ti.com/lit/ds/symlink/ths4503.pdf
> >
> >That example has all the performance data at an even higher load 
> >resistance of 800ohms. Considering that regular ribbon is around 100ohms 
> >diff and even grandpa's old TV cable was 300ohms, what good does that do?
> >
> >I guess now I have to not just find a diff amp but also add an analog 
> >diff buffer IC (if they still exist) or roll my own transistor stuff. 
> >Which will double as a space heater where I really can't use heat. Grumble!
> 
>  In Aoe III, Table 5.10, page 375, covers full differential
>  amplifiers, with diff IN+OUT, listing about 45 parts.  Many
>  are intended to drive ADCs, etc., rather than low-Z cables. 
> 
>  Two differential outputs at +2 and -2 volts, driving a back-
>  terminated 100-ohm differential load, will experience a
>  load current of 4V/200 ohms = 20mA, which isn't too bad.
> 
>  2 volts amplitude at 100mHz requires an wA = 1260 V/us
>  slew rate. That's within the range of the THS4503 specs.
>  I tabulated the Vpp-out max and the slew rates, and many
>  parts are dramatically faster then the THS4503.   I'm
>  sorry I didn't tabulate the maximum output currents.
> 

Do you know of a wideband analog amp that is stable with a cable load? They all require back termination. It seems paradoxical that the typical amp will break into an oscillation frequency at which the cable impedance is resistive, so where did the phase shift come from?


> 
> -- 
>  Thanks,
>     - Win

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#489020

On Thu, 21 Dec 2017 15:48:52 -0800, Joerg <news@analogconsultants.com>
wrote:

>On a design I am working right now I need to drive differential cables 
>at up to 100MHz and with gusto, several volts swing. I'd use coax 
>instead but the customer is king. Looking at the various diff amps du 
>jour I consistently find specs like this:
>
>http://www.ti.com/lit/ds/symlink/ths4503.pdf
>
>That example has all the performance data at an even higher load 
>resistance of 800ohms. Considering that regular ribbon is around 100ohms 
>diff and even grandpa's old TV cable was 300ohms, what good does that do?
>
>I guess now I have to not just find a diff amp but also add an analog 
>diff buffer IC (if they still exist) or roll my own transistor stuff. 
>Which will double as a space heater where I really can't use heat. Grumble!

Could you use a fast dual opamp? One amp inverting, one not.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

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#489021

On Thu, 21 Dec 2017 15:48:52 -0800, Joerg <news@analogconsultants.com>
wrote:

>On a design I am working right now I need to drive differential cables 
>at up to 100MHz and with gusto, several volts swing. I'd use coax 
>instead but the customer is king. Looking at the various diff amps du 
>jour I consistently find specs like this:
>
>http://www.ti.com/lit/ds/symlink/ths4503.pdf

Expensive! $12 at qty 1.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

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#489052

On 2017-12-21 19:59, John Larkin wrote:
> On Thu, 21 Dec 2017 15:48:52 -0800, Joerg <news@analogconsultants.com>
> wrote:
>
>> On a design I am working right now I need to drive differential cables
>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>> instead but the customer is king. Looking at the various diff amps du
>> jour I consistently find specs like this:
>>
>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>
> Expensive! $12 at qty 1.
>

I know. There is also the AD813x series for less money. This project is 
more like yours are where price is secondary and performance is key.

Yes, I could use two opamps but even those often have a hard time 
driving such loads to +/2.5V swings without losing linearity at RF too 
much. Many won't come as duals and that makes it tough in my case (no 
space).

It's weird. Most scenarios I ever came across require driving ribbon, 
Parlex, some other cable or back planes in the 100-200ohms range yet the 
mfgs don't offer much there.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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#489058

On 2017-12-21 15:48, Joerg wrote:
> On a design I am working right now I need to drive differential cables
> at up to 100MHz and with gusto, several volts swing. I'd use coax
> instead but the customer is king. Looking at the various diff amps du
> jour I consistently find specs like this:
>
> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>
> That example has all the performance data at an even higher load
> resistance of 800ohms. Considering that regular ribbon is around 100ohms
> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>
> I guess now I have to not just find a diff amp but also add an analog
> diff buffer IC (if they still exist) or roll my own transistor stuff.
> Which will double as a space heater where I really can't use heat. Grumble!
>

Looking a the modem at my Internet connection, one option might be to 
use DSL or PLC drivers like this:

http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf

Gets iffy though, the GND thermal pad would need to be connected to a 
VCC- plane. They are also not intended to work well down to DC but 
probably will be ok there.

Has anyone used DSL or PLC drivers as differential amps?

-- 
Regards, Joerg

http://www.analogconsultants.com/

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#489063

On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2017-12-21 15:48, Joerg wrote:
>> On a design I am working right now I need to drive differential cables
>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>> instead but the customer is king. Looking at the various diff amps du
>> jour I consistently find specs like this:
>>
>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>
>> That example has all the performance data at an even higher load
>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>
>> I guess now I have to not just find a diff amp but also add an analog
>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>> Which will double as a space heater where I really can't use heat. Grumble!
>>
>
>Looking a the modem at my Internet connection, one option might be to 
>use DSL or PLC drivers like this:
>
>http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>
>Gets iffy though, the GND thermal pad would need to be connected to a 
>VCC- plane. They are also not intended to work well down to DC but 
>probably will be ok there.
>
>Has anyone used DSL or PLC drivers as differential amps?

I've used the ADA4950-1 as a differential ADC driver, but it should be
able to drive a cable. Seems well behaved.

Actually, I designed my gadget with an LT6402, but it was nasty so I
had to redesign with the ADI part.


-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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#489067

On 2017-12-22 12:54, John Larkin wrote:
> On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
> wrote:
>
>> On 2017-12-21 15:48, Joerg wrote:
>>> On a design I am working right now I need to drive differential cables
>>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>>> instead but the customer is king. Looking at the various diff amps du
>>> jour I consistently find specs like this:
>>>
>>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>>
>>> That example has all the performance data at an even higher load
>>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>>
>>> I guess now I have to not just find a diff amp but also add an analog
>>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>>> Which will double as a space heater where I really can't use heat. Grumble!
>>>
>>
>> Looking a the modem at my Internet connection, one option might be to
>> use DSL or PLC drivers like this:
>>
>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>>
>> Gets iffy though, the GND thermal pad would need to be connected to a
>> VCC- plane. They are also not intended to work well down to DC but
>> probably will be ok there.
>>
>> Has anyone used DSL or PLC drivers as differential amps?
>
> I've used the ADA4950-1 as a differential ADC driver, but it should be
> able to drive a cable. Seems well behaved.
>

Thanks, that one looks nice though the data for 2Vpp and 200ohms is 
marginal. I need 5Vpp into 100ohms. Maybe the dual version ADA450-2 
could be run in parallel. Possibly that requires 10ohms or so in series 
with each output, meaning four more resistors.

http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4950-1_4950-2.pdf

Figure 22 seems to contradict figure 23.


> Actually, I designed my gadget with an LT6402, but it was nasty so I
> had to redesign with the ADI part.
>

The distortion specs don't look good. High speed or RF isn't really 
LTC's turf, they are more a switch-mode controller company. Well, now 
it's all AD anyhow.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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#489070

On Fri, 22 Dec 2017 13:29:52 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2017-12-22 12:54, John Larkin wrote:
>> On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
>> wrote:
>>
>>> On 2017-12-21 15:48, Joerg wrote:
>>>> On a design I am working right now I need to drive differential cables
>>>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>>>> instead but the customer is king. Looking at the various diff amps du
>>>> jour I consistently find specs like this:
>>>>
>>>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>>>
>>>> That example has all the performance data at an even higher load
>>>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>>>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>>>
>>>> I guess now I have to not just find a diff amp but also add an analog
>>>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>>>> Which will double as a space heater where I really can't use heat. Grumble!
>>>>
>>>
>>> Looking a the modem at my Internet connection, one option might be to
>>> use DSL or PLC drivers like this:
>>>
>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>>>
>>> Gets iffy though, the GND thermal pad would need to be connected to a
>>> VCC- plane. They are also not intended to work well down to DC but
>>> probably will be ok there.
>>>
>>> Has anyone used DSL or PLC drivers as differential amps?
>>
>> I've used the ADA4950-1 as a differential ADC driver, but it should be
>> able to drive a cable. Seems well behaved.
>>
>
>Thanks, that one looks nice though the data for 2Vpp and 200ohms is 
>marginal. I need 5Vpp into 100ohms. Maybe the dual version ADA450-2 
>could be run in parallel. Possibly that requires 10ohms or so in series 
>with each output, meaning four more resistors.
>
>http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4950-1_4950-2.pdf
>
>Figure 22 seems to contradict figure 23.
>
>
>> Actually, I designed my gadget with an LT6402, but it was nasty so I
>> had to redesign with the ADI part.
>>
>
>The distortion specs don't look good.

THD increases even more when a part breaks into oscillation.


> High speed or RF isn't really 
>LTC's turf, they are more a switch-mode controller company. Well, now 
>it's all AD anyhow.

Some of the LTC stuff is good, but it's all too expensive.

ADI often gets the digital parts of mixed-signal stuff wrong. LTC
seems OK about that.

TI does good work.

-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

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#489073

On 2017-12-22 14:06, John Larkin wrote:
> On Fri, 22 Dec 2017 13:29:52 -0800, Joerg <news@analogconsultants.com>
> wrote:
>
>> On 2017-12-22 12:54, John Larkin wrote:
>>> On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
>>> wrote:
>>>
>>>> On 2017-12-21 15:48, Joerg wrote:
>>>>> On a design I am working right now I need to drive differential cables
>>>>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>>>>> instead but the customer is king. Looking at the various diff amps du
>>>>> jour I consistently find specs like this:
>>>>>
>>>>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>>>>
>>>>> That example has all the performance data at an even higher load
>>>>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>>>>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>>>>
>>>>> I guess now I have to not just find a diff amp but also add an analog
>>>>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>>>>> Which will double as a space heater where I really can't use heat. Grumble!
>>>>>
>>>>
>>>> Looking a the modem at my Internet connection, one option might be to
>>>> use DSL or PLC drivers like this:
>>>>
>>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>>>>
>>>> Gets iffy though, the GND thermal pad would need to be connected to a
>>>> VCC- plane. They are also not intended to work well down to DC but
>>>> probably will be ok there.
>>>>
>>>> Has anyone used DSL or PLC drivers as differential amps?
>>>
>>> I've used the ADA4950-1 as a differential ADC driver, but it should be
>>> able to drive a cable. Seems well behaved.
>>>
>>
>> Thanks, that one looks nice though the data for 2Vpp and 200ohms is
>> marginal. I need 5Vpp into 100ohms. Maybe the dual version ADA450-2
>> could be run in parallel. Possibly that requires 10ohms or so in series
>> with each output, meaning four more resistors.
>>
>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4950-1_4950-2.pdf
>>
>> Figure 22 seems to contradict figure 23.
>>
>>
>>> Actually, I designed my gadget with an LT6402, but it was nasty so I
>>> had to redesign with the ADI part.
>>>
>>
>> The distortion specs don't look good.
>
> THD increases even more when a part breaks into oscillation.
>

Oh yeah. Sometimes oscillation goes unnoticed until a neighbor bangs on 
the door.

>
>> High speed or RF isn't really
>> LTC's turf, they are more a switch-mode controller company. Well, now
>> it's all AD anyhow.
>
> Some of the LTC stuff is good, but it's all too expensive.
>
> ADI often gets the digital parts of mixed-signal stuff wrong. LTC
> seems OK about that.
>
> TI does good work.
>

I am sloshing through DSL drivers now. This was seriously better in the 
olden days where National had their "fast drivers" and "damn fast 
drivers". No kidding, that's what they wrote in the databook for the LH0063.

I guess nobody sends stuff over cable anymore. It's now all Bluetooth 
and apps and virtual-whatever.

One of the problems with AD, TI and some other companies is that they 
have done away with real support at least for smaller companies and 
consultants (even though this project is for a really big client). They 
think that a user forum comprises "support".

-- 
Regards, Joerg

http://www.analogconsultants.com/

[toc] | [prev] | [next] | [standalone]

#489076

On Fri, 22 Dec 2017 14:19:12 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2017-12-22 14:06, John Larkin wrote:
>> On Fri, 22 Dec 2017 13:29:52 -0800, Joerg <news@analogconsultants.com>
>> wrote:
>>
>>> On 2017-12-22 12:54, John Larkin wrote:
>>>> On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
>>>> wrote:
>>>>
>>>>> On 2017-12-21 15:48, Joerg wrote:
>>>>>> On a design I am working right now I need to drive differential cables
>>>>>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>>>>>> instead but the customer is king. Looking at the various diff amps du
>>>>>> jour I consistently find specs like this:
>>>>>>
>>>>>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>>>>>
>>>>>> That example has all the performance data at an even higher load
>>>>>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>>>>>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>>>>>
>>>>>> I guess now I have to not just find a diff amp but also add an analog
>>>>>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>>>>>> Which will double as a space heater where I really can't use heat. Grumble!
>>>>>>
>>>>>
>>>>> Looking a the modem at my Internet connection, one option might be to
>>>>> use DSL or PLC drivers like this:
>>>>>
>>>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>>>>>
>>>>> Gets iffy though, the GND thermal pad would need to be connected to a
>>>>> VCC- plane. They are also not intended to work well down to DC but
>>>>> probably will be ok there.
>>>>>
>>>>> Has anyone used DSL or PLC drivers as differential amps?
>>>>
>>>> I've used the ADA4950-1 as a differential ADC driver, but it should be
>>>> able to drive a cable. Seems well behaved.
>>>>
>>>
>>> Thanks, that one looks nice though the data for 2Vpp and 200ohms is
>>> marginal. I need 5Vpp into 100ohms. Maybe the dual version ADA450-2
>>> could be run in parallel. Possibly that requires 10ohms or so in series
>>> with each output, meaning four more resistors.
>>>
>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4950-1_4950-2.pdf
>>>
>>> Figure 22 seems to contradict figure 23.
>>>
>>>
>>>> Actually, I designed my gadget with an LT6402, but it was nasty so I
>>>> had to redesign with the ADI part.
>>>>
>>>
>>> The distortion specs don't look good.
>>
>> THD increases even more when a part breaks into oscillation.
>>
>
>Oh yeah. Sometimes oscillation goes unnoticed until a neighbor bangs on 
>the door.
>
>>
>>> High speed or RF isn't really
>>> LTC's turf, they are more a switch-mode controller company. Well, now
>>> it's all AD anyhow.
>>
>> Some of the LTC stuff is good, but it's all too expensive.
>>
>> ADI often gets the digital parts of mixed-signal stuff wrong. LTC
>> seems OK about that.
>>
>> TI does good work.
>>
>
>I am sloshing through DSL drivers now. This was seriously better in the 
>olden days where National had their "fast drivers" and "damn fast 
>drivers". No kidding, that's what they wrote in the databook for the LH0063.
>
>I guess nobody sends stuff over cable anymore. It's now all Bluetooth 
>and apps and virtual-whatever.
>
>One of the problems with AD, TI and some other companies is that they 
>have done away with real support at least for smaller companies and 
>consultants (even though this project is for a really big client). They 
>think that a user forum comprises "support".

Some of the semi people have support folks that prowl the forums and
really answer questions; quality varies. I think the idea is that if
support is done inn public, more people get to see it all.

But it's impressive how often nobody has the answer. A recent question
invoked the response "That's an old Burr-Brown part. Nobody knows
anything about it." (dual 14-bit TXDAC.)




-- 

John Larkin         Highland Technology, Inc
picosecond timing   precision measurement 

jlarkin att highlandtechnology dott com
http://www.highlandtechnology.com

[toc] | [prev] | [next] | [standalone]

#489079

On 2017-12-22 14:40, John Larkin wrote:
> On Fri, 22 Dec 2017 14:19:12 -0800, Joerg <news@analogconsultants.com>
> wrote:
>
>> On 2017-12-22 14:06, John Larkin wrote:
>>> On Fri, 22 Dec 2017 13:29:52 -0800, Joerg <news@analogconsultants.com>
>>> wrote:
>>>
>>>> On 2017-12-22 12:54, John Larkin wrote:
>>>>> On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
>>>>> wrote:
>>>>>
>>>>>> On 2017-12-21 15:48, Joerg wrote:
>>>>>>> On a design I am working right now I need to drive differential cables
>>>>>>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>>>>>>> instead but the customer is king. Looking at the various diff amps du
>>>>>>> jour I consistently find specs like this:
>>>>>>>
>>>>>>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>>>>>>
>>>>>>> That example has all the performance data at an even higher load
>>>>>>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>>>>>>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>>>>>>
>>>>>>> I guess now I have to not just find a diff amp but also add an analog
>>>>>>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>>>>>>> Which will double as a space heater where I really can't use heat. Grumble!
>>>>>>>
>>>>>>
>>>>>> Looking a the modem at my Internet connection, one option might be to
>>>>>> use DSL or PLC drivers like this:
>>>>>>
>>>>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>>>>>>
>>>>>> Gets iffy though, the GND thermal pad would need to be connected to a
>>>>>> VCC- plane. They are also not intended to work well down to DC but
>>>>>> probably will be ok there.
>>>>>>
>>>>>> Has anyone used DSL or PLC drivers as differential amps?
>>>>>
>>>>> I've used the ADA4950-1 as a differential ADC driver, but it should be
>>>>> able to drive a cable. Seems well behaved.
>>>>>
>>>>
>>>> Thanks, that one looks nice though the data for 2Vpp and 200ohms is
>>>> marginal. I need 5Vpp into 100ohms. Maybe the dual version ADA450-2
>>>> could be run in parallel. Possibly that requires 10ohms or so in series
>>>> with each output, meaning four more resistors.
>>>>
>>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4950-1_4950-2.pdf
>>>>
>>>> Figure 22 seems to contradict figure 23.
>>>>
>>>>
>>>>> Actually, I designed my gadget with an LT6402, but it was nasty so I
>>>>> had to redesign with the ADI part.
>>>>>
>>>>
>>>> The distortion specs don't look good.
>>>
>>> THD increases even more when a part breaks into oscillation.
>>>
>>
>> Oh yeah. Sometimes oscillation goes unnoticed until a neighbor bangs on
>> the door.
>>
>>>
>>>> High speed or RF isn't really
>>>> LTC's turf, they are more a switch-mode controller company. Well, now
>>>> it's all AD anyhow.
>>>
>>> Some of the LTC stuff is good, but it's all too expensive.
>>>
>>> ADI often gets the digital parts of mixed-signal stuff wrong. LTC
>>> seems OK about that.
>>>
>>> TI does good work.
>>>
>>
>> I am sloshing through DSL drivers now. This was seriously better in the
>> olden days where National had their "fast drivers" and "damn fast
>> drivers". No kidding, that's what they wrote in the databook for the LH0063.
>>
>> I guess nobody sends stuff over cable anymore. It's now all Bluetooth
>> and apps and virtual-whatever.
>>
>> One of the problems with AD, TI and some other companies is that they
>> have done away with real support at least for smaller companies and
>> consultants (even though this project is for a really big client). They
>> think that a user forum comprises "support".
>
> Some of the semi people have support folks that prowl the forums and
> really answer questions; quality varies. I think the idea is that if
> support is done inn public, more people get to see it all.
>

That has gradually become a business driver for me. Years ago larger 
companies could still get someone from the semi mfg who almost lived 
there for a week and helped them with a tricky design. Not anymore, now 
they are on their own.


> But it's impressive how often nobody has the answer. A recent question
> invoked the response "That's an old Burr-Brown part. Nobody knows
> anything about it." (dual 14-bit TXDAC.)
>

Or they might say "You have to go to Golden Sunset Senior Center and ask 
for resident Joe Smith".

Just came across the THS6214 which looks more capabable than the others 
but +/-5V would be at the very bottom supply range for this chip:

http://www.ti.com/lit/ds/symlink/ths6214.pdf

It's also long in the tooth. 2009 is almost ancient for telco chips.

-- 
Regards, Joerg

http://www.analogconsultants.com/

[toc] | [prev] | [next] | [standalone]

#489089

On Fri, 22 Dec 2017 15:16:16 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2017-12-22 14:40, John Larkin wrote:
>> On Fri, 22 Dec 2017 14:19:12 -0800, Joerg <news@analogconsultants.com>
>> wrote:
>>
>>> On 2017-12-22 14:06, John Larkin wrote:
>>>> On Fri, 22 Dec 2017 13:29:52 -0800, Joerg <news@analogconsultants.com>
>>>> wrote:
>>>>
>>>>> On 2017-12-22 12:54, John Larkin wrote:
>>>>>> On Fri, 22 Dec 2017 12:07:12 -0800, Joerg <news@analogconsultants.com>
>>>>>> wrote:
>>>>>>
>>>>>>> On 2017-12-21 15:48, Joerg wrote:
>>>>>>>> On a design I am working right now I need to drive differential cables
>>>>>>>> at up to 100MHz and with gusto, several volts swing. I'd use coax
>>>>>>>> instead but the customer is king. Looking at the various diff amps du
>>>>>>>> jour I consistently find specs like this:
>>>>>>>>
>>>>>>>> http://www.ti.com/lit/ds/symlink/ths4503.pdf
>>>>>>>>
>>>>>>>> That example has all the performance data at an even higher load
>>>>>>>> resistance of 800ohms. Considering that regular ribbon is around 100ohms
>>>>>>>> diff and even grandpa's old TV cable was 300ohms, what good does that do?
>>>>>>>>
>>>>>>>> I guess now I have to not just find a diff amp but also add an analog
>>>>>>>> diff buffer IC (if they still exist) or roll my own transistor stuff.
>>>>>>>> Which will double as a space heater where I really can't use heat. Grumble!
>>>>>>>>
>>>>>>>
>>>>>>> Looking a the modem at my Internet connection, one option might be to
>>>>>>> use DSL or PLC drivers like this:
>>>>>>>
>>>>>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4311-1.pdf
>>>>>>>
>>>>>>> Gets iffy though, the GND thermal pad would need to be connected to a
>>>>>>> VCC- plane. They are also not intended to work well down to DC but
>>>>>>> probably will be ok there.
>>>>>>>
>>>>>>> Has anyone used DSL or PLC drivers as differential amps?
>>>>>>
>>>>>> I've used the ADA4950-1 as a differential ADC driver, but it should be
>>>>>> able to drive a cable. Seems well behaved.
>>>>>>
>>>>>
>>>>> Thanks, that one looks nice though the data for 2Vpp and 200ohms is
>>>>> marginal. I need 5Vpp into 100ohms. Maybe the dual version ADA450-2
>>>>> could be run in parallel. Possibly that requires 10ohms or so in series
>>>>> with each output, meaning four more resistors.
>>>>>
>>>>> http://www.analog.com/media/en/technical-documentation/data-sheets/ADA4950-1_4950-2.pdf
>>>>>
>>>>> Figure 22 seems to contradict figure 23.
>>>>>
>>>>>
>>>>>> Actually, I designed my gadget with an LT6402, but it was nasty so I
>>>>>> had to redesign with the ADI part.
>>>>>>
>>>>>
>>>>> The distortion specs don't look good.
>>>>
>>>> THD increases even more when a part breaks into oscillation.
>>>>
>>>
>>> Oh yeah. Sometimes oscillation goes unnoticed until a neighbor bangs on
>>> the door.
>>>
>>>>
>>>>> High speed or RF isn't really
>>>>> LTC's turf, they are more a switch-mode controller company. Well, now
>>>>> it's all AD anyhow.
>>>>
>>>> Some of the LTC stuff is good, but it's all too expensive.
>>>>
>>>> ADI often gets the digital parts of mixed-signal stuff wrong. LTC
>>>> seems OK about that.
>>>>
>>>> TI does good work.
>>>>
>>>
>>> I am sloshing through DSL drivers now. This was seriously better in the
>>> olden days where National had their "fast drivers" and "damn fast
>>> drivers". No kidding, that's what they wrote in the databook for the LH0063.
>>>
>>> I guess nobody sends stuff over cable anymore. It's now all Bluetooth
>>> and apps and virtual-whatever.
>>>
>>> One of the problems with AD, TI and some other companies is that they
>>> have done away with real support at least for smaller companies and
>>> consultants (even though this project is for a really big client). They
>>> think that a user forum comprises "support".
>>
>> Some of the semi people have support folks that prowl the forums and
>> really answer questions; quality varies. I think the idea is that if
>> support is done inn public, more people get to see it all.
>>
>
>That has gradually become a business driver for me. Years ago larger 
>companies could still get someone from the semi mfg who almost lived 
>there for a week and helped them with a tricky design. Not anymore, now 
>they are on their own.
>
>
>> But it's impressive how often nobody has the answer. A recent question
>> invoked the response "That's an old Burr-Brown part. Nobody knows
>> anything about it." (dual 14-bit TXDAC.)
>>
>
>Or they might say "You have to go to Golden Sunset Senior Center and ask 
>for resident Joe Smith".
>
>Just came across the THS6214 which looks more capabable than the others 
>but +/-5V would be at the very bottom supply range for this chip:
>
>http://www.ti.com/lit/ds/symlink/ths6214.pdf
>
>It's also long in the tooth. 2009 is almost ancient for telco chips.

I designed a couple things around the THS3062, which was claimed as
suitable as a VSDL line driver. What I didn't realize is that it's
*only* safe to use as a VSDL line driver. If you use it as a
general-purpose opamp, it tends to fry itself. The latest version of
the data sheet mentions the problem.

So be careful!

IC data sheets are not universally to be believed. Nor are their Spice
models. I usually breadboard a new part.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

[toc] | [prev] | [next] | [standalone]

#489092

On 2017-12-22 16:30, John Larkin wrote:
> On Fri, 22 Dec 2017 15:16:16 -0800, Joerg <news@analogconsultants.com>
> wrote:
>
>> On 2017-12-22 14:40, John Larkin wrote:

[...]

>>
>>> But it's impressive how often nobody has the answer. A recent question
>>> invoked the response "That's an old Burr-Brown part. Nobody knows
>>> anything about it." (dual 14-bit TXDAC.)
>>>
>>
>> Or they might say "You have to go to Golden Sunset Senior Center and ask
>> for resident Joe Smith".
>>
>> Just came across the THS6214 which looks more capabable than the others
>> but +/-5V would be at the very bottom supply range for this chip:
>>
>> http://www.ti.com/lit/ds/symlink/ths6214.pdf
>>
>> It's also long in the tooth. 2009 is almost ancient for telco chips.
>
> I designed a couple things around the THS3062, which was claimed as
> suitable as a VSDL line driver. What I didn't realize is that it's
> *only* safe to use as a VSDL line driver. If you use it as a
> general-purpose opamp, it tends to fry itself. The latest version of
> the data sheet mentions the problem.
>

That one is a CFB amp. If used as an integrator or even with the 
slightest capacitive feedback it coan oscillate and unsolder itself. 
They also don't like capacitive loads but that goes for most high-speed 
VFB opamps as well.


> So be careful!
>

I will. Sometimes I have the feeling that datasheets are written by 
marketing types. Why else would anyone spec a diff "driver" at 500, 800 
or in some cases even 1000 ohms? It's like specifying a pickup truck's 
performance with a load of two six-packs in the bed.


> IC data sheets are not universally to be believed. Nor are their Spice
> models. I usually breadboard a new part.
>

I might go with the ADA4950 for now and place 10ohms each in series so 
it won't go berserk when it sees a few pF of capacitive load. Cable is 
never ideal. Its drive capabilities aren't much to write home about but 
this is more a proof-of-concept. Later we'll likely try a DSL driver, 
depending on high much THD and IM performance we end up needing. 
Sometimes you have to run a really high voltage supply rail and divide 
the output back down or clamp it.

-- 
Regards, Joerg

http://www.analogconsultants.com/

[toc] | [prev] | [next] | [standalone]

#489114

On Fri, 22 Dec 2017 16:57:21 -0800, Joerg <news@analogconsultants.com>
wrote:

>On 2017-12-22 16:30, John Larkin wrote:
>> On Fri, 22 Dec 2017 15:16:16 -0800, Joerg <news@analogconsultants.com>
>> wrote:
>>
>>> On 2017-12-22 14:40, John Larkin wrote:
>
>[...]
>
>>>
>>>> But it's impressive how often nobody has the answer. A recent question
>>>> invoked the response "That's an old Burr-Brown part. Nobody knows
>>>> anything about it." (dual 14-bit TXDAC.)
>>>>
>>>
>>> Or they might say "You have to go to Golden Sunset Senior Center and ask
>>> for resident Joe Smith".
>>>
>>> Just came across the THS6214 which looks more capabable than the others
>>> but +/-5V would be at the very bottom supply range for this chip:
>>>
>>> http://www.ti.com/lit/ds/symlink/ths6214.pdf
>>>
>>> It's also long in the tooth. 2009 is almost ancient for telco chips.
>>
>> I designed a couple things around the THS3062, which was claimed as
>> suitable as a VSDL line driver. What I didn't realize is that it's
>> *only* safe to use as a VSDL line driver. If you use it as a
>> general-purpose opamp, it tends to fry itself. The latest version of
>> the data sheet mentions the problem.
>>
>
>That one is a CFB amp. If used as an integrator or even with the 
>slightest capacitive feedback it coan oscillate and unsolder itself. 
>They also don't like capacitive loads but that goes for most high-speed 
>VFB opamps as well.
>
>
>> So be careful!
>>
>
>I will. Sometimes I have the feeling that datasheets are written by 
>marketing types. Why else would anyone spec a diff "driver" at 500, 800 
>or in some cases even 1000 ohms? It's like specifying a pickup truck's 
>performance with a load of two six-packs in the bed.
>
>
>> IC data sheets are not universally to be believed. Nor are their Spice
>> models. I usually breadboard a new part.
>>
>
>I might go with the ADA4950 for now and place 10ohms each in series so 
>it won't go berserk when it sees a few pF of capacitive load. 

Why not 50 ohms in series with each output, to source terminate?


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

[toc] | [prev] | [next] | [standalone]

#489145

On 2017-12-22 20:33, John Larkin wrote:
> On Fri, 22 Dec 2017 16:57:21 -0800, Joerg <news@analogconsultants.com>
> wrote:
>
>> On 2017-12-22 16:30, John Larkin wrote:
>>> On Fri, 22 Dec 2017 15:16:16 -0800, Joerg <news@analogconsultants.com>
>>> wrote:
>>>
>>>> On 2017-12-22 14:40, John Larkin wrote:
>>
>> [...]
>>
>>>>
>>>>> But it's impressive how often nobody has the answer. A recent question
>>>>> invoked the response "That's an old Burr-Brown part. Nobody knows
>>>>> anything about it." (dual 14-bit TXDAC.)
>>>>>
>>>>
>>>> Or they might say "You have to go to Golden Sunset Senior Center and ask
>>>> for resident Joe Smith".
>>>>
>>>> Just came across the THS6214 which looks more capabable than the others
>>>> but +/-5V would be at the very bottom supply range for this chip:
>>>>
>>>> http://www.ti.com/lit/ds/symlink/ths6214.pdf
>>>>
>>>> It's also long in the tooth. 2009 is almost ancient for telco chips.
>>>
>>> I designed a couple things around the THS3062, which was claimed as
>>> suitable as a VSDL line driver. What I didn't realize is that it's
>>> *only* safe to use as a VSDL line driver. If you use it as a
>>> general-purpose opamp, it tends to fry itself. The latest version of
>>> the data sheet mentions the problem.
>>>
>>
>> That one is a CFB amp. If used as an integrator or even with the
>> slightest capacitive feedback it coan oscillate and unsolder itself.
>> They also don't like capacitive loads but that goes for most high-speed
>> VFB opamps as well.
>>
>>
>>> So be careful!
>>>
>>
>> I will. Sometimes I have the feeling that datasheets are written by
>> marketing types. Why else would anyone spec a diff "driver" at 500, 800
>> or in some cases even 1000 ohms? It's like specifying a pickup truck's
>> performance with a load of two six-packs in the bed.
>>
>>
>>> IC data sheets are not universally to be believed. Nor are their Spice
>>> models. I usually breadboard a new part.
>>>
>>
>> I might go with the ADA4950 for now and place 10ohms each in series so
>> it won't go berserk when it sees a few pF of capacitive load.
>
> Why not 50 ohms in series with each output, to source terminate?
>

Then I don't get enough swing. The amplitude specs (for any decent 
distortion performance) are just as paltry as the allowed load. Also, I 
need +/-2.5V on each leg for some of the lines and with 5V a source 
termination just isn't possible. If I have to use 24V DSL drivers I can 
supply them with +/12V and then I'd use source termination. Have to, in 
order to protect gear downstream that is on +/-5V if someone forgets the 
end termination.

-- 
Regards, Joerg

http://www.analogconsultants.com/

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