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Re: Diff amps spec at 500ohms plus, why?

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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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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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