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Another op-amp stability question (LT1677)

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  Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-29 00:37 -0700
    Another op-amp stability question (LT1677) pcdhobbs@gmail.com - 2017-03-29 04:50 -0700
      Re: Another op-amp stability question (LT1677) piglet <erichpwagner@hotmail.com> - 2017-03-29 13:23 +0100
      Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-29 10:58 -0700
        Re: Another op-amp stability question (LT1677) tabbypurr@gmail.com - 2017-03-29 11:15 -0700
        Re: Another op-amp stability question (LT1677) Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-03-29 23:29 +0200
          Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-29 17:14 -0700
            Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-29 18:06 -0700
            Re: Another op-amp stability question (LT1677) Gerhard Hoffmann <ghf@hoffmann-hochfrequenz.de> - 2017-03-31 14:40 +0200
              Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-31 14:49 -0700
                Re: Another op-amp stability question (LT1677) Phil Hobbs <pcdhSpamMeSenseless@electrooptical.net> - 2017-03-31 20:39 -0400
                Re: Another op-amp stability question (LT1677) tabbypurr@gmail.com - 2017-04-01 09:16 -0700
                  Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-04-01 15:03 -0700
                    Re: Another op-amp stability question (LT1677) Winfield Hill <hill@rowland.harvard.edu> - 2017-04-01 16:46 -0700
                      Re: Another op-amp stability question (LT1677) John Larkin <jjlarkin@highlandtechnology.com> - 2017-04-01 17:31 -0700
                    Re: Another op-amp stability question (LT1677) tabbypurr@gmail.com - 2017-04-01 20:59 -0700
                      Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-04-01 21:41 -0700
                        Re: Another op-amp stability question (LT1677) tabbypurr@gmail.com - 2017-04-02 04:53 -0700
                          Re: Another op-amp stability question (LT1677) John S <Sophi.2@invalid.org> - 2017-04-02 16:26 -0500
                            Re: Another op-amp stability question (LT1677) krw@notreal.com - 2017-04-02 19:29 -0400
                              Re: Another op-amp stability question (LT1677) John S <Sophi.2@invalid.org> - 2017-04-02 18:34 -0500
    Re: Another op-amp stability question (LT1677) John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-29 05:18 -0700
      Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-29 10:56 -0700
        Re: Another op-amp stability question (LT1677) John Larkin <jjlarkin@highlandtechnology.com> - 2017-03-29 13:02 -0700
          Re: Another op-amp stability question (LT1677) "John Miles, KE5FX" <jmiles@gmail.com> - 2017-03-29 17:57 -0700
            Re: Another op-amp stability question (LT1677) John S <Sophi.2@invalid.org> - 2017-04-02 16:31 -0500
        Re: Another op-amp stability question (LT1677) John Larkin <jjlarkin@highlandtechnology.com> - 2017-04-15 08:58 -0700

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#454102 — Another op-amp stability question (LT1677)

Tim W.'s question about the influence of resistance in the feedback path
of a voltage follower reminded me of a similar problem I ran into a while
back.  

I've used LT1677s as an integrator in several low-noise PLLs in the 
past and been satisfied with its performance.  It's an old part and 
a fairly expensive one, but it's quiet and well-behaved in loop filters, 
with RRIO operation and guaranteed freedom from phase reversal.  It's 
slow (GBP=7 MHz) but not slow enough to be a problem in most RF PLLs.  
You could ask for lower input bias current, I guess.  But overall the 
LT1677 is a good "jellybean" opamp for use between a low-voltage charge 
pump and a high-voltage VCO.

It's also characterized explicitly for use at Av=+1; see page 13 of the
data sheet at http://www.linear.com/docs/2102 .  At one point, I needed 
a voltage follower for something I was breadboarding, and an LT1677 was 
already lying around on the workbench.  When I put it in the circuit, 
I observed a nasty oscillation that didn't make any sense.  It's easy to 
reproduce the setup that misbehaves.  Tie pin 2 to pin 6 as usual, put 
a scope probe on pin 6, and feed a steady DC level or an LF tone to 
pin 3:

http://www.ke5fx.com/1677.jpg 

When the input (and output) level approaches 1.13V, I see this:

http://www.ke5fx.com/1677_osc_sine.png (when fed with sine wave)

http://www.ke5fx.com/1677_osc.png (when fed with DC from trimpot)

Things that don't have a significant effect on the symptom:

- Load resistance, either in series or parallel with the scope probe
- Load capacitance (lots of C will cause instability as expected)
- Source impedance (4.7K in this test bed, but doesn't matter)
- Supply voltage (might be a little less prone to occur at >10V)
- Resistance in feedback path (too much causes instability as expected,
and it might get a bit better with some values of resistance, but
I can always make it happen)
- The usual suspects like power supply impedance and bypassing, layout,
etc.

Things that do have an effect:

- The frequency of the input signal.  If I increase the frequency from
the function generator beyond about 1 kHz, the oscillation goes
completely away (?!)

- Temperature.  R134a spray lowers the amplitude of the oscillation,
while a heat gun makes it worse.

- Using a different opamp.  No other parts rated for Avmin=+1 within easy
reach exhibit this behavior.  Any given LT1677 from my parts drawer will.

So... what's up with this?  Can anyone else repro the behavior?  Maybe 
I just got a bad batch of parts, or I'm doing something dumb.  Hope so.

-- john, KE5FX

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#454107

>So... what's up with this?  Can anyone else repro the behavior?  Maybe 
>I just got a bad batch of parts, or I'm doing something dumb.  Hope so. 

Where's the supply bypass? First guess is that 100 nF to ground will fix it. Keep the leads less than 1/4 inch long. 

Not everything in the amp is as slow as 7 MHz--it looks like the two input stages (required for RRI operation) are fighting each other at the crossover point.

Cheers

Phil Hobbs

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#454111

On 29/03/2017 12:50, pcdhobbs@gmail.com wrote:
>> So... what's up with this?  Can anyone else repro the behavior?  Maybe
>> I just got a bad batch of parts, or I'm doing something dumb.  Hope so.
>
> Where's the supply bypass? First guess is that 100 nF to ground will fix it. Keep the leads less than 1/4 inch long.
>
> Not everything in the amp is as slow as 7 MHz--it looks like the two input stages (required for RRI operation) are fighting each other at the crossover point.
>
> Cheers
>
> Phil Hobbs
>

Datasheet indicates input stage NPN/PNP handover occurs at "approx 1.5V" 
above negative rail so that figures with your observed 1.2 threshold.

piglet

---
This email has been checked for viruses by Avast antivirus software.
https://www.avast.com/antivirus

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#454135

On Wednesday, March 29, 2017 at 4:50:57 AM UTC-7, pcdh...@gmail.com wrote:
> Where's the supply bypass? First guess is that 100 nF to ground will fix it. 

Big glass-encapsulated axial ceramic in photo.  I think it's 330 nF but it 
might be 100.  Makes no difference.

-- john, KE5FX

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#454140

On Wednesday, 29 March 2017 18:58:27 UTC+1, John Miles, KE5FX  wrote:
> On Wednesday, March 29, 2017 at 4:50:57 AM UTC-7, pcdh...@gmail.com wrote:
> > Where's the supply bypass? First guess is that 100 nF to ground will fix it. 
> 
> Big glass-encapsulated axial ceramic in photo.  I think it's 330 nF but it 
> might be 100.  Makes no difference.

I don't know either. I presume you've tried slotting in other opamps, were they ok?


NT

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#454148

Am 29.03.2017 um 19:58 schrieb John Miles, KE5FX:
> On Wednesday, March 29, 2017 at 4:50:57 AM UTC-7, pcdh...@gmail.com wrote:
>> Where's the supply bypass? First guess is that 100 nF to ground will fix it.
>
> Big glass-encapsulated axial ceramic in photo.  I think it's 330 nF but it
> might be 100.  Makes no difference.
>
> -- john, KE5FX
>

I have just built and tested it.
The setup is in

< 
https://www.flickr.com/photos/137684711@N07/32912433303/in/dateposted-public/ 
 >

I used a HP3325B as signal source, 500 Hz +- .
The built-in offset voltage is nice here.
There is a 49R9 termination resistor from Pin3 to gnd.

No problems whatsoever with Vcc from 3 to 15V.
But with Vcc < 2.4V hell breaks loose.
You can get any distortion or oscillation you can imagine
by just playing with the voltages.

But the spec says 3V min, so that is ok.

regards, Gerhard

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#454157

On Wednesday, March 29, 2017 at 2:29:23 PM UTC-7, Gerhard Hoffmann wrote:
> There is a 49R9 termination resistor from Pin3 to gnd.
> 

Thanks, that seems to be a good clue.  I was mistaken when I said the 
source Z "didn't matter" -- I must not have tried anything very low.  
Can you take the 49.9 ohm termination out, and add a 10K series 
resistance from the function generator instead?  Set the 3325B to 
50 Hz, 1V peak, 1V offset, and run the LT1677 from +5V.  See if it 
looks like this:

http://www.ke5fx.com/3325B_1Vpk_1Voff_10K_series_R_Vcc5.png

When I use a termination under a couple K, the instability goes away.

-- john, KE5FX 

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#454168

On Wednesday, March 29, 2017 at 5:14:52 PM UTC-7, John Miles, KE5FX wrote:
> http://www.ke5fx.com/3325B_1Vpk_1Voff_10K_series_R_Vcc5.png

By the way, don't use any so-called "high resolution" modes or other 
averaging buttons on your DSO when you look for this effect.  They clean 
up the oscillation nicely.

-- john, KE5FX

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#454395

Am 30.03.2017 um 02:14 schrieb John Miles, KE5FX:
> On Wednesday, March 29, 2017 at 2:29:23 PM UTC-7, Gerhard Hoffmann wrote:
>> There is a 49R9 termination resistor from Pin3 to gnd.
>>
>
> Thanks, that seems to be a good clue.  I was mistaken when I said the
> source Z "didn't matter" -- I must not have tried anything very low.
> Can you take the 49.9 ohm termination out, and add a 10K series
> resistance from the function generator instead?  Set the 3325B to
> 50 Hz, 1V peak, 1V offset, and run the LT1677 from +5V.  See if it
> looks like this:
>
> http://www.ke5fx.com/3325B_1Vpk_1Voff_10K_series_R_Vcc5.png
>
> When I use a termination under a couple K, the instability goes away.
>
> -- john, KE5FX
>


Hi, John,

you have found a real bug with the LT1677 !

< 
https://www.flickr.com/photos/137684711@N07/33598135802/in/album-72157662535945536/ 
 >

cheers,

Gerhard, DK4XP

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#454485

On Friday, March 31, 2017 at 5:40:10 AM UTC-7, Gerhard Hoffmann wrote:
> Hi, John,
> 
> you have found a real bug with the LT1677 !
> 
> < 
> https://www.flickr.com/photos/137684711@N07/33598135802/in/album-72157662535945536/ 

Thanks very much for checking, Gerhard. I was hoping I just got a bad 
batch of LT1677s or some midnight-shift specials out of Shenzhen.

Maybe this is common/expected behavior when driving RRI opamps from 
high-Z sources, but manufacturers need to document it if so.

-- john, KE5FX

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#454536

On 03/31/2017 05:49 PM, John Miles, KE5FX wrote:
> On Friday, March 31, 2017 at 5:40:10 AM UTC-7, Gerhard Hoffmann wrote:
>> Hi, John,
>>
>> you have found a real bug with the LT1677 !
>>
>> <
>> https://www.flickr.com/photos/137684711@N07/33598135802/in/album-72157662535945536/
>
> Thanks very much for checking, Gerhard. I was hoping I just got a bad
> batch of LT1677s or some midnight-shift specials out of Shenzhen.
>
> Maybe this is common/expected behavior when driving RRI opamps from
> high-Z sources, but manufacturers need to document it if so.
>
> -- john, KE5FX
>

Nah, there are high-Z things in the datasheet's applications circuits.

That sort of mistake is pretty rare for LTC in my experience--they seem 
to have learned from the LT1028 noise peak debacle.  (Early LT1028 
datasheets had noise plots that cut off at 20 kHz or so, neatly 
disguising the big peak at ~300 kHz that disfigures what would otherwise 
be almost the universal low noise, low-Z op amp.

(I much prefer the ADA4898 because it doesn't have the big peak and is 
much less squirrelly generally.  Not as fast, though, nor quite as quiet 
outside of the peak.)

Cheers

Phil Hobbs

-- 
Dr Philip C D Hobbs
Principal Consultant
ElectroOptical Innovations LLC
Optics, Electro-optics, Photonics, Analog Electronics

160 North State Road #203
Briarcliff Manor NY 10510

hobbs at electrooptical dot net
http://electrooptical.net

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#454620

On Friday, 31 March 2017 22:49:50 UTC+1, John Miles, KE5FX  wrote:
> On Friday, March 31, 2017 at 5:40:10 AM UTC-7, Gerhard Hoffmann wrote:
> > Hi, John,
> > 
> > you have found a real bug with the LT1677 !
> > 
> > < 
> > https://www.flickr.com/photos/137684711@N07/33598135802/in/album-72157662535945536/ 
> 
> Thanks very much for checking, Gerhard. I was hoping I just got a bad 
> batch of LT1677s or some midnight-shift specials out of Shenzhen.
> 
> Maybe this is common/expected behavior when driving RRI opamps from 
> high-Z sources, but manufacturers need to document it if so.

It's a known issue with some opamps, but I've stayed quiet cos I can't remember where a datasheet (IIRC) talks about it. IIRC the advise was to keep source impedance under 10k, and the cause is stray capacitive coupling from output to input plus a faster input stage than output stage. But as I say, I can't remember where this was from so ICBW.


NT

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#454664

On Saturday, April 1, 2017 at 9:16:52 AM UTC-7, tabb...@gmail.com wrote:
> It's a known issue with some opamps, but I've stayed quiet cos I can't 
> remember where a datasheet (IIRC) talks about it. IIRC the advise was 
> to keep source impedance under 10k, and the cause is stray capacitive 
> coupling from output to input plus a faster input stage than output 
> stage. But as I say, I can't remember where this was from so ICBW.

Doesn't seem like a typical phase-margin gotcha, or one that involves external
coupling or feedback of any kind.  There's no sensitivity to stray C at 
all.  There must be a thermal component to the phenomenon, because it's only
noticeable at very low frequencies and is quite sensitive to temperature.  The
oscillation itself is around 700 - 800 kHz but I've never seen it appear on
signals above a few kHz.  No electrical time constants are anywhere near that.

Given that people don't usually use pricy low-noise opamps with high-
resistance sources, I guess it's not too surprising that the issue isn't
well-known.  That's no excuse for manufacturers not knowing about it and
warning accordingly, though.

-- john, KE5FX

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#454677

John Miles, KE5FX wrote...
>
>On Saturday, April 1, 2017 at 9:16:52 AM UTC-7, tabb...@gmail.com wrote:
>> It's a known issue with some opamps, but I've stayed quiet cos I can't 
>> remember where a datasheet (IIRC) talks about it. IIRC the advise was 
>> to keep source impedance under 10k, and the cause is stray capacitive 
>> coupling from output to input plus a faster input stage than output 
>> stage. But as I say, I can't remember where this was from so ICBW.
>
>Doesn't seem like a typical phase-margin gotcha, or one that involves external
>coupling or feedback of any kind.  There's no sensitivity to stray C at 
>all.  There must be a thermal component to the phenomenon, because it's only
>noticeable at very low frequencies and is quite sensitive to temperature.  The
>oscillation itself is around 700 - 800 kHz but I've never seen it appear on
>signals above a few kHz.  No electrical time constants are anywhere near that.
>
>Given that people don't usually use pricy low-noise opamps with high-
>resistance sources, I guess it's not too surprising that the issue isn't
>well-known.  That's no excuse for manufacturers not knowing about it and
>warning accordingly, though.

 Yes, John.  But I think its safe to say opamps going through
 input-pair transitions at certain voltages are suspect to be
 used at those voltages.  Your experience illustrates that point.


-- 
 Thanks,
    - Win

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#454681

On 1 Apr 2017 16:46:00 -0700, Winfield Hill <hill@rowland.harvard.edu>
wrote:

>John Miles, KE5FX wrote...
>>
>>On Saturday, April 1, 2017 at 9:16:52 AM UTC-7, tabb...@gmail.com wrote:
>>> It's a known issue with some opamps, but I've stayed quiet cos I can't 
>>> remember where a datasheet (IIRC) talks about it. IIRC the advise was 
>>> to keep source impedance under 10k, and the cause is stray capacitive 
>>> coupling from output to input plus a faster input stage than output 
>>> stage. But as I say, I can't remember where this was from so ICBW.
>>
>>Doesn't seem like a typical phase-margin gotcha, or one that involves external
>>coupling or feedback of any kind.  There's no sensitivity to stray C at 
>>all.  There must be a thermal component to the phenomenon, because it's only
>>noticeable at very low frequencies and is quite sensitive to temperature.  The
>>oscillation itself is around 700 - 800 kHz but I've never seen it appear on
>>signals above a few kHz.  No electrical time constants are anywhere near that.
>>
>>Given that people don't usually use pricy low-noise opamps with high-
>>resistance sources, I guess it's not too surprising that the issue isn't
>>well-known.  That's no excuse for manufacturers not knowing about it and
>>warning accordingly, though.
>
> Yes, John.  But I think its safe to say opamps going through
> input-pair transitions at certain voltages are suspect to be
> used at those voltages.  Your experience illustrates that point.

But what's the use of a rail-to-rail opamp if, well, you can't use it
rail to rail? I mean, really!

Sounds like LTC scrooed up that one.


-- 

John Larkin         Highland Technology, Inc

lunatic fringe electronics 

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#454718

On Saturday, 1 April 2017 23:03:18 UTC+1, John Miles, KE5FX  wrote:
> On Saturday, April 1, 2017 at 9:16:52 AM UTC-7, tabby wrote:

> > It's a known issue with some opamps, but I've stayed quiet cos I can't 
> > remember where a datasheet (IIRC) talks about it. IIRC the advise was 
> > to keep source impedance under 10k, and the cause is stray capacitive 
> > coupling from output to input plus a faster input stage than output 
> > stage. But as I say, I can't remember where this was from so ICBW.
> 
> Doesn't seem like a typical phase-margin gotcha,

what I described isn't. It's what happens when the input stage moves fast but the output stage doesn't.

> or one that involves external
> coupling or feedback of any kind. 

You've tested for that?

> There's no sensitivity to stray C at 
> all. 

that too?

> There must be a thermal component to the phenomenon, because it's only
> noticeable at very low frequencies and is quite sensitive to temperature.  The
> oscillation itself is around 700 - 800 kHz but I've never seen it appear on
> signals above a few kHz.  No electrical time constants are anywhere near that.
> 
> Given that people don't usually use pricy low-noise opamps with high-
> resistance sources, I guess it's not too surprising that the issue isn't
> well-known.  That's no excuse for manufacturers not knowing about it and
> warning accordingly, though.

If my memory serves me correctly, they have.


NT

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#454729

On Saturday, April 1, 2017 at 8:59:59 PM UTC-7, tabb...@gmail.com wrote:

> You've tested for that?
Yep.

>that too? 
Yep.

I routinely play with 10 GHz GBW parts.  It's been a while since I stubbed my
toe on a 7-MHz one.

>If my memory serves me correctly, they have. 

Not in the data sheet I linked to, they don't.

If a data sheet from Linear Technology tells me that a part can be used as a
unity-gain buffer, I'm inclined to believe it.  I agree with the subtext 
behind Win's comment, in that ignorance of fundamentals is no excuse, but I
don't agree that this is a "fundamental" topic.  I believe that 99 out of 100
working EEs would have made the same ass-umption, so there needs to be an
explicit warning in the data sheet.

-- john, KE5FX

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#454754

On Sunday, 2 April 2017 05:41:32 UTC+1, John Miles, KE5FX  wrote:
> On Saturday, April 1, 2017 at 8:59:59 PM UTC-7, tabby wrote:
> 
> > You've tested for that?
> Yep.
> 
> >that too? 
> Yep.
> 
> I routinely play with 10 GHz GBW parts.  It's been a while since I stubbed my
> toe on a 7-MHz one.
> 
> >If my memory serves me correctly, they have. 
> 
> Not in the data sheet I linked to, they don't.
> 
> If a data sheet from Linear Technology tells me that a part can be used as a
> unity-gain buffer, I'm inclined to believe it.  I agree with the subtext 
> behind Win's comment, in that ignorance of fundamentals is no excuse, but I
> don't agree that this is a "fundamental" topic.  I believe that 99 out of 100
> working EEs would have made the same ass-umption, so there needs to be an
> explicit warning in the data sheet.

Found it.
https://www.onsemi.com/pub/Collateral/LM339-D.PDF
page 6


NT

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#454801

On 4/2/2017 6:53 AM, tabbypurr@gmail.com wrote:
> On Sunday, 2 April 2017 05:41:32 UTC+1, John Miles, KE5FX  wrote:
>> On Saturday, April 1, 2017 at 8:59:59 PM UTC-7, tabby wrote:
>>
>>> You've tested for that?
>> Yep.
>>
>>> that too?
>> Yep.
>>
>> I routinely play with 10 GHz GBW parts.  It's been a while since I stubbed my
>> toe on a 7-MHz one.
>>
>>> If my memory serves me correctly, they have.
>>
>> Not in the data sheet I linked to, they don't.
>>
>> If a data sheet from Linear Technology tells me that a part can be used as a
>> unity-gain buffer, I'm inclined to believe it.  I agree with the subtext
>> behind Win's comment, in that ignorance of fundamentals is no excuse, but I
>> don't agree that this is a "fundamental" topic.  I believe that 99 out of 100
>> working EEs would have made the same ass-umption, so there needs to be an
>> explicit warning in the data sheet.
>
> Found it.
> https://www.onsemi.com/pub/Collateral/LM339-D.PDF
> page 6
>
>
> NT


There is an error in the schematic on that page. I could tell you where 
it is, but then I'd have to kill you.

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#454820

On Sun, 2 Apr 2017 16:26:08 -0500, John S <Sophi.2@invalid.org> wrote:

>On 4/2/2017 6:53 AM, tabbypurr@gmail.com wrote:
>> On Sunday, 2 April 2017 05:41:32 UTC+1, John Miles, KE5FX  wrote:
>>> On Saturday, April 1, 2017 at 8:59:59 PM UTC-7, tabby wrote:
>>>
>>>> You've tested for that?
>>> Yep.
>>>
>>>> that too?
>>> Yep.
>>>
>>> I routinely play with 10 GHz GBW parts.  It's been a while since I stubbed my
>>> toe on a 7-MHz one.
>>>
>>>> If my memory serves me correctly, they have.
>>>
>>> Not in the data sheet I linked to, they don't.
>>>
>>> If a data sheet from Linear Technology tells me that a part can be used as a
>>> unity-gain buffer, I'm inclined to believe it.  I agree with the subtext
>>> behind Win's comment, in that ignorance of fundamentals is no excuse, but I
>>> don't agree that this is a "fundamental" topic.  I believe that 99 out of 100
>>> working EEs would have made the same ass-umption, so there needs to be an
>>> explicit warning in the data sheet.
>>
>> Found it.
>> https://www.onsemi.com/pub/Collateral/LM339-D.PDF
>> page 6
>>
>>
>> NT
>
>
>There is an error in the schematic on that page. I could tell you where 
>it is, but then I'd have to kill you.

A dot connecting the wire crossing the +15V line.  How long did it
take you to fin it?

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