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Weird Attenuator Problem
Weird Attenuator Problem
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Old 26th February 2020, 06:28 AM   #11
simonra is offline simonra  United Kingdom
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Thanks for all the pointers, the distortion seems to be coming from OP1- I'll confirm this evening. OP1's feedback network resistors should be 1k (mistake on schematic).
Series resistance seems an obvious thing to try, I'll see what I can do without affecting the attenuator operation (and without wrecking the board).
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Old 26th February 2020, 02:24 PM   #12
Mark Tillotson is offline Mark Tillotson
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Quote:
When I started to probe the circuit I found that the distortion disappeared as soon as the probe touched the signal circuit.
Oscillation or instability is immediately suggested - an RF spectrum analyzer with a E-field probe would be one way to track it down non-invasively.


Try 10pF in parallel to R16, that's possible to tack on top temporarily. What's driving the non-inverting input of OP1?
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Old 26th February 2020, 05:12 PM   #13
simonra is offline simonra  United Kingdom
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Thanks Mark, 10pF in parallel with R16 did the trick... plus I can just stack the components without butchering the PCB, so result all round.
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Old 29th February 2020, 02:21 AM   #14
JohnW is offline JohnW  Hong Kong
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Quote:
Originally Posted by sgrossklass View Post
BJTs do not like predominantly inductive source impedance, their beta(f) curve tends to turn it into negative impedance at the emitter.
This!!!

As the original poster suggested add say 50Ohm resistor as close as possible to the opamps' inverting input to isolate it from external stray capacitance... Just like Grid stopper resistors on Tube Grids / Mosfets Gates
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Old 29th February 2020, 11:23 PM   #15
sgrossklass is offline sgrossklass  Germany
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It's a bit unusual for input capacitance to push things into oscillation with a 1k / 1k feedback network, but whatever - if it's fixed, it's fixed. You normally don't see much in terms of parallel compensation caps until Rf approaches 10k. Perhaps the layout has plenty of groundplane under the inverting input pin?
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Old 1st March 2020, 11:06 AM   #16
simonra is offline simonra  United Kingdom
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The problem is fixed in all cases other than when all attenuator stages are bypassed. I think adding 50Ω in series with OP2 input (or the attenuator output) is a sensible thing to try. At the moment I've just just set the minimum attenuation to 0.5dB in software which is not really a problem.

I'd like to understand what's going on but also don't want to do anything too invasive to the PCB as it's 380x225mm and it'll take a bit of work to recover some of the more expensive components if I mess up (and I often do).

This is a four layer board (1 Signal, 2 AC Gnd, 3 Power, 4 DC Gnd) which is a consolidation of a number of (tested) designs onto one PCB. The only differences are that previous versions have used a 1k attenuator, input, attenuator and output circuits were on separate boards and I have 2x gain on the input.

Thanks for all the suggestions.
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Old 1st March 2020, 02:20 PM   #17
Mark Tillotson is offline Mark Tillotson
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Originally Posted by simonra View Post
Thanks Mark, 10pF in parallel with R16 did the trick... plus I can just stack the components without butchering the PCB, so result all round.
Yay! I wasn't actually very confident that that was the issue ...
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