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Nulling Input Offset Voltage
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TinyMirrors:
Thanks for the replies. I'll see what I can do about adding a buffer between the low pass filter and the input. This seems like the easiest thing to try so it shouldn't be too hard to do. I'll also try matching the resistors for both inputs.

R1 a metal thin film resistor. Not sure if it's the best type to use in this case. Part number RG2012P-105-B-T5. I'll look into using several smaller value resistors.

I believe the design is a mosfet based opamp. I'll have to read into how this would drift. It is possible the design consists of an opamp and a high voltage gain stage. In that case it might be the hv gain stage that is drifting and not the input offsets.

Sorry nick_d for the blanket statements. I'll try to keep that in mind for the future. The capacitor is in silicon and the cutoff frequency is fixed for this application so that's why it would be hard to change.


nick_d:
Cool. By the way, when I said NE5534 I was mistakenly thinking this was a FET input, I meant something like TL072. And in regards to the 1M resistors I was forgetting that the voltage across them is some 100V not 5V. So a significant current will develop in them compared with the gate input current making them less susceptible to inaccuracy than I thought. Still it could be a useful test to swap it to say 500k and see if this halves the drift. Anyway. In much fewer words David Hess summed up the idea of offsets affecting the positive input more than the negative, like I said I am not super experienced in this. I hope you get it sorted out!
cheers, Nick
lowimpedance:

--- Quote from: Gyro on December 12, 2018, 09:35:40 pm ---
--- Quote ---I've cleaned the boards using detergent 8 and dionized water in an ultra sonic bath so there should be virtually no residue. There isn't any that I can see at least.
--- End quote ---

Hmmm, if you've got non-hermetic epoxy packages involved, not to mention the PCB, it may not be as 'humidity controlled' as you think. My bet is that there's a good deal of drying out still going on. After a wash like that you can probably expect a good deal of settling time before it stabilizes.

FR4 PCB substrates in particular take a long time to dry out and you've got high voltages involved.

--- End quote ---

I also have a suspicion that the PCB has some effect, have you considered a different dielectric besides FR4 (making the assumption that the circuit is constructed on FR4 of course  :P).
Would this mystery 'op amp' be from APEX by chance.

Is it possible to bodge up the same circuit with point to point wiring ,ie no PCB , to eliminate PCB effects ?.
David Hess:

--- Quote from: TinyMirrors on December 13, 2018, 11:00:46 pm ---I believe the design is a mosfet based opamp. I'll have to read into how this would drift. It is possible the design consists of an opamp and a high voltage gain stage. In that case it might be the hv gain stage that is drifting and not the input offsets.
--- End quote ---

The parts I am familiar with come from Apex Microtechnology and use hybrid construction with MOSFET input and output stages.  They easily have the levels of drift that you mentioned and would be used with a separate error correcting operational amplifier for higher precision.
TinyMirrors:
Hey guys! I have tried the PA441! Though maybe I should try it again since I've had some lessons learned since my first attempt with it last year. I couldn't get the output to drift less than 60mV at the time. Another opamp for some applications is the HV264. It also has two power supply inputs and has issues drifting.

You guys bring up a good point about having two different error corrections for the different gain stages. Maybe this is just an issue with most designs based on a mosfet high voltage gain stage.

I could go another route and add a gain stage with a good opamp front end. Something from Application Note 18 by Jim Williams. Any suggestions on that route?
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