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I am designing a circuit which involves taking readings down in the uV range, anywhere between 90uV and 2mV. I am then needing to sum 2 voltages together to give me my resultant voltage.

I need this as accurate as possible so I decided to use a chopper stabilised op-amp for buffering the low voltage and also as the summing amp as I was impressed with the extremely low offset/drift etc. I thought this would be a much better solution than to use a standard op amp which could introduce potentially large errors in my readings.

The problem I am getting is that the chopper op amps seem to be giving me quite a bit of noise in my readings. Annoyingly, if I switch to a standard op amp, I get clean readings, but they aren't as accurate. If I use the choppers, then I get a noisy reading.

I know they use capacitors to offset the errors, so I would assume that maybe has something to do with it? If it is to do with that, why are these things advertised as having such low offsets if they are noisy in the lower voltage ranges?

If anyone has any suggestions on making it less noisy, I would love to hear it! The things I have tried so far:

1 - Swapping the chopper amps for normal op-amps: Cleaner output, but with larger error.

2 - RC and LC filters on the outputs: Didn't seem to do much at all.

3 - Changed decoupling capacitor values (higher and lower): No difference

4 - Placed different values of capacitor on the output: As the capacitor value increased, the error seemed to get larger annoyingly (perhaps it would have then needed another buffer as I am unsure how good they are at driving capacitive loads)

Any help would be appreciated!

The op amp I am using is the TLC2652AC (which apparently has a 1uV max offset which is what made it so appealing).

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    \$\begingroup\$ Are your input signals DC only or do you require a certain BW ? Chopper opamps should provide better 1/f noise which mainly manifests itself at low frequencies. What chopper opamp are you using ? Maybe your filtering was not suppressing the right frequencies ? Low-pass filtering should always reduce noise as it is the same as averaging. If this is not true in your application, it is not real (random) noise ! It could be a spurious signal. \$\endgroup\$ Commented Sep 7, 2017 at 13:25
  • \$\begingroup\$ @Bimpelrekkie Apologies for not providing all the info! I am using an TLC2652AC and I am using DC signals, no AC whatsoever in this design. I will add the datasheet to the question \$\endgroup\$
    – MCG
    Commented Sep 7, 2017 at 13:55
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    \$\begingroup\$ If it is DC, then a lowpass filter will get rid of the noise... Can you post more info about the noise, frequency, amplitude, scope trace?... \$\endgroup\$
    – bobflux
    Commented Sep 7, 2017 at 14:06
  • \$\begingroup\$ The noise has no set frequency, just randomly jumps about by a few tens of uV here and there. No discernable pattern in frequency or amplitude \$\endgroup\$
    – MCG
    Commented Sep 7, 2017 at 14:10

2 Answers 2

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The noise of the amplifier you are using is typically several uV p-p from 0-10Hz according to the datasheet.

Chopper op-amps also use switches right at the front end connections so there are huge (relatively speaking, compared to the average) transient currents present at the input connections. If your source impedance is large, that can affect the signal. Note the specsmanship present in the datasheet:

enter image description here

Assuming you have a non-inverting amplifier configuration and don't need a lot of bandwidth you can try a large-ish low leakage capacitor from the non-inverting input to ground and across the feedback resistor.

I assume you've bypassed the supply properly as you refer to it, but double check. Adding capacitance to the output will probably not do anything that is helpful- you are decreasing the stability so any noise will cause ringing.

Edit: Keep in mind also that chopper op-amps tend to have high sensitivity to EMI- since you have a switching supply this could contribute to the noise. There are some newer models of chopper amplifier that feature EMI filtering on the inputs.

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  • \$\begingroup\$ Thanks for your answer, please see the comment I left on analogsystemsrf's post, think I may have figured it out. But you made some great points so thanks for the extra knowledge! \$\endgroup\$
    – MCG
    Commented Sep 7, 2017 at 17:39
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Add a 160 Hertz passive RC filter on the output. 1Kohm and 1uF is 1milliSecond tau, or 160Hz.

As Spehro suggested, make sure the supplies are well bypassed TO A GROUND PLANE.

You are using a ground plane, right?

And to provide the input surge currents, do as Spehro suggested with caps at the inputs.

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  • \$\begingroup\$ Thank you very much for the suggestion. I've now figured it out. It was picking up noise from my SMPS. I had this running off separate battery power with its own ground plane and it seems I didn't make the isolation gap large enough between the 2 circuits so I think it was picking up noise from that. I built just that part of the circuit on a separate PCB and connected the outputs with wires. Everything was stable. Still need to confirm tomorrow but pretty sure that was it! \$\endgroup\$
    – MCG
    Commented Sep 7, 2017 at 17:37
  • \$\begingroup\$ Thus inductive Hfield coupling? Or Efield coupling? \$\endgroup\$ Commented Sep 7, 2017 at 17:42
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    \$\begingroup\$ I'll admit that's the first I've heard of those terms.... could you elaborate? Sounds like useful info! \$\endgroup\$
    – MCG
    Commented Sep 7, 2017 at 17:44
  • \$\begingroup\$ @analogsystemrf how would one check to determine if were due to inductive or capacitive coupling? \$\endgroup\$
    – Mike
    Commented Sep 9, 2017 at 13:34

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