Hi everyone.
I’m trying to desing a simple low noise amplifier but I’m not able to reach low enough noise, and I suspect I’m missing something. I would appreciate some help to track the problem down.
The circuit is built around a TL972 (this opamp is 4 nV/sqrt(Hz)).
I used a non-inverting 101x voltage gain, with two poles.
C2 (with R2) sets a high pole at about 40 kHz.
C1 (with R1) sets a low frequency pole at about 7 Hz.
Both poles set the passband.
Circuit works perfectly (built and tested), except for the noise expectations.
When I short input to ground, I measure 1 mV – 5 mV (rms) at the output, suggesting an EIN of 10-50 uV rms (due to the approx 100x voltage amplification).
There’re two problems:
1)- Probably 10 uV is not that bad, considering the circuit is not inside a metal enclosure (I was aiming for 5 uV max).
2)- The real issue is that noise level varies with apparently irrelevant things. Sometime the noise is 10-20 uV (quite stable), but then I move some meters away or closer, or move some wires around, and the noise gets 40-50 uV (stable again). And it’s possible to revert and partially control these changes, suggesting there’s a very concise reason behind which I couldn't find after several hours of trial and error.
Of course I did not use a protoboard. I soldered all components as tight as I could (no flying wires), and used 1000 uF close to each supply pin and ground. I also tried to short C1, which (maybe) improves very little. The power supply is linear, and no close SMPS was powered on.
Any tip? Anything I could try to achive lower noise with the same op-amp?
Thank you!
I’m trying to desing a simple low noise amplifier but I’m not able to reach low enough noise, and I suspect I’m missing something. I would appreciate some help to track the problem down.
The circuit is built around a TL972 (this opamp is 4 nV/sqrt(Hz)).
I used a non-inverting 101x voltage gain, with two poles.
C2 (with R2) sets a high pole at about 40 kHz.
C1 (with R1) sets a low frequency pole at about 7 Hz.
Both poles set the passband.
Circuit works perfectly (built and tested), except for the noise expectations.
When I short input to ground, I measure 1 mV – 5 mV (rms) at the output, suggesting an EIN of 10-50 uV rms (due to the approx 100x voltage amplification).
There’re two problems:
1)- Probably 10 uV is not that bad, considering the circuit is not inside a metal enclosure (I was aiming for 5 uV max).
2)- The real issue is that noise level varies with apparently irrelevant things. Sometime the noise is 10-20 uV (quite stable), but then I move some meters away or closer, or move some wires around, and the noise gets 40-50 uV (stable again). And it’s possible to revert and partially control these changes, suggesting there’s a very concise reason behind which I couldn't find after several hours of trial and error.
Of course I did not use a protoboard. I soldered all components as tight as I could (no flying wires), and used 1000 uF close to each supply pin and ground. I also tried to short C1, which (maybe) improves very little. The power supply is linear, and no close SMPS was powered on.
Any tip? Anything I could try to achive lower noise with the same op-amp?
Thank you!