Yes, it is a high pass, to remove very low frequencies. If those frequencies are not (that much) amplified, why should that add noise?you suppress the \(\frac1{2πRC}≥7Hz\) from your feedback making it a high pass ?
Yes, it is a high pass, to remove very low frequencies. If those frequencies are not (that much) amplified, why should that add noise?you suppress the \(\frac1{2πRC}≥7Hz\) from your feedback making it a high pass ?
That's kind of disturbing. I see a lot of shot noise, aka popcorn noise-- the sudden apparent jump from one level to another. I also see a fair amount of flicker noise (1/f noise), which I would expect with this op amp. There's also a hint of external interference; it looks a bit like some radio station in the background. Altogether, it's definitely not pure "white" noise.
I would suggest step down transformer at the input and coupled inductor at the output...i did this before and it worksHi 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.
View attachment 215755
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.
View attachment 215756
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!
Thank you. I'm currently reading at a good speedAttached is some reading material on noise; I think you're going to have to do some digging, here.
I tried my setup with a 220 kohm resistor between input and ground (so the source impedance is high), to test how the TL972 performs.3) The TL972 lists its input equivalent noise voltage density as 4 nV/√Hz at 100 kHz, but it says nothing about input current noise density. And if this circuit is going to be driven from a high-impedance source such as 10 kΩ, it's likely the current noise that will cause the biggest problems rather than the voltage noise.
If the thermal noise of your source resistor is dominating, and the op amp's current noise is not causing any significant added voltage noise by flowing through it, then your op amp's current noise is not an issue. My initial guess that it would be very high, was apparently off the mark.So, why would a very low noise current opamp be essential (with high source impedance) if the thermal noise of the source resistor is much higher and dominates??
These look much better, don't they?That's kind of disturbing.
Much, much, MUCH better.
. . .or the TL972, using the exact same setup,
IIRC the datasheet by Nat Semi detailed a cure for that distortion. IIRC again, it was a resistor from somewhere going to the negative rail.The noise images look different because the 'scope timebase and voltage settings are completely different.
The lousy LM358 was mentioned. Some have ordinary noise level and others have very high noise.
in addition to the low bandwidth they produce horrible crossover distortion like this:
I do like the LM358 for some applications: it is able to go down to 0 V using single supply, current consumption is very low, it's cheap, and some applications benefit from the low bandwidth (from a stability point of view). OP07 is also an old low bandwidth opamp, but some specs are still great for me, like its low offset voltage and drift.If course, you can add a DC output resistor so that one output transistor conducts a fairly high current all the time in class-A.
But the resistor does not fix the high noise level and poor high frequency response, Use a modern audio opamp instead.
by Aaron Carman
by Duane Benson
by Jake Hertz
by Jake Hertz