I'm building a guitar audio processor. I inherited this circuit from someone else's project. I am seeing a significant amount of noise on the input. I digitize the input using a 16b ADC and clip to 12b to eliminate quantization noise. However, I was still seeing a lot of noise (+/- 7mV) even when the input signal prior to the preamp is clamped to 0V. If I bypass the preamp and clamp the input to the ADC to 0V I get a clean 0V signal. So, I know the source of the noise is the preamp. The schematic is attached. As you can see the analog input (SW_JACK_IN) is passed through a CAP to remove DC and then a 1.65V DC is added to put the signal mid-rail of a 3.3V range. The input is a basic unamplified audio signal from a guitar, so not much more than a few 10's of mV. It then passes through a low-pass filter set for about 20KHz, a opAMP amplifier and a second low-pass filter also at about 20KHz. The final analog signal is then digitized through an ADC at 96Khz.

When I do a SPICE simulation of this circuit I see a huge sensitivity to noise. I'm trying to figure out how to reduce the noise. I suspect the problem is in the opAmp amplifier. Would I be better off just doing a voltage follower prior to the amplier to get a high-impedance input prior to amplifying it?

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  • \$\begingroup\$ C20 would do more good on 1V65 than 3V3. Your C4 says 68nF but you meant 68pF. TL972 has a rather large input bias current - even nominal 200nA will generate 0.2V offset after flowing through R11 and then that gets amplified by your gain. What gain do you have (1 + R4/(RV1+R5) when you observe this 7mV noise? Look for a rail-to-rail opamp with much lower input bias current. \$\endgroup\$
    – td127
    Commented Apr 6, 2023 at 2:34
  • \$\begingroup\$ Do you get mains hum or what kind of noise? Can you show a trace? And also a sidenote: "I digitize the input using a 16b ADC and clip to 12b to eliminate quantization noise." - What ? If you clip to 12 bit, you get 12 bit with the quantization noise of 12 bit. \$\endgroup\$
    – tobalt
    Commented Apr 6, 2023 at 7:23
  • \$\begingroup\$ Do not use a ceramic cap like a X7R for C1 as hi-k capacitors are generally piezo-electric. This means they can work as microphones. \$\endgroup\$
    – Kartman
    Commented Apr 7, 2023 at 16:13

1 Answer 1

  1. If you look into the data-sheet, then you can see that the noise is going up very fast if the output voltage is higher. With a load resistor of 10k, the noise is lower over a wider voltage area. The best voltage area is around 0.1V till 0.9V with a load of 10k Ohm.

Page 6 - https://www.ti.com/lit/ds/symlink/tl972.pdf - Figure 4. Total Harmonic Distortion + Noise vs Output Voltage

  1. The resistor values are very high and your circuit is possibly not shielded. You have magnetic fields in your environment and they can create a current in the traces of the PCB or breadboard. You can try to use a shield with a metal canister or a mesh net what is on ground potential.

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