My partner and I are designing a guitar tuner based on a CYD (cheap yellow display with 2.8" TFT) that runs on ESP32. The basic audio circuit is below. As you can see it runs on 5v, has gain, an offset voltage to put the guitar signal in the center of the 3.3v ADC, and has low and high pass filters appropriate for stringed instruments. GPIO35 is the input to the CYD where it processes the signal.

It has been suggested to put a large capacitor (>100u) on the input of the CYD because it's a screen that puts off EMI noise. The problem is that if I put a large capacitor in parallel with C6, this will lower the cutoff of the LPF too much unless I got rid of R15. I suppose that is an option to forgo the LPF and filter it with software? Can the signal go straight from IC1 to GPIO35 without a resistor?

I'd appreciate any suggestions.

 

It seems the output of the LM358 would support that.

 

It would make more sense to put a 100Pfd on the LM358 output, and put the 100MFd capacitor on the VFD POWER supply terminal.

 

It would make more sense to put a 100Pfd on the LM358 output, and put the 100MFd capacitor on the VFD POWER supply terminal.

The power supply section has 100u and 100n caps:

 

Looking at this in LTspice. I wondered if using the other side of the LM358 would benefit any. I'm not sure. I tried putting another big cap on the output of U1B, but it then acts like a LPF too.

But @MisterBill2 is suggesting a 100p instead of 100n for C1? Then change R3 to 1Meg for the same cutoff?

 

When figuring the filter, the question is what frequencies do you want to be counting??

 

When figuring the filter, the question is what frequencies do you want to be counting??

Yes I am happy with a band pass of 34-1600 Hz. I was just wondering if the LM358 at the end is of any benefit.

 

Considering that the LM358 is a dual amplifier device, and that probably it is not needed elsewhere in the project, it does provide a good buffer between the amplifier/filter and the analog input circuit, whatever that may be. And it is a low-cost feature, not needing any added components. Plus, it takes care of the otherwise unused amplifier section. So it is actually an excellent choice benefit.

 

Considering that the LM358 is a dual amplifier device, and that probably it is not needed elsewhere in the project, it does provide a good buffer between the amplifier/filter and the analog input circuit, whatever that may be. And it is a low-cost feature, not needing any added components. Plus, it takes care of the otherwise unused amplifier section. So it is actually an excellent choice benefit.

A question that came up in another thread, does the 2nd side of a dual amplifier need to be biased like the 1st side?

 

Short answer: NO! Aside from the power source they are independent. For some types there might be a limitation on maximum power dissipation, but not most op-amps.