Analog: External Amplifier Noise (publ. 2025-03-18)

Work continues on building my DIY analog computer. Next to the voltage reference mentioned in the last post, I added an amplifier/inverter:

photo of breadboard with voltage reference and amplifier/inverter

I write "amplifier/inverter" because an inverter is just an amplifier where the feedback resistor equals the input resistor. So, I'm still not really clear on whether I want to have dedicated inverter modules, or just a way to easily switch over an open amplifier to inverter mode. The latter makes more sense in principle since open amplifiers can be used in conjunction with multiplication to get division.

To try to do that (the latter) I have the raw output of the amplifier going to one terminal block pin, and then there is a 100k resistor going from there to the other output terminal block pin. My idea is that I could have the 100k resistor output on its own patch panel socket, and then I could just patch the resistor output to the feedback (IN-) port, to get the inverter.

With the amplifier setup as inverter, I immediately notice this very strong noise:

inverter output with noise

The overall waveform is coming in at around 120 Hz, but looks to be constructed of much higher frequencies. I did some quick Internet research. Some sites indicated that a "120 Hz" hum is caused by a power supply with failing filters, like bad caps. This seems plausible enough, since my bipolar power supply is quite old. But something confusing there is that I don't see the noise when I hook up to a different, simpler circuit, or with the scope hooked directly to the PS.

I tried putting in big caps between GND and the +/- rails. I found that a 1000uF cap in between GND and + did attenuate the noise some, but not nearly enough. Adding additional caps did not help.

I spent a while trying to look into the possibility of ground loops, but I didn't make any progress on identifying one.

At one point, I put my finger on the wire connecting the 100k feedback resistor to the feedback (IN-) port, and the noise pattern doubled in amplitude. This made me wonder if interference was coming into the feedback circuit, like a little antenna. I added a (decoupling?) capacitor from the output of the feedback resistor, to GND, and the noise signal dropped dramatically:

inverter output with noise mostly suppressed

I can still see the noise signal if I look closely, but this is much better.

My understanding is that capacitance at this location in the circuit is normally a bad thing, since it affects stability of the op amp. But it seems to be helping in my use case, an analog computer working at low frequencies.

Last Friday, I put another $50 toward my envelope savings for an Analog Thing computer. That gets me about 2/5ths of the way to a purchase.

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