Analog: Adder, Interference (publ. 2025-03-25)

I added another integrator module, and then an adder module. The adder module produces a direct sum of up to four input signals. One of the signals is 10x — smaller resistor — while the others are all 1x.

Regarding the integrators, I saved some board space by sharing a relay between two integrators, since each is SPDT. The relays handle switching between Initial Condition and OP/RUN mode.

Now I have the following modules:

- +/- 10V reference

- Inverter/Open-Amp

- Adder

- Multiplier

- 2x Integrators

On the adder module, I observed HF interference just like on the Inverter/Open-Amp module. This interference seems to be present on all modules that use just a resistor for the feedback. Zooming in on the interference signal, it appears to be 4.3 Mhz with a 120 Hz beat.

As with the Inverter, I added in 100 nF bypass capacitor, right after the feedback resistor (i.e., at the inverting input) and that made the interference go away. However, I noticed that if I am using a square wave signal for input, that I get enormous amounts of ringing, which I felt was unacceptable. With experimentation, I found that if I reduced the bypass capacitor value down to 100 pF, that the interference noise still is (mostly) eliminated, but also the ringing is reduced dramatically. So I kept the 100 pF capacitor on the adder, and on the inverter I replaced the 100nF capacitor with a 100 pF capacitor.

It still seems dubious to be adding in capacitance here — like there should be some better way. But it works for now.

Copyright

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