My optical pulse monitor (with a phototransistor, a transimpedance amplifier, a high-pass filter, and a second stage to increase gain) was having problems with capacitively coupled 60Hz interference. Because the combined gain of the two stages is over 700MΩ at the gain setting I usually use, even very tiny currents at the input cause large output signals. On the design I had in the lab to demo the method, I was getting 60Hz interference that was almost a large as the pulse signal (though grounding myself reduced it by a factor of about 4).
I have been relying on a low-pass filter in the transimpedance amplifier (a capacitor in parallel with the feedback resistor that sets the gain) to reduce the 60Hz interference, and I realized last week that I could set the corner frequency much lower, from around 15Hz down to about 2.2 Hz. The signals I’m interested in are blood pulses, so the interesting signals are about 0.3–3Hz. I don’t need to keep the detailed shape of the pulse waveform, so a little attenuation at the highest and lowest parts of the passband is a good tradeoff for reducing the 60Hz interference over 6-fold.
Here is a plot of the gain of the minimum gain of the two-stage amplifier (with pot set to be 10kΩ):
Although the minimum gain for the amplifier is about 55MΩ, I usually need to set the gain higher (up to around 700MΩ) to get a clear pulse signal to record with the PteroDAQ data acquisition system.