Thank you for your answer odissey!
I've assembled the circuit like one in the attachment image. Opamps and PGAs are both configured for the high power, vrefs are all auto-enabled, the code only includes the initialization of the components.
10K resistor and 47nF should produce a cutoff frequency of ~320Hz, which is high enough for decent attenuation of the mains hum, and still is well below the signal frequency (tens of kHz range). I'm scoping between Pin2 and Pin4, and I get nothing. Do you possibly see where I am wrong?
(I've also tried adding about 1uF between Pin3 and GND, but it made no difference)
Circuit_0.PNG 166.2 K
F_cutoff=1/RC=10k x 0.047u =2.1kHz. Note that PGA_Inv input impedance is low (~5k), and depends on gain setting, see datasheet.
Increase C1 to 1u, R1 to 100k, remove OpAmps 1 and 2, remove PGA_2, set PGA_1 gain to 1. Provide few kHz signal from signal gen (or WaveDAC). You should observe signal on input pin and output pin with a scope (between pin and ground) If none, then input pin is damaged, change to another pin/port. PSoC inputs can't tolerate more than ~5.5V, since AC noise was previously larger than that, pins may be damaged.
What range signal amplitude / frequency we are talking about?
You are absolutely right, I just checked and two pins I was using were damaged. I moved the configuration to the other pins and now it works perfectly (signal is relatively small, <1V, but the hum happened to be about 6-7V, the signal frequency is between 10 kHz and 100 kHz).
Thank you very much!