i'm using a Coto Tech C347S SSR. Datasheet attached.
just testing it for on resistance, i measure >50M when the LED (switch) is off, and 0.2 ohm when the switch is on. just like the datasheet says. great for DC.
my test AC signal is a 1kHz sine wave, 100mVpp. with the switch on, i measure 100mVpp at the output. with the switch off, i measure 60mVpp. this also scales to 1V, where i measure 600mVpp. so, 60% of the signal is getting through.
i was initially sharing the ground with power and signal. i changed that to have a different signal ground, even though in my actual circuit they will be shared. then i went further and used a differential probe on the output signal. so only the signal and input probe share a ground. same results!
The C347s looks like it is a standard back-to-back MOSFET so it should be ok for AC signals, shouldn't it? I have the LED pins floating, no pulldown resistor, but this should be ok I think.
Even though I measured DC off resistance at >50M, I do notice that an RC filter with 1M / 190pF (the output capacitance of the C347S) has -3dB frequency of 840Hz. So I played with the frequency and I see that by 5kHz the SSR is passing the full signal even in the off state, and at 20Hz it's down to 20% of the signal. I guess I could do a bode plot later, but anyway it appears to be acting as a high pass filter in the off state.
this doesn't seem right, but at the same time, i can't imagine what's wrong with my measurement setup.