Does the 7i92-7i76-7i74 firmware support SSI ?

Like Andy, i suspect the noise in the the SSI channels is bit errors, probably due to a ground loop putting current through the 5I25 --> 7I74 cable and crosstalk in the cable (especially if you are using a flat cable)

I suspect its my fault as the SSI input has no digital filter so may capture short
noise pulses even though the SSI data rate is typically fairly slow (400 KHz being a common rate)

I'll make up a test bitfile with a digital filter on input as a test, what is your current clock rate?

The 7I77 / 7I92 error is very odd, I suspect its a driver bug of some kind since the sserial firmware
is identical on 5I25s and 7I77s and only the driver knows what a 0 values is

What LinuxCNC version is this? By notch do you mean the output does not change while the
analog voltage in in hal changes?

freeby.mesanet.com/ssid200.zip

7i92 and 5i25 7i77_7I74 SSI configs plus changed sources

These have a 200 ns fixed digital filter on the SSI data

On the analog output problem, this may be a solved issue, are you using the latest version of linuxCNC?, there was a sserial race bug found in the driver a couple of months ago (Doit Issued before all transfer data registers written) this would likely show up as corrupted data on a particular data register and would explain the 5I25/7I92 difference (different register write timing)