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The "step" we see at DAC~1200 is probably the result of crosstalk from FHE discriminator (in normal channel it is seen at DAC~30, but at approximately the same ADC signal), confirming that FHE discriminator is also alive.
The following plot shows the change in RMS of ADC measurements for the same DAC value (which gives the electronic noise):
For HEX8 and HEX1 ranges noise didn't change much, while for LEX8 and LEX1 ranges noise became more than 2 times bigger.