The MAX9924EVKIT interface circuit provides a fixed gain of 1V/V and used to evaluates sensor-signal amplitudes of 40mVP-P to 300VP-P with a frequency range of 5Hz to 25kHz. It makes no sense to evaluation this situation (0V differential input).
Also, the COUT is up to the operation mode and external threshold in Mode B and C.
We can confirm that in Mode A1/A2, the COUT output should be HIGH, because in A1/A2, the threshold voltage is 1/3 of the peak of the previous cycle of the input VR signal. If the input signal voltage remains lower than the adaptive peak threshold for more than 85ms, an internal watchdog timer drops the threshold level to a default minimum threshold (VMIN_THRESH, about 15mV) at compartor "+" input.
In mode B, the internal adaptive peak threshold can be disabled and directly fed from the EXT input. This EXT voltage is typically generated by filtering a PWM-modulated output from an onboard microcontroller (μC). Supply a digital PWM signal to the FILTER and GND pads. The signal must provide a 5V logic-high frequency in the range of 25kHz to 100kHz. In this case, the threshold voltage will be > 0V input as well, COUT = HIGH.
In mode C, need to confirm the threshold setup.