Debugging multiplexing AD converter ADC0809

[eric appla]

Hello fellow technicians.

I have an old instrument based around two Z80 processors which is using 4 60mm slider pots to list through menus. I have a problem that in some menus slider won't change anything for say first 20% of the travel then it would list through all the values and towards the last 20% it would do nothing again. When you have say 100 items in the menu it's quite annoying that only small area of the slider is being utilised.

4 sliders are linear 10k pots connected as a voltage divider. The AD converter is ADC0809 and Vref + is 5V and equal to Vcc and equal to the slider voltage before division. Vref - is tied to GND and again there are no noticeable variances in the GND potential. It's all within 10mV which is less then 1/2 LSB resolution regardless.

I have confirmed the input voltage to the ADC scales linearly and correctly and the references are OK and there is no significant ripple to cause this issue.

I have tested 3 different ADC0809 chips to ensure there isn't a hardware fault.

I would like to use Logic Analyser to confirm the ADC is doing it's job and that on fact the fault is in the software implementation.

If I connect all 8 bit wires to the LA and the 3 address wires I should be able to start the capture and move the slider in either of the "dead zones" and observe if I can see the value change. I wonder if there is a better way I could in situation test the AD linearity?

Any ideas are highly appreciated