Hi,
I'm operating in a configuration where I've got a fixed pull-down resistor on VIN (4.7kohm), and then several different value pull-up resistors that are switched in/out to basically form a variable voltage divider.
I'm using an 8V positive rail, which is also tied to VREF. All the pull-ups go to that rail.
When the ADC isn't connected, the voltage divider network is working as expected, giving all the correct voltages. When I connect to VIN, I noticed that lower voltages, starting around VREF/2, start to get a little higher than what the expected voltage should be. The effect gets worse as the voltage gets lower. I took the following measurements:
I found the 0.35V particularly strange, so I actually disconnected everything except the pull-down resistor, and it still gave the same result. 0.35V/4.7k would correspond to a current of about 75uA, and I couldn't find any spec in the datasheet that seemed to fit that range. Curious as to what's causing this loading effect, and what a suggested solution would be? I imagine some kind of buffer might help, perhaps an op amp, but would have to find one with rail-to-rail inputs and outputs at 8V. Alternatively, I thought of adjusting the resistance values to compensate for the loading effect, but I would have to be sure that it'd be consistent from chip to chip as I only have a margin of about +/-50mV accuracy for my input voltage.
Thanks,
Ryan
