K8061 analogue input not behaving as expected

I’m having a problem with the analogue inputs on a k8061

The card is supplied from a dual psu, 12v for the card and 5v for the breadboard.
I have a simple voltage divider on the breadboard, negative of the external supply is connected to the k8061 GND.

I am using a 100k thermistor (@25"C) and a 100k balance resistor (measured with a meter), when these are connected together I get 2.5v/-2.5v at the junction of the thermistor/resistor pair and pos/neg, all this is fine.

When I then connect an analogue in from the k8061 to this junction the voltage goes to approx .7v between neg and junction, 4 point something between junction and positive. Any idea whats causing this? I’ve tried all 8 inputs.

Take a look at page 14 of the K8061 manual: vellemanusa.com/downloads/0/ … 1_rev1.pdf

The analog inputs have a voltage divider, built from two 10K resistors, going into an operational amplifier.

The resistance of that voltage divider is the problem. It builds a 20K resistor that is in parallel to your 100K balance resistor. This turns the second half of your thermistor voltage divider effectively into a 16K7 resistor, not a 100K. If you do the math, 5V * (16.7 / 116.7) = 0.7155.

This is the same problem as discussed here: viewtopic.php?f=3&t=8041&start=0


Jan, thanks for pointing me in the right direction!

I have use an opamp as a follower after the voltage divider and it works perfectly.

Thanks again.