# Interfacing With a Sensor on a Separate Circuit

First, I would like to apologize if somewhere there is a similar topic but I've searched high and low to no avail.

This is probably a silly question, but I'm trying to get analog readings from a sensor that uses a full wheatstone bridge. I'm taking the output signals from the wheatstone bridge and running them through a single supply differential amplifier. Since there is a single supply, the op amp that is the differential amplifier is referenced to a virtual ground. I know that the sensor and the amplifier are working since I have verified its function with a multimeter. My problem is when I connect the output of the amplifier to my arduino. An analog reading from pin A0 jumps all over the place. The connection into the A0 is the only connection the arduino has with the amplifier circuit, and I think that may be my problem. I'm fine with the default 5V reference for ADC that the arduino provides as I have adjusted the amplifier gain accordingly, but does the arduino ground need to be tied to a ground within the amplifier circuit (virtual or actual)?

Some other information that may be useful: The excitation voltage of the wheatstone bridge is 15V so my virtual ground is at roughly 7.5V. I'm mainly just curious if there are any connections besides the amplifier output that needs to be made between the arduino and the amp circuit, that's why no schematics or code have been posted.

The connection into the A0 is the only connection the arduino has with the amplifier circuit, and I think that may be my problem.

Yes it is.

You need also a common signal reference point. Normally this is ground but you are using 15V so that would blow your arduino.

Is the bridge power supply independent of the arduino, if so connect your virtual ground to the arduino ground and make sure the input to the arduino is between this virtual ground and 5 volts above it.
So no exceeding this and no negative voltages.

The simplest way however would be to connect the arduino ground to your -ve excitation voltage and put a potential divider on the op amp output to keep the maximum voltage below 5V.

This worked great! Sorry for the late reply but today was the first time I was able to test this out. Thanks so much for the help!