Differential Voltage Channel Ghosting?
I am having an extremely odd issue with some data acquisition. I'm currently collecting ai signals on 2 channels (channel 0 and 1). They are differential voltages from a pressure transducer amplifier that outputs a certain V/mmHg read by the pressure sensors. I wanted to add an additional sensor to the DAQ. It is a simple limit switch set to transmit 5 V when closed and nothing it the sensor is open (In the setup I currently have it will normally be closed and then open when a door is opened and stops applying pressure to the switch). I have powered the switch off of the same power supply that powers the pressure transducer amplifier. I am sampling the switch at the same Hz as the pressure sensors (2 samples at 2 Hz - 1 sample every 0.5s). The limit switch is not working. When the switch is open the signal jumps around close to zero (as expected). I can then close the switch and the voltage jumps up to 5 (great that is what it should be doing). When I let the switch open again the voltage does not drop back down to 0 - it stays up around 5.
The first thing I did was take out my pressure probes and hooked only the switch to the DAQ. And using the default differential settings on MAX started problem solving the switch alone. Same thing happens using MAX that happened in my project - so it isn't a programming problem in terms of my block diagram. Now the really weird thing (to me) is when I take my voltmeter probes and touch the screw terminals that the switch is connected to, it "fixes" the voltage on MAX. So if I try to use the switch with only MAX I get the issue, but if I touch the terminals with my voltmeter (black to - and red to +) I get the right readings. I tried it with the DAQ grounded and with it ungrounded (DAQ's COM channel grounded to the ground of an electrical outlet).
I feel like I am missing something really simple!