I'm trying to set up a PCIe-6361 to supply a voltage and measure the voltage drop over two resistors. The device actually needs two voltage inputs with a common ground, but for now, I'm just setting it up with one to make sure it works. Attached is a simplified schemicatic as well as the breadboard for the DAQ connections.
What I'm getting is that, even without a device, there is somehow a drop in voltage across the upper resistor. Why is this happening? Measuring with an external voltmeter, the drop only happens when I have attached the AI0 leads across the resistor, so it isn't anything funky with my breadboard. I have the LabVIEW program configured for a differential measurement, and the supply voltage is only in the range of 0.2-1V so I don't think it's above the common mode voltage limit.
Any thoughts? I also notice that the noise in the AI voltage measurements increases significantly when I connect the sourse... if there's any way to reduce the noise, that would be really helpful too.
Resistances are 39.5kOhm (top) and 609 Ohm (bottom.)
The problem I'm have seems to be related to the sampling rate.
To recap, I have ao0 connected to ai0+l and one side of a resistor, with ai0- connected to the other side. On a separate area of the board, I have ao ground connected to ai1- and one side of a resistor, with ai1+ connected to the other side. There are no connections between the two resistors.
When I have the sampling rate set to around 10k Hz or less, I don't get any voltage drop across either resistor (as I would expect.) When I have the sampling rate set to 1MHz, I see a large voltage drop across the upper resistor (connected to ao0 and monitored by ai1) and can confirm with an external ammeter that around 5mA of current is flowing through ai0+ (pin 66).
I tried backing down on the sampling rate, but the issue only seems to really disappear around 10kHz, but the noise level is still very high.
Any thoughts? I need to do frequency analysis with this setup and would really like a sampling rate higher than 10kHz!