I'm using the following setup to measure a voltage across a resistor. I measure it both by a DAQ system and an oscilloscope. Below is the illustration for the setup:
DAQ is the data acquisition hardware which is: http://www.mccdaq.com/PDFs/manuals/USB-1616HS-BNC.pdf
I1 is a current regulator which is set to 4mA constant current. OS is an oscilloscope. Mains is the mains multiple outlet for power supplies. A1 is the DC power supply for the DAQ. A2 is the DC power supply for the I1. R is the resistor through which current passes.
Voltage across the R is fed both to the DAQ and the OS.
Here is the problem:
When i observe the voltage across the resistor with the oscilloscope, it is around constant 2 Volt with a very very low ripples. Voltmeter also measures 2 Volt. This is expected, since R = 500 ohm.
But when I make data acquisition I saw a very low mean value around 0.7 Volt. Then I plotted the data from the data acquisition. Below plot is what I obtained:
It looks like a 100 Hz pulse with 2V max 0V min. DAQ is obviously wrong. What might be the problem here?