How to measure voltage divider output using analog input board?

[epsilon-delta]

I am using LabVIEW 8.6.1 with a PXI chassis containing a PXI-6733 and PXI-6289.  I am applying a DC voltage to an amplifier, the output of which has a simple two resistor voltage divider.  The DC voltage originates from an analog output of the PXI-6733.  I am using two input channels to the 6289 to measure the voltage across the two divider resistors.  My VI displays these measurements and sums them to get the total voltage across the divider, which is the amplifier output voltage.  The analog input channel  (AI1) across the lower resistor is configured as RSE and uses the AI GND.  For the upper resistor I am using AI0 and AI8 as a differential input channel to measure the voltage across the upper resistor.  But when I connect DVM's across each of the two voltage divider resistors,  I get values different than what the AI channels are indicating.  What could be causing this discrepancy?  Thank you.

[Henrik_Volkers]

Why measure the voltage on both resistors?

Build your voltage divider and take into account:

- Input impedance. If you measure AC parts you will need an additional (tuneable) capacitor

- Thermal drift , use resistors of same tco and mount them close ( Vishay VD200 is nice 🙂

Now use an appropriate power supply and calibrate your voltage divider as part of the system.