Difference between ADC otuput and voltage measured with voltmeter

[nickagian]

Hi!

I would like to ask a question that troubles me. I am trying to test and calibrate a data acquisition system and I am facing the following problem :

I am using an InAmp that amplifies the output voltage of a strain bridge and the output of the InAmp goes to a 12-bit ADC integrated in an MSP430 uC. I have several test configurations and the problem is that I am facing a difference between the output voltage of the InAmp as measured from the PCB with a voltmeter and the voltage that is measured from the ADC. For example, the voltage with the voltmeter is found to be 1.464V and the measured voltage from the ADC is 1.486. This last voltage is the mean value of approximately 60 measurements, with a sampling rate of 400Hz. Additionally, I can see that the measurements of the ADC vary a lot, between 1.432V and 1.527V

What is the reason of this error? Is it accurate to measure the voltage on the PCB with the voltmeter?

Thanks,
Nikos

 

[MikeMl]

Could be several things; most likely how you routed the analog ground traces vs the digital ground traces (You did keep them separate, didn't you?)