DAQ card voltage offset problem
DAQ card voltage offset problem
(OP)
Hi,
I'm not the most knowledgeable about electronics so hopefully this will be an easy problem to solve. I am trying to aquire a voltage signal from across a wheatstone bridge pressure sensor using a National Instruments PCI6024E DAQ card, and I'm having huge problems with a voltage offset. I've built my own bridge, and circuit is as below:
+5V from voltage calibrator ---------------------------
| |
| |
\ \
/ /
R1 \ R2 \
/ /
\ \
/ /
Connection to ch 2 ADC---| |
A | | B
| |
Connection to ch 10 ADC---------------------|
\ \
/ /
\ \
R3 / R4 /
\ \
/ /
| |
| |
0V connection and AIGND ---------------------------
connection on DAQ card |
and earth on power supply |
_____
___
_
I thought the problem might be with my pressure sensor so I built my own bridge to test, but the problem is the same with this - I get a large voltage offset.
For example: With the circuit above with R1 = R2 = R3 = R4 = 562 ohms, I connect the circuit up without the DAQ channel connections, and then get the expected voltages of 2.5V and 2.5V when probing location A and location B respectively. However when I connect my DAQ channels and reprobe, I get the expected 2.5V at location A but a value of 2.37V at location B. thus when sampling within labview in differential mode I get an offset value of 0.13V. Given that the FSO of the pressure transducer is 50mV, this is a substantial problem.
I know for definite that I'm in differential mode with the DAQ card so the signal should be fed straight into a floating source instrumentation amp and I've tried virtually every combination of earthing possible.
This has been driving me nuts for a week so any help much appreciated!
Rob.
I'm not the most knowledgeable about electronics so hopefully this will be an easy problem to solve. I am trying to aquire a voltage signal from across a wheatstone bridge pressure sensor using a National Instruments PCI6024E DAQ card, and I'm having huge problems with a voltage offset. I've built my own bridge, and circuit is as below:
+5V from voltage calibrator ---------------------------
| |
| |
\ \
/ /
R1 \ R2 \
/ /
\ \
/ /
Connection to ch 2 ADC---| |
A | | B
| |
Connection to ch 10 ADC---------------------|
\ \
/ /
\ \
R3 / R4 /
\ \
/ /
| |
| |
0V connection and AIGND ---------------------------
connection on DAQ card |
and earth on power supply |
_____
___
_
I thought the problem might be with my pressure sensor so I built my own bridge to test, but the problem is the same with this - I get a large voltage offset.
For example: With the circuit above with R1 = R2 = R3 = R4 = 562 ohms, I connect the circuit up without the DAQ channel connections, and then get the expected voltages of 2.5V and 2.5V when probing location A and location B respectively. However when I connect my DAQ channels and reprobe, I get the expected 2.5V at location A but a value of 2.37V at location B. thus when sampling within labview in differential mode I get an offset value of 0.13V. Given that the FSO of the pressure transducer is 50mV, this is a substantial problem.
I know for definite that I'm in differential mode with the DAQ card so the signal should be fed straight into a floating source instrumentation amp and I've tried virtually every combination of earthing possible.
This has been driving me nuts for a week so any help much appreciated!
Rob.





RE: DAQ card voltage offset problem
TTFN
RE: DAQ card voltage offset problem
Cheers for the math though. Rob.
RE: DAQ card voltage offset problem
TTFN
RE: DAQ card voltage offset problem
A common trick to cancel measurement offsets in a balanced measurement circuit is to use an input mux to sample twice but in opposite polarity. The results are subtracted thus cancelling the offset error and doubling the result. If the offset is from your mux or before, say due to static damage, then this solution will not work. Some protective diodes are known to cause similar problems as static damage with light shining on them. I have designed automated fault location equipment with individual bridges spred out over 6 miles creating serious problems because of all these issues. It is not easy to balance a bridge to a few milivolts with over 100 volts of noise and a few microamps of stimulus current.
Good luck.
RE: DAQ card voltage offset problem
RE: DAQ card voltage offset problem