SCXI-1520 Zero Offset Changes with Variable Loads

We are using the SCXI-1520 to calibrate sensors based on strain gauge bridges. The problem I'm having is that the offset changes when changing the bridge resistance. For example, the output reads 0.0000 mV/V for 350 ohm bridges. It reads 0.0112 mV/V for 700 ohm bridges. It reads 0.0135 mV/V for 1000 ohm bridges. The signal input is a constant zero for all three of these examples. The span (gain) is very consistent. In other words, changing the bridge resistance does not affect the gain. I know the excitation sense lines are working and they are applied very near the bridge. Enabling or disabling the autozero mode makes no difference. What can be done to eliminate the variable offset when changing the bridge resistance?

Dear Dj143,
Are you calibrating your strain gauge lines in the Measurement and Automation Explorer (MAX) or in your LabVIEW program?
In MAX you can create a task and use the calibrate tab in the task to calibrate your line. You would then call the task in LabVIEW. If you are using this method, make sure that you are specifying the correct Gage Resistance (gauges in an unstrained position). The default is 350 so it would make sense that you are getting 0 V with this resistance.
In LabVIEW you can use DAQmx Perform Bridge Offset Nulling Calibration.vi to calibrate your lines. There is an example in the Example Finder that shows how to use this VI. (LabVIEW Help menu>>Find Examples. Search calibration. Cont Acq Strain Samples (with Calibration).vi). In this case make sure that you are specifying the correct Nominal Gage Resistance (gauges in an unstrained position). Once again the default is 350.
If you still get incorrect voltages when specifying the correct resistance, let me know how you are calibrating your lines and I will see what I can think of. Have a great day!
Sincerely,
Marni S.

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LabVIEW 6.1
SCXI-1000
SCXI-1520
SCXI-1314
PCI-6031E
Windows 2000
Greetings-
I'm trying to determine how the gain that is settable on a given channel of an SCXI-1520 module is combined with the gain that NI Max assigns to a Virtual Channel.
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How are you applying gain if not through a virtual channel?
When you set the input signal ranges in MAX for your virtual channel, SCXI and DAQ board gains are applied. For example, if you create a voltage channel to read the mV output of a sensor, and set the range to +/- 10mV, the SCXI Module would apply a gain of 1000 to the signal and send the resulting +/-10V to the MIO DAQ device. The DAQ device will then attenuate this signal to +/-5V (the range of the ADC) by applying a gain of 0.5. Now, if you set the input signal range to +/-1mV, the SCXI Module will apply its maximum gain of 1000 and send the resulting +/-1V to the MIO device. The DAQ device will then amplify this signal to +/-5V by applying a gain of 5.
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Hi all,
I am using SCXI-1000 chassis and PCI-1024E DAQ card.
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grte,
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You can convert almost any current output to a voltage output by adding a resistor across the output terminals. Calculate the resistor such that the voltage across the resistor at max current does not exceed max input voltage of the acquisition system. Also, check the specs for max output voltage of the signal source. Don't expect the circuitry to supply more than the circuit supply voltage minus, say, 20 p.c.
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SCXI-1520 Zero Offset Changes with Variable Loads

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