fluctuations in analog input-NI usb 6221 DAQ

[Ice93]

Hello,

We are trying to interface the thermocouple to the labview software using the NI usb 6221 DAQ. 

The settings in daq assistant are:

max and min voltages are: 10 and -10

1 sample(on demand)

When the output is viewed, the voltage keeps fluctuating in the software and doesnt seem to be accurate.

We tried  this for both differential as well as RSE connections. However when the output of the thermocouple is given to a multimeter, it gives constant output.

The output range of the thermocouple is 1 - 50mV. Kindly assist us.

Thank you.

[Dennis_Knutson]

How much fluctuation - microvolts, millivolts, volts? What is the range of readings? The resolution at +/- 10 is the worst you can have. Are you doing any sort of averaging like a DMM does?

[crossrulz]

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Ice93 wrote:The output range of the thermocouple is 1 - 50mV.

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Then use the -200mV to +200mV range.  That will give you much better resolution (112uV instead of 3.1mV).

As Dennis already said, DMMs have a tendency of averaging stuff out.  A DAQ has a much higher bandwidth.  Try getting 100 samples at 10kHz and take an average of them.