use of 9219 module for 4 RTD measurement with excitation current in series

[DidierM]

Is it possible to use the 9219 module for the measurement of 4 RTDs using a single excitation current through the 4 resistors in series and measuring voltage with the 4 channels as on the appended sketch.

I am trying to limit the number of lines as space on the PCB is limited

Thanks

[mehdi.afif · Accepted solution]

Hello.

This connection is possible, except that the current will be generated by one of the 4 channels, between EX+ and EX-.

Best regards

[DidierM]

Thanks,

I was wondering that the module would not accept this configuration.

For the CH1, configured in 4W resistance,  it will be as if the wire resistance is 3 times the measured resistance.

CH2-4 will be configured in simple voltage measurement (value of resistance is obtained by dividing by the current of CH1)

Am I correct ?

Regards

Didier Maillefer

[mehdi.afif]

Hello.

You're right.

The first measurement will be a 4W with a wire resistance equal to 3 times the value to measure, and the 3 other measurements will be a 2W.

Best regards.

[DidierM]

Hello,

Thank you for your valuable help.

I have received the 9219 module from NI and started to experiment.

The measurement does not work the way I expected.

I have configured the first measurement (CH0) in 4W resistance and the three others (CH1-3) in voltage.

You tell me to configure CH1-3 in 2W resistance but then, they will have their own current source and I will lose the benefit of the 4W configuration.

I can measure the voltage on CH1-3 but it does not correspond to the value of the resistance given the excitation current set at 500uA.  When I measure the actual excitation current through the 4 resistors, I read 396uA and it varies with the measured resistance.

I have tried to use the property node giving the AI.Excit.ActualVal thinking that it was a measured value of the actual current but it outputs the theoretical value of 500uA.

At this point, I can't figure out how to obtain an accurate resistance value on Ch1-3

Can you help me?