Internal Excitation Source in USB-6289

[SJS]

I have a NI USB-6289 and would like to use a thermistor to log temperature throughout experiments. I'm having problems with using an internal excitation for the thermistor. I understand ports APFI0 and APFI1 can be used as excitation sources. However, I get an error 200161 that says my device does not have an available internal excitation. My questions are:

1) Hardware: how do I connect the thermistor to measure either resistance or the actual temperature?

I've connected the thermistor as a differential sensor (e.g. connecting to ports ai0 and ai8) and I can get a voltage reading from that (only if I use an external constant voltage source). I can then use an ammeter to measure change in current so I can calculate the resistance and therefore temperature change. This is too long-winded-is there a way of measuring resistance or temperature directly with this DAQ card?

If so:

2) Hardware: how do I connect APFI0 or APFI1 in the DAQ card to the thermistor to get an internal excitation signal?

3) Why does Labview SignalExpress not recognise an excitation signal from APFI0/APFI1? Should I be using an analog output to create an excitation signal instead?

4) What am I doing wrong on Labview SignalExpress? When logging an analog signal (Resistance), should I be using the 'Triggering' tab to identify the trigger source (e.g. APFI0)? Or is it enough to specify the current of the internal excitation source (Eix) on the 'Configuration' tab? 

Cheers!

[James_McN]

Hi SJS,

The USB-6289 cannot provide an excitation voltage or current for the thermistor.  You could look at using AO channels at certain ranges but these can only drive up to 5mA so you must consider the resistances you expect in the temperature ranges you are using.  The APFI lines that you refer to are for analog trigger lines or AO references for offsets.

Other excitation options depend on your set up.  If you have the mass termination device then you could look at connecting it to some signal conditioning front end to make a current output channel such as the SCC or SCXI range (depending on channel count).  You would then set this up seperatly and use external excitation options in the configuration or just use another source as you have mentioned.

There is a document that talks about connecting thermistors to DAQ cards at https://www.ni.com/en/shop/data-acquisition/sensor-fundamentals/measuring-temperature-with-thermocou.... In terms of the simplest software methods you can set up channels as either Vex (Voltage Excitation) or Iex (Current Excitation) Thermistor channels.  Once you have done this you can enter the A,B, C and excitation levels of your set up.  You can also look at the connection diagrams tab of the channel window to show you the ideal connections.

Regards,