PCI 6225 differential VS RSE

[BenETH]

Hi there,

We are trying to acquire multiple PT100 sensors monitoring our system temperature (in total 52 sensors) and we have a NI PCI 6225. We therefore need to use RSE mode to acquire all the signals and then do the substraction in Labview.

I have tested the two operation mode on a set of 4 sensors:

- acquire all signals and do the substraction in LabView

- acquire the signals in differential mode

The sensors are the only elements now connected to the 6225 board via SCB 68 + cable SHC68-68. The sensor-loop is fed by a current source of 1mA.

The results however show a difference of 5-6mV between the "substracted data" and the differential measurement.

Any idea where this come from ? any chance to solve the problem ?

I have attached a schematic of the simple circuit.

Regards

Benoit

[reschri]

Hello,

I am trying to understand a little better what you mean with substracting.

At the moment, what I think your doing is: measuring in RSE-Mode the total voltage drop over all your serial resistances ( three R(T) and one 2kOhm resistance) to ground.

When you measure the voltage drop of every single resistance differentially and substract that from the result of the RSE-measurement, you do not get zero, is that it? Or rre you calculating the theoretical voltage drop at the 2 kOhm resitance?

Regards

René

[BenETH]

Hello,

Thks for your answer. I see that I am not really clear so I will maybe describe a litte more...

I'm doing the following in a LabView program:

step1: a/ acquire signal1, 2, 3 and 4 in RSE mode (voltages at the points between the resistances) -> I use the DAQ assistant: 2kSamples @ 20kHz

          b/ get the mean of every signal (*)

          c/ do the "substraction": mean(signal1)-mean(signal2) = voltage drop PT100_1

                                              mean(signal2)-mean(signal3) = voltage drop PT100_2

                                              etc.

           (*) I tried to do the substraction before the mean and vice-versa but results are the same

step2: acquire directly the voltage drop accross the PT100 using the differential mode

=> the results of step 1 and 2 differs by 5-6mV depending on the PT100

At the moment I substituted the PT100 by simple resistances of 100ohms and I measured the resistances using an multimeter. There are all 100+/-0.1ohm.

Hope this helps

Benoit

[reschri]

Hi,

Look at this Link:

http://www.ni.com/white-paper/7113/en

In Figure 5 there are typical examples. I think what your are doing in RSE Moed is not recommended. So, the differentially aquired results should be good.

Regards

René

Figure 5