Single ended signals to differential DAQ

[Shazlan]

Dear all,

I have an SCXI 1102C with 1303 terminal block and and three RSE signals. Initially, I thought that SCXI 1102C can take DI and RSE signals. But, I think I'm wrong since I cannot change the Differential option in MAX to RSE - The RSE option is greyed out. If this is true, please advice me on how should I connect the SCXI with my sensors?

I've tried to connect one of the single ended signal to the +ve of Channel 0, and the ground cable to the -ve. But it seems that the measured voltage is wrong. Any suggestions?

Thanks.

Shazlan

[Robert_F]

Hello Shazlan,

The SCXI-1102C is only capable of measuring signals differentially.  You can have ever set up your signals in a pseudo single ended mode.  Connect ground to the negative terminals and your signal to the positive terminal.  This method will allow you to measure all of your signals in a single ended mode. 

[Shazlan]

Dear Rob,

Thanks for your reply.

I've tried the configuration you mentioned. However, when I verified the voltage measured in MAX with several DMMs, the numbers differed by the 100 (Max showed 0.1V while the DMM showed 1 mV). Any suggestions?

Thanks.

[Robert_F]

Hello Shazlan,

How are you measuring your signal with the DMM?  Are you measuring the difference between your signal and ground or are you measuring the differential input of your signal?

When you connect the negative input terminal to ground as well as the positive terminal to your signal, the SCXI-1102C will measure the difference between your signal and ground.    This could be the reason why you are getting such a big difference between your two signals. Ground is a reference voltage, you need to make sure that your signal and DMM are both referenced to the same voltage as the SCXI-1102C. 

[Shazlan]

Dear Rob,

The way I measured the signal is by touching the positive probe of the DMM to the positive screw terminal of Channel 0 and the negative probe of the DMM to the negative screw terminal of Channel 0. Any other ideas?

Thanks and I really appreciate the help.

Shazlan

[Sandra_T]

Hello Shazlan,

I see that you are using the SCXI 1102C with 1303 terminal block to three analog voltage measurements.

Firstly to troubleshoot an issue like this we look at the signal you are trying to measure.

What is the source of the voltage you are trying to measure? Is it from a thermocouple sensor or is it a voltage across a load (example resistor)?
Is the signal a floating signal source or grounded signal source?

The input signals on the SCXI-1102 is either ground-referenced or floathing. The way you connect each of these types of signals is different, refer to Figure 2-1 and Figure 2.2 of the SCXI-1102/B/C User Manual [april 2006] to see how signals are connected. The SCXI-1303 is often used for floating signal sources and contain bias resistors, refer to the installation guide for the terminal block for more information.

After you have connected the signal correctly,  we can now check your signal in MAX. One thing to take a note of is the Gain/Input Range, this is a software-configurable setting that allows you to choose the appropriate amplicifications. Setting the input range allows the driver to compute the gain.

Lastly, if you are making a temperature measurement, create a DAQmx Task to test your hardware.

Regards,
  Sandra T.

Applications Engineer | National Instruments

[Shazlan]

Hi,

What I have are floating signals from ICP sensors connected to SCXI-1102C via SCXI-1303 terminal block. I've looked at the documentations available at ni.com and in SCXI-1303 manual on connecting floating signals to differential DAQ input channels. Both says that I need to use bias resistors. My questions are:

1. All online tutorials say that I need to connect  bias resistors between Ch+ to ground and Ch- to ground. For example, in table 1 here (http://zone.ni.com/devzone/cda/tut/p/id/3344). This is what I adopted. The SCXI 1303 manual however says that I need to connect the bias resistor between Ch- to ground only. Which one is true or the best approach?

2. The value for the bias resistors are adviced to be somewhere between 100k Ohm to 10M Ohm. Does it matter which value I choose? If yes, how do I determine the best resistor for my applications?

Please advice. 

[S_Hong]

Hi Lan 78,

If you take a look at Table 2 in the SCXI-1303 manual, you can see the different configuration settings available to you for your SCXI-1303 and you will notice that our recommended settings for the SCXI-1303 when used with the 1102 is the 10 Mohm pull-up (CH+ to ground) and bias resistor (CH- to ground) setting. Note that unlike other terminal blocks, your SCXI-1303 has pins in place for the connection of resistor networks for biasing should come with both the 10 ohm and 10 Mohm resistors networks (Figure 3), therefore, there should be no need to externally connect your own resistors to the both CH+ & CH-.

I hope this helps.