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FPGA: Measured Voltage with NI9215 is wrong

Hello, 

My equiptment is:

- NI-PXI8108

- FPGA Card PXI-7842R

- R-Series Expansion Chassis cRIO-9151

- Analog Output Module: NI9263

- Analog Input Module: NI9215 (BNC)

 

I am trying to output a voltage via the NI9263 Module and read the same voltage via the NI9215 Module. In Labview I then want to show those two values in a graph. In the screenshot you can see my Host-VI and my FPGA-VI. 

 

My problem:

When there is no wire connected to the input module, it still shows a voltage of 10V on some pins and sometimes also 5V. Why is the voltage not 0V?

Also when I connect the output module to the input module, it does not read the values as I am outputting.

For example, when I output 0V, the measurement jumps from 0 to 5V. 

When I output 1V, the measurement shows 6V. 

When I output 2V the measurement shows 8V. 

With some positive Values, the measurement even jumps to negative Values. 

As you can see the offset is not constant. 

 

How can I solve this? 

 

Thanks in advance Smiley Happy

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Message 1 of 11
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I would suggest as a starting point that you work to remove all of the coercion dots.

Carry out the conversion to FXP from DBL on the RT system before setting the FPGA terminal, and ensure that the values you read have the same FXP configuration as the terminals you're writing them to.

 

If that doesn't work, perhaps attach the project and VIs using a zip file so we can take a look at the actual configurations (rather than a collection of images).

GCentral
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Message 2 of 11
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First of all, thanks for the reply. 

I removed all of the coersion dots and put all the Conversions to the Host-Diagramm. 

Unfortunately there are still very wrong measurements. 

I tried to simplify it as much as I could, so now I am only using the FPGA-VI without a Graph. Just one Control to output a Voltage and four indicators to read this same voltage. 

I have attached the VI. 

 

Is it possible, that there ist a Hardware-Malfunction with a module or the FPGA? Or do I have to Calibrate the Input modules?

 

 

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So I have started a measurement of the Input Voltage with a Output Voltage ranging from -10V to +10V. 

As an Input Module I used the NI9215. I know, that the Output Voltage works fine, because I have tested it with a Multimeter.

 

Why does it read such random numbers on the Input?

 

 

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Well, your VI is indeed the minimum example, so I'm guessing in the dark a little, but...

Can you try adding a short-ish wait? Perhaps on the order of 25us?

I can see the specifications list 100kHz sampling rate, but perhaps giving a little more settling time could ensure there isn't some strange effect there...

I'm not sure this will help, but I can't see any other/real problems now...

GCentral
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I just tried to add a wait. But unfortunately it doesn't change anything. Thanks anyway. Smiley Happy

 

I read about a calibration procedure for these modules. Maybe there is something wrong with them. 

 

Does anybody have any experience with the calibration of NI Modules? 

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Message 6 of 11
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There's a guide here (which you probably already found): NI 9215 Calibration Proceedure

 

You can use the analog output module as the "Calibrator" probably, if you're happy with the accuracy and stability of the output.

I haven't needed to do this, but perhaps I was lucky with my AI C-Series module, or perhaps it's a specific problem for the 9215, or perhaps you were unlucky.

GCentral
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Message 7 of 11
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Hi username,

 

When there is no wire connected to the input module, it still shows a voltage of 10V on some pins and sometimes also 5V. Why is the voltage not 0V?

This effect is called ghosting, due to the muxed ADC in the 9215 module.

Don't read open inputs or ground them...

Best regards,
GerdW
CLAD expired, using LV2011SP1 + LV2017 (+LV2019 sometimes) on Win7+cRIO
Kudos are welcome Smiley Wink

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Message 8 of 11
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Hello GerdW, 

Thanks I will do that. But could you tell me how to ground the inputs? What do I have to wire specifically? 

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There's a description and guide to troubleshooting ghosting here: How Do I Eliminate Ghosting from My Measurements?

 

However, I *think* (see datasheet) that the NI 9215 uses separate ADCs for each of the (simultaneously sampled) channels.

 

The Getting Started guide has some connection diagrams for different sensor (or source, more generally)/wiring configurations that might be of use.

GCentral
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Message 10 of 11
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