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NI USB-6211 corrupted diff. measurement

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Hi! I have a NI USB-6211 that I want to use for measuring a signal in differential mode. However there is problems acheiving the correct results. In the figure below I measure over a single 1,5 V AAA battery, with the device set for differential measurement (and terminated AI0 & AI8). As you can see the signal is quite noisy, and also for each 20 ms the signal drops to 0 V for some milliseconds. The same behaviour occurs for all channels!

diff_batt.PNG

If using RSE (connected AI0 & AI GND or AI8 & AI GND), the DAQ shows the expected measurement without any noise as shown below.

rse_batt.PNG

What do I do wrong, or have the DAQ failed?

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Accepted by topic author andreskareberg

Hello Andres,

 

If you check out Pseudodifferential versus Differential Input Configurations, you can see that for floating signal sources, we actually recommend you to use pseudodifferential connections when you have a floating signal source, like a battery. This means that you need to connect a small resistor between AI(-) (AI8, in your case) and AI GND. Try that and see if it works.

Regards,
Daniel REDS
RF Systems Engineer

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Thank you, that solved my problem!

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