Voltage measurement (AC) using cDAQ-9172 with NI 9221 (+- 60 V)

[fredheim]

Hi,

I must read a voltage signal (0-16 V AC across the terminals), and I have the NI-9221 at hand (datasheet: http://www.ni.com/pdf/manuals/375905a_02.pdf). However, this can only read single-ended signals, and I need to measure across the two terminals. My question is, can I ground one terminal and plug the other into one of the channels and perform a normal single-ended measurement?

Best regards,

Fredrik

[ikaiser]

 Hi Fredrik,

Welcome to the forum!

Depending on the bigger picture of your setup this measurement might be tricky, as you are trying to measure AC with a DC module.

• What is the frequency of your AC signal?
• Is the frequency constant?
• What other measurements are you doing in the NI-9221? I am asking this because the NI-9221 can measure up to 800k Samples/second, split among all channels used. As you need a relatively high sample rate to see the full waveform of your signal, this might be a limitation.
• Do you know where you want to proceed when you have measured the AC waveform?

The part about grounding one terminal I do not understand. Generally, you need to connect the COM terminal to the common ground of the signals you want to measure. As your AC signal is self-referenced (at least this is what I understand from "across the terminals"), you need to measure both these signals simultaneously and subtract them from each other. If one of the AC lines is "0V" all the time, then you could use this as COM.

However, when using a general GND as COM and measuring your AC signal on two input terminals, be aware that the signal could float away from your COM. The NI-9221 cannot stand more than 60V.

This might he hard to understand, therefore I'll give an example: Your AC signal lines are 100V and 116V, relatively to a "0V" that you use as COM. The difference is 16V, this is what you want to measure. However, connecting 100V to AI will break the module. So:

• Is your AC signal referenced to a GND?
• If so, is that the same as your COM?