AC Resistance measurement (ACR)

[JamesLai]

I have a customer re-questing for an application to measure AC resistance.
Basically, a 1 Khz signal is passed through the circuit & the resistance is
measured (ACR)
Can the PXI-4072 or any of our products make this kind of measurement? Also,
they are looking into a switching solution for up to 15A DC.

[Claudia]

Hi JamesLai,

The PXI-4072 will return the value of C or L of your DUT, but not the value of the resistance (R) or reactance (X).
I assume you know the characteristics of your excitation signal. Let's say is a 1kHz voltage signal with a known amplitude that you have calibrated before.
If your DUT is mostly resistive, then you could use a 407x (either 4070, 4072 or 4071) in digitizer mode to acquire the current waveform signal. In your program you can run an FFT and look at the 1kHz peak. The resistive part will be V/I taken at 1kHz.
If your DUT has a resistive and reactive part, you could use one or two 407x in digitizer mode to measure the voltage and current waveforms, and then calculate in your program the phase difference and measure the amplitude. Whether you use one or two depends on whether you know the phase at which your excitation signal starts and if you can trigger both the excitaiton signal source and the 407x with the same external trigger.
At this moment I feel I am speculating too much, so in order to help you better, I would need to know what are you measuring (description of the DUT), what are the values you expect to get (i.e. in the order of kOhms, or mOhms, or Mohms, etc) and if possible a description of your application (feel free to add diagrams and sketches).

Now let's get to the second part of your question.

When switching large currents we highly recommend using external shunts. For more information about switching high currents refer to the following sections of the NI Digital Multimeters Help:

• "Devices >> NI 4071 >> DMM Measurements >> DC and AC Current >> High-Current Measurement Considerations"
• "Integration and System Considerations >> Using Switches >> Load Switching >> Switching Current"

In your case I think external shunts are specially necessary for the following two reasons:

1. You will need a shunt anyway, because the 4070/4072 can measure up to 1A without a shunt, and the 4071 can measure up to 3A wihtout a shunt. The shunt we provide for current measurements larger than this limits is the CSM-10A, which allows the user to measure up to 10A. In your case, you will need to provide your own shunts.
2. Not many switches can handle large current values. Most high current switches can switch up to 1A (refer to the
   Switch Product Selection Guide for a list with this information).

Please reply to this message if you have any questions,

Claudia L
DMM R&D
National Instruments