Psuedo AC current measurement

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tempneff
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Joined: March 25th, 2016, 9:54 am
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Psuedo AC current measurement

Post by tempneff » January 10th, 2018, 11:01 am

I hoped to use a Keithley 2400 source meter to generate a ~120Hz AC signal by outputting a 2-point sweep from 1A to -1A on an infinite trigger. The waveform image is attached.

The end goal is a resistance measurement for bulk semiconductor materials, we prefer AC measurement to avoid thermoelectric drifts. To validate my input current I am measuring AC amperage through a verified 1.01 ohm resistor with a Keithley 2000-20 multimeter.

The meter measures .94A AC during the sweep. If I send 1A without sweeping and read DC current, it measures 1.01A; as expected.

I'd like to better understand the difference in magnitude. How is the measurement impacted by not sending a sine wave, nor a bipolar square wave?
current_waveform.PNG
current_waveform.PNG (51.34 KiB) Viewed 2976 times

Dave W
Tektronix Applications
Tektronix Applications
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Joined: April 26th, 2010, 12:01 pm
Country: United States

Re: Psuedo AC current measurement

Post by Dave W » January 10th, 2018, 11:33 am

Hi tempneff,

120 Hz AC is pretty quick for a 2400 to output, so yes it will definitely be tricky to get a good sine wave. The quickest you will likely be able to do would be about 1 point per ms so you could program a list sweep with about 8 points per cycle to get 120 Hz. It won't be a great sine wave, but it will be better than what you get with 2 points per cycle.

Since this is not a true sine wave, you cannot do a simple peak measurement and then divide by the square root of 2 to get the RMS value. Are you using a scope to measure waveform? Assuming you are using a TekScope, the RMS measurement on the scope makes a true RMS measurement, integrating the area under the curve, so you will get an accurate RMS measurement, whether it is a sine wave or not.

tempneff
Posts: 2
Joined: March 25th, 2016, 9:54 am
Country: United States

Re: Psuedo AC current measurement

Post by tempneff » January 10th, 2018, 12:53 pm

Hey Dave, thanks for answering.

The graph above was AC voltage measured by a 500kS/sec DAQ unit. What I really want to know is how the Keithley multimeter 2000-20 arrives at its display number. Is it also integrating under the curve? It doesn't make sense to me why AC (0.94 amps) and DC (1.01 amps) numbers are so different...

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