Understanding current measurement on an irregular waveform

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Discussion Overview

The discussion revolves around the measurement of current in an irregular waveform, specifically using a Keithley 2400 source meter to generate an AC signal for resistance measurement in bulk semiconductor materials. The focus is on understanding the discrepancies in current readings when using AC versus DC measurements and the implications of waveform shape on these measurements.

Discussion Character

  • Technical explanation, Experimental/applied, Debate/contested

Main Points Raised

  • One participant describes using a Keithley 2400 source meter to generate a ~120Hz AC signal and measures AC amperage through a resistor, noting a discrepancy between AC and DC readings.
  • Another participant questions the presence of dwells at 0A during polarity switching and suggests this might contribute to a 7% error in measurements.
  • A different participant speculates that the timing of the source meter could be causing the observed error, despite having the delay set to zero.
  • One participant inquires about the possibility of configuring the source to output a 2App square wave with a -1A offset instead of using a two-point sweep.

Areas of Agreement / Disagreement

Participants express uncertainty regarding the impact of waveform shape on measurement accuracy and the source meter's timing, indicating that multiple competing views remain on the causes of the discrepancies observed.

Contextual Notes

Limitations include potential assumptions about the waveform shape and its effects on measurement, as well as unresolved questions about the source meter's configuration and timing settings.

Who May Find This Useful

Readers interested in experimental methods for current measurement, waveform analysis, and semiconductor material testing may find this discussion relevant.

tempneff
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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 below.

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 an "AC" current. Is there something problematic math-wise by not sending a sine wave.

current_waveform.PNG
 

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Why are there dwells at 0A when switching polarity? Is that maybe where the 7% error is coming from?
 
Could be. I think it is inherent to the timing of the source meter. I have the delay set to zero, but we know that isn't possible.
 
Is there a way to set the source to be 2App square wave with a -1A offset, instead of it being the two point sweep? I'm not familiar with that generator...
 

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