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I have been building a source-measure unit based on a design similar to the DC2591A. I unfortunately cannot share the design, but largely it replicates the DC2591A using slightly different components here and there. The one major change is that I am using a multiplexer and some MOSFETs to switch the input, and hence I have 8 channels which I can measure. So where the DC2591A has the banana plug, I have the input into a multiplexer, which outputs to 8 channels which I select.

Largely the design works as expected, however the additional resistance from the multiplexer and MOSFETs amounts to an estimated 1.4 Ohm. I am using this setup to characterize solar cells (via IV Curves), and this additional series resistance is affecting my IV curves.

IV curves for the same solar cell under AM1.5

I suppose there are two issues here.

  1. If the effect was purely series resistance, I would expect the measurements to have identical ISC and VOC. This tells me something isn't quite right in that regard. This may just be to slightly different connectors
  2. The slightly lower current at a given voltage is indicative of the additional series resistance.

My question is whether there would be any way to "remove" this effect and adjust my readings to reflect the additional series resistance. If I know the voltage, current and additional series resistance - is there a way I can say what the current would have been without the additional series resistance?

As an aside, when I measure the current from a Keithley 2450 sourcing say 10mA of current, my reading is accurate to the micro-amp level - so as far as I can tell the circuit itself is accurate, but it isn't perfect for characterization.

EDIT: Adding a block diagram

Block Diagram

Tiaan Stals
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1 Answers1

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I assume you probe the "voltage" on the IN- terminal of the LT1970? If yes, then indeed the cell current will drop a slight voltage over the 1.4 Ohm excess resistance. Your current reading will be correct but the voltage reading will be somewhat off.

The solution is to probe the voltage directly at the Solar Cell positive output.

By the way, that same voltage error likely also exists for the SMU unless you use 4-wire sensing (at the cell terminals). But its internal switches that set it up for current measurements are probably mechanical relays with very small resistance of ~10 mOhm, so its voltage error will be much smaller than for your PCB MUX with 750 mOhm.

tobalt
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  • Yes, I am measuring voltage at the IN terminal of the LT1970. My interpretation of the issue is then that I set a voltage for the DAC, which I have calibrated using an external voltmeter on the terminals. Then I align these "set" voltages with the ADC input, and this works when no current is flowing, but when current is flowing the voltage at the terminals is slightly different to the voltage at the "in" terminal. – Tiaan Stals Jul 13 '22 at 12:15
  • Yes, and it is a substantial error of 10s of mVs, but maybe it is not the only error: You would expect that the voltage error becomes 0 at zero current. But your curve shows disagreement even at 0 current. – tobalt Jul 13 '22 at 12:36
  • Thanks for the help. I will try and fix the voltage issue first and see where I stand. – Tiaan Stals Jul 13 '22 at 12:42