[DCPower] Cable length is affecting voltage measurement

[Basjong53]

I have a PXIe 4139 source meter. I use it to do measurements on solar cells. You apply a voltage sequence on a solar cell and measure the current response resulting in a typical shape (JV curve). Now, I have a calibrated cell with its correct current response. I connect it to the SMU using 4 wires (Remote sensing). But there are slight differences in the voltage response. Moreover, the voltage response is different when increasing the cable length. 

Here is the response I get compared to what I shoud measure. Increasing the cable length reduces the voltage even more. The illumination source is calibrated as well so that should be fine.

I'm using the DCPower Measure Step Resonse.vi example shipped with LabVIEW as it seems quite a complete example. I only added the Remote sensing option. I have measured the transient response due to a votlage step, but the current stabalizes after a couple of milliseconds, so that's not a problem. Playing with the scan rate doesn't affect it either. I've played around with settings of the DCPower like, transient response, autoranging and source delay, but the results are always the same. I don't know what I can change more to have the correct measurement.

[santo_13]

4-W connection is the correct way to do and you do seem to do that.

1. source delay
2. aperture time
3. Do you have graphs for different lengths of wires?
4. what AWG wires do you use?
5. If you shorten the wire, does the measured curve get closer to the expected or away?
6. does the accuracy differences explain any part of the measurement error?

[Basjong53]

1. 50ms

2. 50ms

4. AWG22 if I remember correctly

Other points: lengthening the wire moves the graph farther away from the reference graph. The reference graph was performed with the same wires as I'm using, and the graph is still not correct. There is no noticeable noise in the measurement, so I don't think it is an accuracy issue.

Here is the transient behaviour of the current in time. The measurement is very clean, but it's still different from what it should be.

[santo_13]

What if you attempt a CC sweep instead of CV sweep?

Like, sweep the Forced current from 5mA to 125mA and see if the plot resembles the CV sweep.

BTW, how did you obtain the reference calibrated curve?

[Basjong53]

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@santo_13  ha scritto:

What if you attempt a CC sweep instead of CV sweep?

Like, sweep the Forced current from 5mA to 125mA and see if the plot resembles the CV sweep

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I will try it tomorrow when I'm back in the lab. 

The reference data was provided by the supplier company.

[Basjong53 · Accepted solution]

Turns out one of the cables was not well connected to the DCPower connector. Simple mistake, cost me 2 days of troubleshooting.

[santo_13]

Glad you found the problem as this was puzzling given logically everything should be perfect as 4139 is one of the best in NI's SMU lineup.