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4200 SCS as voltmeter for uV ranges problem

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Constantinos
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Joined: March 29th, 2016, 8:39 am
Country: United Kingdom

4200 SCS as voltmeter for uV ranges problem

Post by Constantinos » March 29th, 2016, 8:48 am

I am using the 4200-SCS model to measure the potential difference between two points of my device.
This is for thermoelectric characterisation. Hence, if the temperature of my dut is uniform and constant, then I should not get any potential difference. However, what happens is that I get a measurement value of about 10uV. (This is like 10% error since most of my values to be measured are around 100uV)
I tried to put the two probes on the same pad and still get a value similar to the one above.

I have taken a 2wire-resistor project and modified it with the following settings:
I set SMU1 as common and SMU2 as Current bias (0A).
Then, I measure the voltage from SMU2.
Voltage compliance I am using is 0.22V (much higher than my measurements)
I use the sampling mode, and get an average of about 5 measurements.
The SMUs are connected through preamplifier units.

[I guess that this 10uV is a machine's noise.]

brian.d.smith
Keithley Applications
Keithley Applications
Posts: 402
Joined: June 10th, 2013, 6:00 am
Country: United States

Re: 4200 SCS as voltmeter for uV ranges problem

Post by brian.d.smith » March 31st, 2016, 6:50 am

You may have an offset problem in your circuit. In addition there can be an offset in the 4200.

The lowest voltage measure range of the 4200 is 200mV. The uncertainty is 0.012% of reading plus 100 uV. The actual offset can be much smaller than 100uV. You have to measure it for your particular 4200. Each one will have a different actual offset.

For very low voltage measurements, care must be taken to account for all of the individual microvolts of offset created by dissimilar metal junctions.

Once you assemble the test circuit including all of the connections to the 4200, anything you have touched will be warmed far above ambient temperature. There will be microvolts at those places you have touched that have to cool down to ambient. This can take a very long time, as in an hour or more. I have performed this exercise and observed the offsets drifting back to a stable state. I used a model 147 nanovoltmeter with a 30 nV range and a manual zero offset control. Its very instructive to witness how offsets behave.

Does your device generate voltage with no stimulus? If so, is there a way to make the device generate no voltage such as make the environment dark?

If you can power off the device, you can measure the offset with the 4200. Then you can subtract that offset from your subsequent measurements.

Please elaborate about the device and how it behaves. That will help.

The model 2182A is a better instrument for measuring low voltage. The lowest measure range is 10mV. There are 7 1/2 digits of resolution vs 5 1/2 digits for the 4200. The uncertainty of the 2182A is much smaller at 0.005 % of reading and 0.0004% of range.

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