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Issues faced in measure voltage/source current mode when range is changed

Posted: May 24th, 2017, 5:16 am
by parth.jinwala
Sir
I am using 2420 SMU. In measure voltage and source current mode (actually I am sinking current) whenever I change my current range from 100 mA to 1 A range while keeping the current same, SMU clamps (reaches the voltage compliance value set). The compliance value is higher than the voltage expected at that node (much higher actually so that is not the problem).

For example when my current is -80 mA (negative 80 mA because it is in sink current mode) in 100 mA range, the voltage output is as expected. But when I change my range at that moment from 100 mA to 1 A without changing the current value, the SMU reaches compliance value and is unable to sink that much current.

However, when I start in 1 A range (from 10 mA upto 80 mA) it does not reach compliance (doesnt clamp). That is, it works perfectly in the 1 A range when I start increasing current from a smaller value in 1 A range.

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 24th, 2017, 6:27 am
by brian.d.smith
The analog behavior during a range change is not continuous. If you consider how the instrument has to make a range change, it is apparent why. There is a feedback resistor in an op amp circuit that has to be changed from one value to another value. This requires opening and closing relays that perform the resistor substitution for the resistor in the feedback position of an op amp amplifier circuit.

This substitution process will cause some disruption to the current flow.

If your circuit under test cannot tolerate a disturbance, the solution is to use only one fixed range as you have discovered experimentally. The downside is that the smaller the current, the range uncertainty becomes a larger error component.

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 24th, 2017, 6:39 am
by brian.d.smith
Try increasing the voltage compliance to see if that cures the problem. The range change behavior may contribute to the compliance issue.

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 25th, 2017, 9:40 pm
by parth.jinwala
Thanks a lot Mr. Smith I would try that.
Can you tell me how much time does the relay take to perform the resistor substitution and output gets stabilized again. Approximate (max) value?
Thanks.

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 26th, 2017, 3:29 am
by parth.jinwala
brian.d.smith wrote:
May 24th, 2017, 6:39 am
Try increasing the voltage compliance to see if that cures the problem. The range change behavior may contribute to the compliance issue.
I increased the compliance. Problem is cured now. Range change doesn't affect my circuit. I am still curious as to why compliance has any effect on range change? Can you please enlighten me about that?
Thanks,
Parth

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 26th, 2017, 6:09 am
by brian.d.smith
If your circuit is inductive, or behaves like an inductor, a step in current will cause a voltage spike, V = L * di / dt.

The range change can be thought of as a step in current. The derivative of a step is an impulse. The impulse can be high enough to trigger the voltage compliance detection.

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 28th, 2017, 9:40 pm
by parth.jinwala
brian.d.smith wrote:
May 26th, 2017, 6:09 am
If your circuit is inductive, or behaves like an inductor, a step in current will cause a voltage spike, V = L * di / dt.

The range change can be thought of as a step in current. The derivative of a step is an impulse. The impulse can be high enough to trigger the voltage compliance detection.
I haven't used any inductor. Bondwire is used though (~ 1nH range). But don't you think it is small enough?

Thanks,
Parth

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: May 29th, 2017, 9:14 pm
by parth.jinwala
Just an observation I made today.
I am using 2420 in meas V/ sink I mode. I observed my circuit getting clamped at 21 V when I changed current range from 100 mA to 1 A. (21 V is the max compliance I can set in 20 V range). But just as I increase my 20V range to 60 V range, and set the compliance voltage to 21.1V, the circuit does not clamp when I increase my range from 100 mA to 1 A. Any explanation for this behaviour. I mean, 21 -> 21.1 V is not much difference in compliance voltage. I think it has to do something with Measure voltage range when it is increased from 20V to 60V range.
Thanks,
Parth

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: June 4th, 2017, 5:30 pm
by Andrea C
The source accuracy of the the two current ranges are also something to be considered.
The offset error term increases from 20uA to 900uA when you change from 100mA to 1A source range.
Perhaps the actual sourced current is changing enough to account for the voltage compliance clamping difference.

When was your instrument last calibrated?

Re: Issues faced in measure voltage/source current mode when range is changed

Posted: June 5th, 2017, 4:44 am
by parth.jinwala
My instrument was last calibrated in November 2016.