Author Topic: Guard signal on SMU and VI instruments.  (Read 1079 times)

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Offline desertTopic starter

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Guard signal on SMU and VI instruments.
« on: April 13, 2023, 01:04:08 pm »
Guard is widely used on SMU and VI instruments to avoid leakage current on force high and sense high.
Where guard voltage should be generated, before or after current measurement resistor?

If generating guard voltage before current measurement resistor, then there is a voltage difference between guard and force/sense high. Sometimes the voltage drop on a current measurement resistor is as high as 2V (it’s current depend).
If generating guard voltage after current measurement resistor, then leakage current on guard lines will impact current measurement accuracy.

What's the typical hardware implementation on SMU and VI instruments? And why?
 

Offline alm

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Re: Guard signal on SMU and VI instruments.
« Reply #1 on: April 13, 2023, 08:30:50 pm »
I checked the SMU service manuals I had in front of me, Keithley 237 and Keithley 2400, and both take it after the current sensing. The 237 from the current sensing amplifier and the 2400 via the sense line where ever sense is connected (at the binding posts on the back or at the load). See attached block diagrams. I'm sure this (together with the current sensing itself) will introduce some bias current that affects the current measurement, but as long as the bias current is stable, this can be corrected during adjustment. I think your explanation that there could be a substantial voltage drop over the sense resistors is the most obvious one.

I would consider asking a moderator to move your topic to the test equipment section (use the "report to moderator" link).

Offline desertTopic starter

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Re: Guard signal on SMU and VI instruments.
« Reply #2 on: April 14, 2023, 05:57:38 am »
According to Keithley 237 schematic, the voltage drop on current measurement resistor is as high as 10V, then it’s absolutely the right choice to generate guard after the resistor.
But I also see some schematics show that guard is generated before the resistor.
Any detailed analysis and calculation in this area?

I checked the SMU service manuals I had in front of me, Keithley 237 and Keithley 2400, and both take it after the current sensing. The 237 from the current sensing amplifier and the 2400 via the sense line where ever sense is connected (at the binding posts on the back or at the load). See attached block diagrams. I'm sure this (together with the current sensing itself) will introduce some bias current that affects the current measurement, but as long as the bias current is stable, this can be corrected during adjustment. I think your explanation that there could be a substantial voltage drop over the sense resistors is the most obvious one.

I would consider asking a moderator to move your topic to the test equipment section (use the "report to moderator" link).
 

Offline Kleinstein

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Re: Guard signal on SMU and VI instruments.
« Reply #3 on: April 14, 2023, 06:16:57 am »
With the relatively high drop on the shunts the guard should be from behind the shunts.  Some of the simplified schematics or block diagrams may be simplified a bit too much.

It is usually OK to not have separate guards for the drive and sense lines: the separate sense lines are mainly a thing for higher currents. At low currents the difference is small. The guard lines are mainly a thing for very low currents. Having separate gurad lines would be no real issue - not much effort and not that confusing.
 


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