Use a source meter (Keithley 2636A) as a voltmeter only (low voltages)? - Page 1
Forum section: electronics / testgear

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 15, 2014, 06:07:02 pm
content: Laid my hands on a Keithley 2636A and there hasn't been much sleep latelyThere is something funny going on when I try to use it as a voltmeter only though. The manual says "the instrument is configured to measure voltage only by setting it to source 0 A and measure voltage" (see attached image as well). Fine, I do that but even when set to source 0.00nA,a tiny (a few mV) voltage always appears on the test cables(trying to avoid "input/output" terminology here) and when the DUT has sufficiently hight impedance it throws off the whole thing.The particular test I was trying is - characterize the uCurrent Gold in it's lowest range (1mA/1nA) by:- sourcing current (a few values in the nA range) from channel A of the SMU- measuring/logging voltage with channel B of the SMU at the uCurrent outputThis does work on the 1mV/1uA range of the uCurrentbut the results are all over the place on the 1mV/1nA range. With the femto Amp, blah, blah precision of the instrument I was a bit surprised by this. Do you think I am doing it wrong, or should I try adding a buffer to lower the output impedance or something (but then am I characterizing the uCurrent?)...

author: robrenz
author_rank: Super Contributor
timestamp: May 15, 2014, 06:14:20 pm
content: IMO you should be using a single channel to source the currentandmeasure the voltage,notsource with A and measure with B. My 2450 single channel SMU does this on the uCurrent Gold just fine.

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 15, 2014, 06:44:52 pm
content: Thanks for the reply, robrenz! I am struggling to visualize how the connections look like when you source to the input of the uCurrent (which is a four terminal device, OK technically three) and measure the output voltage at the same time with just two cables. Or do you just measure the voltage drop on the input?But also in general, is the expected behavior some tiny voltage to appear on the terminals when the thing is set to source 0.00nA (not used to my voltmeters to present voltage to the DUT)?

author: plesa
author_rank: Frequent Contributor
timestamp: May 15, 2014, 08:03:11 pm
content: Voltages above about 100uV is strange. What type of cables are you using for measurement? I suspect them, I'm using low noise Keithey cables without any problem.

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 15, 2014, 08:17:13 pm
content: I use the standard triax to red, green, black crocodiles Keithley cables which came with the instrument. When I attach a DMM (Fluke 289) and try to measure this voltage it immediately drops from milivolts to microvolts (edit: looking at the SMUs own voltmeter for that channel), so it's a very low energy thing (if a few mega ohms of 'load' kill it) but it does seem to affect sensitive circuits. Will try and measure and post the exact numbers I am seeing tonight when I get to it.

author: robrenz
author_rank: Super Contributor
timestamp: May 15, 2014, 08:58:18 pm
content: 4 wire mode, source current measure voltage 10mA on the mA range of the µcurrent. reads ok4 wire mode, source current measure voltage 100µA on the µA range of the µcurrent. reads ok4 wire mode, source current measure voltage 1µA on the µA range of the µcurrent. reads okThe nano volt range on the µcurrent gives good voltage out when read by my 8846A but the 2450 gives bad voltage readings even though set up just like the other ranges

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 15, 2014, 09:47:58 pm
content: Wow, thanks for this, robrenz! Yes, the nanoamp range is the funny one (the uCurrent itself works fine on that range if I just connect an external DMM) on the other ranges my scheme also seemed to work. I'll try your way tonight (hope the 2636A can do it, wonder why didn't they suggest exactly this in the manual when it comes to measuring voltage alone). I also didn't realize that in 4 wire mode the SMU voltmeter gets completely disconnected and can be used standalone via the sense inputs (or does it? I don't think my other PSUs do that in 4 wire mode). Anyways, thanks for educating me, I'll post what happens next...

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 16, 2014, 08:17:34 am
content: OK, one channel, four wire setup. Attached are the characteristics from the three ranges:- uCurrent-1mV-1nA.png - a sweep from 1nA to 1uA- uCurrent-1mV-1uA.png - a sweep from 1uA to 1mA- uCurrent-1mV-1mA.png - a sweep from 1mA to 1A1mV/1uA and 1mV/mA seem to be working, 1mV/1nA is completely off.Still don't quite get how in four wire mode it measures an independent voltage (and not what's on the load). Tried sourcing some current, giving stable 1V on the sense wires and the voltmeter did not "see" the 1V (shows some arbitrary voltage). Yet when I connect it to the uCurrent output it measures properly just like robrenz said. Time to read the whole manual I guess

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 29, 2014, 02:02:25 am
content: Finally did some more measurements here. I think I understand now what the 2636A is doing while sourcing current, measuring voltage, 4-wire mode. It *can* characterize the uCurrent in this mode but only because the uCurrent has the negative (current) input and the negative (voltage) output connected internally (see first picture). If they weren't connected the only way the voltmeter measures correctly is ifyou connectForce LO and Sense LO (red connection on second picture, apologies for the unconventional diagram, hope it's legible). Put another way you *can not* characterize devices this way if Force LO and Sense LO can not be galvanically connected. Good to know, thanks again for the help!@robrenz, wonder how you figured it out, sir? Not finding this use case in the 2450 documentation either...

author: robrenz
author_rank: Super Contributor
timestamp: May 29, 2014, 02:55:38 am
content: I figured since it must be able to source current on the force leads and measure voltageindependentlyon the sense leads when in resistance mode it should work the same if set for source current and measure voltage in 4 wire mode.I wonder now if the cause of the lowest range on the µcurrent not working on my 2450 setup could be thatpossiblythe commons on the µcurrent arenotconnected on the lowest range. When I get a chance I will connect the commons externally on the lowest range and see if it reads properly

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 29, 2014, 03:42:25 am
content: Got it, thanks for the explanation. This is the exact test I did (picture), the SMU sources *some* set current, say 100uA and the Force leads are just shorted.

author: TiN
author_rank: Super Contributor
timestamp: May 29, 2014, 04:51:27 am
content: I usually connect sense on DUT side, not SMU, as that tracks system measurements better.

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 29, 2014, 06:11:23 am
content: Sure, but the point is that the Sense HI can be left disconnected from Force HI and can actually measure something completely independent. When/how do you do that?

author: TiN
author_rank: Super Contributor
timestamp: May 29, 2014, 10:56:54 am
content: I meant Low side red line on your drawing, as its more like 3wire in that way on picture.

author: ivaylo
author_rank: Frequent Contributor
timestamp: May 29, 2014, 03:27:45 pm
content: Yes, the subject was "Use a source meter ... as a voltmeter only" and this seems to be one of the ways.