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2450 SMU - periodic glitching when sourcing voltage to resistor

Models 2450, 2460, 2461, 2470
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bwilkins
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Joined: April 16th, 2021, 8:05 pm
Country: United States

2450 SMU - periodic glitching when sourcing voltage to resistor

Post by bwilkins » April 16th, 2021, 8:39 pm

Hello,

I recently started using a 2450 source meter. I've encountered some strange yet consistent results from a basic test and hope someone here can help me.

Test setup:
- The 2450 is set to source 1.1mA through a ~100ohm resistor, and measure voltage.

Meanwhile I have a digitizing multimeter (50kHz sample rate) measuring the voltage across the resistor. Based on Ohms law I expect ~110mV (it is not a precision resistor) + random noise.

The multimeter data (see plot below) shows ~110mV + noise as expected, but also a burst of high-low glitches occurring at 500ms intervals.
- The glitches alternate high then low at exactly 0.5ms intervals.
- Each burst of glitches lasts 16ms.
- There is a 3.5ms delay between each burst of glitches.
- There are 3 such bursts of glitches.
- The pattern repeats precisely every 500ms.

2450_glitches.png
2450_glitches.png (447.79 KiB) Viewed 126 times

Each plot zooms in on the horizontal axis, which is time in milliseconds.

Everything is good except for these glitches, which you can see occur in bursts of 3 groups, and is periodic every 500ms.


While trying to find where this interference is coming from, I found if I set the SMU output to OFF the glitches are still there, however if I turn the SMU itself off, the glitches are gone! Also if I disconnect the cable from the SMU the glitches are gone. So, somehow these glitches seem related to the SMU itself.

The glitches look like it could be a relatively low 1kHz signal that is "clocking" 16-bits x 3 groups, every 500ms. Could it be related to something happening internally of the SMU near the front panel or a sampling circuit?

I wonder if I have something wrong with my setup, or using the instrument incorrectly, but this seems to be an elementary test.
- I've tried sourcing voltage, measuring current and setting a 1.1mA limit and the problem is also observed.

If anyone has ideas what I could do differently or test, I am most willing to try.

Thank you!

bwilkins
Posts: 5
Joined: April 16th, 2021, 8:05 pm
Country: United States

Re: 2450 SMU - periodic glitching when sourcing voltage to resistor

Post by bwilkins » April 17th, 2021, 6:19 pm

Addendum: To show this more directly from the setup, and that the digitizing multimeter is itself not the problem, I've tested with both Keysight 34465A and Keithley DMM7510 and took photos.

- The 2450 SMU is sourcing 0.5mA in this case, so the voltage across the resistor is nominally 50mV.

keithley.jpg
keithley.jpg (621.38 KiB) Viewed 117 times
keysight.jpg
keysight.jpg (699.5 KiB) Viewed 117 times

Andrea C
Keithley Applications
Keithley Applications
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Re: 2450 SMU - periodic glitching when sourcing voltage to resistor

Post by Andrea C » April 20th, 2021, 5:20 pm

We are looking at this.

bwilkins
Posts: 5
Joined: April 16th, 2021, 8:05 pm
Country: United States

Re: 2450 SMU - periodic glitching when sourcing voltage to resistor

Post by bwilkins » April 20th, 2021, 5:33 pm

Andrea C wrote:
April 20th, 2021, 5:20 pm
We are looking at this.
Thanks for the reply. Just want to clarify whether your comment means this is a known issue you can reproduce, or this is the first report and you're looking into this as a result?

bwilkins
Posts: 5
Joined: April 16th, 2021, 8:05 pm
Country: United States

Re: 2450 SMU - periodic glitching when sourcing voltage to resistor

Post by bwilkins » April 23rd, 2021, 11:11 pm

I did another test:

1. Keithley 2450 SMU source 0.95mA into the 100 ohm resistor.
2. Keithley DMM7510 measures current in series with resistor (digitize mode; 50kHz sample rate) (I connected the +/- wires such that it's showing negative current below.)
3. Keysight 34465A is measuring voltage across the resistor (digitize mode; 50kHz sample rate)

Here are photos from the instrument setups showing the same "spikes" as earlier:

smu.jpg
smu.jpg (556.49 KiB) Viewed 72 times
current.jpg
current.jpg (630.33 KiB) Viewed 72 times
voltage.jpg
voltage.jpg (853.53 KiB) Viewed 72 times

One of the outcomes I found from this test is that the 2450 Source I RANGE setting (Auto, 1mA, 10mA, 100mA, etc.) has an effect on the severity of the spike artifact.

Andrea C
Keithley Applications
Keithley Applications
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Joined: October 15th, 2010, 10:35 am
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Re: 2450 SMU - periodic glitching when sourcing voltage to resistor

Post by Andrea C » April 26th, 2021, 8:19 am

The periodic nature of it was curious to me. I checked with the factory/design engineering. The periodic nature of the increased source current is due to some internal sensor monitoring that the instrument performs. We must have a little internal coupling into the signal path. They were not alarmed by the observation.

They reminded me to keep in mind: our performance specs are all defined at 1NPLC settings using an integrating A/D converter.
Using a relatively wide bandwidth tool such as the 1MHz digitize feature of DMM7510 may show signal characteristics that are well above the effective bandwidth of any IV data the 2450 will return.

To illustrate, compare the DMM7510 reported current when the DMM7510 is set for DCI using the integrating A/D at 1 NPLC.
Using integrating A/D at 1NPLC on either the 2450 or the DMM7510, both will show current values that are well within the allowed sourcing error of the 10mA range of the 2450.
2450_10mA_SrcCurrent_measured_with_DMM7510_1NPLC.PNG
measure at 1NPLC setting
2450_10mA_SrcCurrent_measured_with_DMM7510_1NPLC.PNG (56.66 KiB) Viewed 62 times

bwilkins
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Joined: April 16th, 2021, 8:05 pm
Country: United States

Re: 2450 SMU - periodic glitching when sourcing voltage to resistor

Post by bwilkins » April 28th, 2021, 9:18 am

Hi Andrea, thanks for investigating this issue with the factory/design engineering. I appreciate that and the follow-up here.

I understand that measuring with 1NPLC averaging would yield values within the product specification. However at least in terms of average value, that would also be the case even if the glitching were continuous. (Any periodic signal -- sine wave, glitches, etc. -- that fits an integer number of times within 1NPLC and having an average value over one period matching what is desired, would overall satisfy the spec in terms of the average value.)

Considering your comment "The periodic nature of it was curious to me." it seems you are equally surprised at the finding, and probably can appreciate from a users point of view that this is not anticipated. The average value is great -- no problem at all. On the other hand the glitches are unexpected and easily confound tests and measurements.

I think it's worth highlighting two things:
- The glitches far exceed surrounding levels of noise.
- The period of glitches is 1kHz. This is significantly lower "bandwidth" than the 1MHz digitize feature of DMM7510. The results I posted above were at a 50kHz sample rate. I expect the glitches will be easily observable at 10kHz or even 5kHz.


Where to from here? Is Keithley design/engineering able to consider solutions to reduce the artifact? Is it possible to resolve by FW update at all?

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