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6221/2182 pulse delta noise level

2182 Series Support
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sfostner
Posts: 6
Joined: May 21st, 2013, 2:51 pm
Country: New Zealand

6221/2182 pulse delta noise level

Post by sfostner » May 6th, 2014, 5:49 pm

As mentioned in a previous post, we have this pair set up to short pulse delta measurements (~ settings below) with 55us pulses. However, we've had a few odd issues with the noise level (among other things, but let's just start with this).

With the setup we currently have we tend to have a noise floor somewhere on the order of 10-8 to 10-6 V depending on other factors in our measurements (while NOT using the pulse delta, and typically on medium measurement rate on the 2182 and the lowest measurement range possible).

However, after switching to the pulse delta mode we tend to measure voltage noise on the order of 10-4 to 10-3 V. This is the case regardless of whether we alter the voltage ranging, compliance range, current source range, etc. I have also tried fiddling around with the pulse length (to a point, though I need to toy with that a bit more to be sure that I've covered our bases)

I know that for the 2182 the noise level is expected to be worse for faster measurement rates (eg Fig 3-1 in the manual), however it doesn't really make it clear how MUCH worse is typical? Ie is there simply no way to beat this for fast pulses or is there a fundamental problem in the arming settings and code?

Typical settings (Voltage range, high, and a few other settings are variable, but given as fixed numbers here just for comparison).

*******
*RST
SOUR:PDEL:HIGH %.2E
SOUR:PDEL:LOW 0
SOUR:PDEL:WIDT 55e-6
CURR:COMP 20
SOUR:PDEL:COUN 1
SOUR:PDEL:RANG BEST
SOUR:PDEL:INT 5
SOUR:PDEL:SWE OFF
SOUR:PDEL:LME 1
TRAC:POIN %D
SYST:COMM:SER:SEND "VOLT:RANG 1"
SOUR:PDEL:SDEL 16e-6
SOUR:PDEL:ARM

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

Re: 6221/2182 pulse delta noise level

Post by brian.d.smith » May 15th, 2014, 7:27 am

Have a look at the next page of the manual. The fastest integration time for remote operation is 0.01 PLC whch is 200 uS for 50 Hz and 166.67 uS for 60 Hz.

I would try to run the test at 0.01, 0.02, 0.05. 0.1, 0.2, 0.5, 1.0 PLC to create the curve of FIg 3-1 for your device and your cabling in your environment.

sfostner
Posts: 6
Joined: May 21st, 2013, 2:51 pm
Country: New Zealand

Re: 6221/2182 pulse delta noise level

Post by sfostner » May 15th, 2014, 7:05 pm

I find that puzzling. If the device is incapable of integration times faster than 200 us, then what exactly does the specified minimum pulse width being 50 us in a paired pulse delta mode with the 6221 actually mean (the default width being 110 us according to the manual)? Obviously we set the source delay to something less than pulse width (usually the 16us default), but what effect does this timing then have? My impression was that the integration time was set by width and source delay (presumably width-delay or something to that effect).

Or does the minimum integration time specifically relate to the remote operation and not to RS232 mode?

I will go and have a measure and see how the noise looks at these different pulse lengths, to be sure we are on the same page here, but it didn't seem to help much as I recall last time (over a range of ~55us to 1ms). It's worth checking to be sure.

sfostner
Posts: 6
Joined: May 21st, 2013, 2:51 pm
Country: New Zealand

Re: 6221/2182 pulse delta noise level

Post by sfostner » May 15th, 2014, 9:31 pm

Had a quick measure, as the system is warming up I can't actually measure the sample in its true low temperature state. However, with a few hundred ohm sample the noise level is reasonable compared to the normal measurements (~1uV at 55us to say <50nV at 10ms, just ballpark numbers), the same is true while measuring a short, basically a junction T with essentially 0 resistance. Most of my tests previously were on samples at low temperatures and with very low resistance (<100 ohm normally, fractions of an ohm at most in the superconducting state, similar to the shorted sample I also just tested).

These numbers suggest to me that maybe it's an impedance issue or something related.

In summary:
1) PULSED noise scales with pulse width for high R (in system), or low R (outside the system, different cabling/leads/etc)
2) PULSED measurement, with low R sample ~3-4 orders of magnitude higher noise for short pulses (~55 us)
3) DC measurement: similar noise to longer pulses with identically cabling/setup

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

Re: 6221/2182 pulse delta noise level

Post by brian.d.smith » May 16th, 2014, 6:07 am

The key wording is "remote mode". Using the instrument in front panel mode should allow the faster pulsing and measure.

Almost all Keithley instruments require one hour warm up to meet the data sheet performance. Measuring noise floor would be better after the requisite warm up.

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