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Keithley DMM7510, SMU 2450 & 2460 problems

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niner_007:
could one in theory implement better (custom) AZ, by using an external ultra precise reference and custom scripting on the DMM, so when you do the AZ, you switch to the reference (relays), measure, offset (on the data read from the DMM), then continue measuring? without modifications to the DMM, of course, this would all be external

niner_007:

--- Quote from: Kleinstein on May 18, 2018, 08:00:07 am ---There are already similar curves for the Keysight meters earlier in this thread. The suitable test would be to collect data for a reasonable long time (e.g. 1-3 hours) under stable conditions. Ideally with a very stable source (e.g. LTZ1000 or better) in 10 V range, but the extra noise is also visible with just a short.

The Alan variance curve is kind of related to the frequency spectrum of the noise. The curve from the custom AZ script is about the normal shape expected. In some metrology applications the minium value reached is an important quality criterion, especially if this is not at too long a time scale. That 10-100 seconds time scale is just the one needed for doing test with dividers or similar. The custom AZ script can do better in this range, but as one can see it is rather noisy on the short time scale, and an internal solution if done right could likely do considerably (about a factor of 1.5-4) better. So the extra script is kind of a work around but not a full solution.

Just using the 5 PLC mode does not solve the problem. Once you average those data taken at 5 PLC to get even lower noise that extra noise comes back and noise even goes up when averaging more than about 100 samples. This might also lead to wrong conclusions if the normal statistics is used to estimate noise. One would normally assume the data are not correlated over more than maybe the next neighbors - but with the Keithley meters (to some extend a similar bump is even visible with the K2002 and may very well effect more meters) there seem to be some correlation up to 100 seconds.

From the analysis done so fat in this thread, it looks like Keithley is using some averaging on the zero readings when doing AZ, even if digital filtering is turned off. This sounds like a good idea at first sight and could reduce the short time scale (high frequency) noise a little. However it comes at the price of not so good suppression of 1/f noise of the ADC/amplifier. So generally this is not a good idea and it would be really nice if Keithley would also offer the normal simple difference AZ mode.

--- End quote ---
could one use the AZ script as a workaround? what is the problem with using it, beside the noise in the shorter time scale? speed?

Kleinstein:
The script is a kind of workaround at the costs of higher noise for shorter time scales. In addition the timing is a little different (slightly slower). Depending on the application this can help, though an upgraded ("fixed") software in the DMM would be better (lower noise) and easier to use.

The allan deviation curve looks so odd that the the first though was of a software bug that might find a quick fix. However as a similar effect is found with other meters too  - this "feature" looks more like part of very old software at the heart of the measurement system, that might not be changed so fast.

niner_007:

--- Quote from: Kleinstein on May 18, 2018, 11:57:39 am ---The script is a kind of workaround at the costs of higher noise for shorter time scales. In addition the timing is a little different (slightly slower). Depending on the application this can help, though an upgraded ("fixed") software in the DMM would be better (lower noise) and easier to use.

The allan deviation curve looks so odd that the the first though was of a software bug that might find a quick fix. However as a similar effect is found with other meters too  - this "feature" looks more like part of very old software at the heart of the measurement system, that might not be changed so fast.

--- End quote ---
thank you; and that all in the context of the AZ implementation, correct?

Kleinstein:
We don't know for sure, but the likely cause of the extra noise for the 10 s time scale is likely due to the AZ implementation. The script for the workaround also supports this.

My suspicion is that they use some kind of filtering on the zero readings used in AZ mode. So instead of simply using the last zero reading the suspicion is that some kind a average zero reading from the past 10-30 seconds is used. I also have a faint memory (was some 20 years ago I used one) of such a description in one of the older Keithley meters (like 196) to do it like this, but I could not find it in the manuals.

This sounds like a good idea at first because it would hide much of the white noise if the zero reading. However the downside is that the 1/f noise in the time frame for averaging is no longer suppressed. Depending on the noise spectrum this might have been OK with the old (lower resolution) meters with high white noise, but for the DMM7510 and most other modern ADCs  this is a really poor choice.

Let's hope they get this one fixed when they bring out an DMM85xx - it at least would make it much easier to get 8 digit resolution, when you get about a factor of 2-4 improvement from a simple software change.

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