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Keithley DMM7510, SMU 2450 & 2460 problems

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niner_007:

--- Quote from: maxwell3e10 on April 26, 2017, 08:58:59 pm ---Here is an interesting plot of Dr.diesel's data measuring 731B 10V voltage reference using a custom AZ script that I wrote (lownoise.txt). I compare it to the data that I took earlier using a standard 10PLC measurement of a 10V battery. 
https://www.eevblog.com/forum/metrology/dmm-adc-noise-comparison-testing-project/msg1094322/#msg1094322

One can see that in the time range of 10 to 100 sec the custom AZ script removes the noise bump in the Allan variance. At longer times both measurements are affected by the drift of the voltage references in the meters or by the voltages sources themselves.

This confirms that when measuring a finite voltage the autozero problem of DMM7510 is significant and limits the performance on a time scale of 10 to 100 sec.

They have not attempted to fix this problem with the newest firmware release.  To me this problem was obvious within 5 minutes of getting the meter (at first I thought it was defective until I found data from others posted by TiN). Even though the meter meets the specs since there are no specs for longer than 5 PLC, the time domain data look crappy.  I have already spent a lot of time on corroborating and diagnosing this problem, but at this point it's probably better to just stick to Keysight meters.

--- End quote ---
if it was designed that way deliberatly it's not a bug, and it can't be considered noise; from all the information available, it seems like they optimized it specifically at 5PLC; also, since the script removes the bump I don't see how it is a problem, or just use it at 5PLC; what am I missing? it seems like this whole thing was blown out of proportions :) (PS sorry in advance for being a noob)

niner_007:

--- Quote from: maxwell3e10 on April 26, 2017, 09:15:22 pm ---
--- Quote from: HighVoltage on April 26, 2017, 09:06:26 pm ---How does a comparable chart look like from a Keysight meter?

--- End quote ---

I posted earlier a comparison for Keysight 34465A. For finite voltages it does not look great, but I think its because of the voltage reference. I need to get a 34470A for a fair comparison. Or perhaps someone else here has a 34470A and a decent voltage reference.

--- End quote ---
I have a 34470A, tell me what to do and I'd love to do it

Kleinstein:
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.

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 ---
Thank you for explaining it. Is the 10 - 100 seconds here referring to the integration time, or the overall time at which the sample was taken? If the latter (unlikely), couldn't one just wait 100 seconds?

Kleinstein:
The 10-100 seconds time scale where additional noise appears refers to the timeframe used for comparing data or the length of record to calculate noise. The Allan deviation is averaging data over a given time and than compared adjacent averages. So it is a little like taking 10 second average reading of a voltage of interest and than 10 seconds of zero reading to compensate for an offset that is effected by the noise like the 10 second point of the Allan deviation.
With many normal meters the 10-100 seconds range is about the sweet spot, with the lowest Allan deviation. Thus this time frame is often used in sensitive tests - it also kind of matches what is practical with manual switching and reading. So for such manual operated tests the Allan deviation at some 10 seconds is the relevant noise parameter, not 5 PLC noise.

Waiting more than 100 seconds not really a solution as here noise sources like slow thermal drift will come in. So at 100 seconds it is only back to the 2 seconds value.
The script to force a kind of manual triggered measurement is a workaround, but due to extra waiting times involved it would add noise.

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