Electronics > Metrology

Scanner/Multiplexers for voltage references

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alm:

--- Quote from: aronake on October 02, 2023, 05:53:27 am ---- For each switching of 34904a relay do 7 measurements at NPLC 100 with both 3458a. First one 3458a measure, then the second, then the first again.

--- End quote ---
Nice test and write-up, thanks for posting! If you're not aware, a trick to do long measurements more efficiently is to set the 3458A to single trigger mode, then trigger each 3458A (using TRIG SGL command) so they start the measurement, and then poll each meter until the ready for instruction status bit is set. See this code for an example of reading both non-blocking (max_time = -1), polling for a certain amount of time (max_time > 0) or a plain blocking read (max_time = 0). The advantage of polling is that it keeps the GPIB bus free for other thing. This is especially a big deal with resistance readings with OCOMP and DELAY which can take minutes.


--- Quote from: aronake on October 02, 2023, 05:53:27 am ---- ACAL on 3458a for every third round of switching of the 34904a. The first round after each ACAL discarded in final analysis, as the 3458a take some time to settle in after ACAL.

--- End quote ---
Interesting, I have observed this for resistance but not for DCV. But then I normally use the 3458A on the 10V range, so maybe it's only the lowest DCV ranges?

aronake:
Hi Alm,

Thanks for comments.


--- Quote from: alm on October 02, 2023, 08:34:47 am ---
--- Quote from: aronake on October 02, 2023, 05:53:27 am ---- For each switching of 34904a relay do 7 measurements at NPLC 100 with both 3458a. First one 3458a measure, then the second, then the first again.

--- End quote ---
Nice test and write-up, thanks for posting! If you're not aware, a trick to do long measurements more efficiently is to set the 3458A to single trigger mode, then trigger each 3458A (using TRIG SGL command) so they start the measurement, and then poll each meter until the ready for instruction status bit is set. See this code for an example of reading both non-blocking (max_time = -1), polling for a certain amount of time (max_time > 0) or a plain blocking read (max_time = 0). The advantage of polling is that it keeps the GPIB bus free for other thing. This is especially a big deal with resistance readings with OCOMP and DELAY which can take minutes.


--- End quote ---
Background to why I have one 3458a to wait while the other is measuring is that I first had them to measure at the same time. But then saw some mysterious result that I thought could have to do with the meters somehow causing some interference with each other. So then changed. I later figured out this likely was related to making measurements straight after an Autocal.

My usual method for nonblocking reads looks like this:

while not (MySTB & (1<<4)):
            MySTB = 3458a.read_stb()
            time.sleep(2)

What you point at looks better. Do you have some example code where this is used?


--- Quote from: alm on October 02, 2023, 08:34:47 am ---
--- Quote from: aronake on October 02, 2023, 05:53:27 am ---- ACAL on 3458a for every third round of switching of the 34904a. The first round after each ACAL discarded in final analysis, as the 3458a take some time to settle in after ACAL.

--- End quote ---
Interesting, I have observed this for resistance but not for DCV. But then I normally use the 3458A on the 10V range, so maybe it's only the lowest DCV ranges?

--- End quote ---

As mentioned in my writeup I did one ACAL for each 3d round of scan across all switches. In the graph below (not very nicely made) I have marked when ACAL happens (3 times for this part of the measurement). Each horizontal line is 100 uV and I added some offset to actual measurements to keep measurements from the two meters more apart. The faint" digital looking line" is what relay I was meaning on.

One meter seems to take a jump upward and the other a bit lower after ACAL compared to for the relay sweeps for where no ACAL was made. But it looks like it is very small, maybe 20 nV on the one that jumps upward and maybe 10 nV for the one that jumps downward. 20nV when measuring 10 V would be 2 counts on the last digit at 8.5 digits so hidden well below the noise floor. So for 10V measurements ACAL stabilization should not matter. And 3458a not really the right tool for nV level measurement. But what to do if not having anything better at hand (yet)?

This behavior looks very similar for all other times ACAL (maybe 7 more) was done over these measurements.

Purpose of this test was to see TEMF on the scanner, so do not have too good data on ACAL stabilization time, so not very conclusive.. But thanks for giving me an idea on something to dig into. I have 5 3458a, so not a huge sample from a statistical point of view, but still quite some 3458a to test. Its intresting that the two I used here seems to move in different direction after ACAL.

alm:

--- Quote from: aronake on October 02, 2023, 04:52:42 pm ---What you point at looks better. Do you have some example code where this is used?

--- End quote ---
Here you have an example of starting a measurement on two 3458As and then polling both meters (in series) until they're ready.


--- Quote from: aronake on October 02, 2023, 04:52:42 pm ---Purpose of this test was to see TEMF on the scanner, so do not have too good data on ACAL stabilization time, so not very conclusive.. But thanks for giving me an idea on something to dig into. I have 5 3458a, so not a huge sample from a statistical point of view, but still quite some 3458a to test. Its intresting that the two I used here seems to move in different direction after ACAL.

--- End quote ---
I misspoke, I did not observe any deviation after ACAL, but I did observe a deviation after running the TEMP? command, which I run regularly to check if the internal temperature changed enough to require ACAL. When measuring 10 MOhm standard resistor, I observed about 3 ppm drops in the reading the sample after executing TEMP? (see attached graph, blue is the resistance reading and red is temperature reported by TEMP?). I did not observe any difference in the second sample, so I just discard the one sample after TEMP?. I haven't observed any effect on the sample after ACAL that was above the noise floor, but I indeed haven't used the 3458A down to these levels.

aronake:
Interesting. Thanks for this.

I realized i misspoke too. 20 nV which was kind of my rough estimate of ACAL TEMF drift of the most impacted meter is 0.2 of last digit at 10V and 8.5 digits. So not only below noise floor but also below number of digits that can be read. So seems fair to say that DCV measurements can be done straight after an ACAL with no impact. Apparently not resistance though from your findings. I will investigate this further.

aronake:
Time for some modifications to see if the 34904a can be improved. Wrapped the relays in two layers of copper foil, added some insulation and added grounds copper foil as shield around the card. Previously had the 34904a and a 34901a multiplexer in the 34970a. Have added one more card now to fill out the space.

A bit too many changes to know what change will make a difference, but running tests now.

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