Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
joeqsmith:
Here I tried not to disrupt the test and let it set at one level for a little over 2.5 hours. The average is 9.9999929 volts DC. Histogram is again looking at the two LSDs.
90 - 1 count
91 - 13 counts
92 - 273 counts
93 - 1439 counts
94 - 0 counts
95 - 86 counts
96 - 2 counts
Without any added noise (in my case added by my changing the loading resistance), I will see buckets without any counts.
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Wanted to show the zoomed out histogram.
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I have attached the data in ASCII format if you want to have a look.
dietert1:
Since you have about 1800 entries after 9000 seconds, does this mean a Fluke 8506A needs 5 seconds for the 1 uV resolution measurement?
With our 8502As i am currently looking at averages of 20 samples and they need 13.3 seconds to take them. To get a 10 sample average it would take 6.6 seconds, very similar.
Regards, Dieter
joeqsmith:
--- Quote from: dietert1 on January 28, 2021, 10:43:55 pm ---Since you have about 1800 entries after 9000 seconds, does this mean a Fluke 8506A needs 5 seconds for the 1 uV resolution measurement?
With our 8502As i am currently looking at averages of 20 samples and they need 13.3 seconds to take them. To get a 10 sample average it would take 6.6 seconds, very similar.
Regards, Dieter
--- End quote ---
I haven't looked into it. I empirically came up with the five seconds. When I put the software together, if I triggered the meter then request the data too quickly, the meter doesn't have time to respond. Four must be on the edge as it will miss every so often, so I padded it out to five.
The manual shows Average selects S10 (H2) or 2^10 or 1024 samples/reading which they show requires 4.3 seconds. The filter is set to slow with the timeout disabled. Not sure if this has any effect. It would be very easy to measure the time in software.
One thing I did try was just sticking a cap across the meter, charging it to 10V and then allowing it to discharge. I would expect if I monitor the LSD, it should be a fairly even spread. Shown is about 2.6 hours of data.
SilverSolder:
--- Quote from: joeqsmith on January 28, 2021, 08:59:22 pm ---Here I tried not to disrupt the test and let it set at one level for a little over 2.5 hours. The average is 9.9999929 volts DC. Histogram is again looking at the two LSDs.
90 - 1 count
91 - 13 counts
92 - 273 counts
93 - 1439 counts
94 - 0 counts
95 - 86 counts
96 - 2 counts
[...]
--- End quote ---
Looking back at some of my notes, I studied the section in the manual that explains how the A/D converter works a while back, and decided that it is a 21 bit converter, which is just enough for a 6.5 digit display of a 20V range (positive or negative - the converter can flip polarity and uses 21 bits on either side).
The A/D converter therefore has 2^21 = 2,097,152 discrete steps.
So the converter divides the 7 volt reference over 2^21 steps, meaning each LSB = 3.337860107421875uV. But the 7V readings have to be scaled up to the 20V range, around a factor three. (Done mathematically/perfectly by the CPU.) Now, each step is pretty much spot on 10uV, which makes sense with the 6.5 digit capability.
The 7.5 digit mode generates an extra digit by taking an average of a number of readings. It seems intuitive that the number of readings need to be 10, in order to generate an extra digit.
So if the meter is running relatively stable, and the A/D converter is only flipping between two different readings at the 10 uV level occasionally, perhaps it is only natural that the average of those readings end up having missing values?
[Edit:] In other words: the readings are not noisy enough to give a good average?
bdunham7:
So if that theory is right, I will get my missing numbers if I increase the sample size by enough?
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