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| Some old school instruments showing how it's done (HP 3325A and Fluke 8506a) |
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| SilverSolder:
--- Quote from: bdunham7 on January 29, 2021, 02:43:51 am ---So if that theory is right, I will get my missing numbers if I increase the sample size by enough? --- End quote --- If you have everything settled so it moves less than 10uV, the averaging algo won't have anything to work with. This is a limitation of the fact that the meter is "better" than its A/D converter can resolve... So I suspect running very long sample periods may help, but may not be enough to get the distribution of values to become a clean bell curve. It's almost like the A/D converter's own noise needs to be 5uV in order for the LSB to randomly "trip" to either side of the real value, if the real value is exactly between two 10uV steps, if you see what I mean. In other words, if your measured voltage is stably parked between two steps of the A/D converter, so it is 'stuck' on one value, no amount of averaging can rescue that situation... Adding noise to the voltage would cause the A/D to trip to both sides of the measured voltage - and it will trip more often on the side it is closer to, so averaging can find the value we are looking for. Obviously there is going to be some noise in the circuit that will lead to this effect naturally... but there may not be enough noise, is the problem! |
| dietert1:
Yesterday night i had some time to fix the guard problem in one of our 8502As. My mistake - while replacing lots of caps i forgot to cut short the wires of one of them and it touched the guard inside the A/D module. Lucky enough it was the autozero memory, so nothing else happened. Now i have two diagrams to show how i am using the DVMs. First one shows a log of this night with a drift of about 1.5 ppm. The reference and the repaired DVM got turned on at about 23:00. Vertical raster is 1 ppm. Each DVM has "sticky" levels even after averaging, so effective p2p noise is less than 1 ppm. The levels aren't the same for both instruments, because i am recalibrating the measurements in software, such that both DVMs report similar values. When i average both instruments, the result looks like a 7 digit DVM, with a noise level at about 1 or 2 uV at those time scales. After the 8:00 bump the measurement will continue during the day without further drift or bumps. During working hours the lab will be at roughly constant temperature. Regards, Dieter |
| joeqsmith:
When I was reading the manual, I thought they stated that the fast R^2 converter was 24 bits including the sign. With that 10Gohm input impedance my little cap is not discharging very fast. After 15 hours its down to 7.4 volts. The LSD bins continue to look fairly even. |
| bdunham7:
--- Quote from: joeqsmith on January 29, 2021, 12:59:33 pm ---When I was reading the manual, I thought they stated that the fast R^2 converter was 24 bits including the sign. With that 10Gohm input impedance my little cap is not discharging very fast. After 15 hours its down to 7.4 volts. The LSD bins continue to look fairly even. --- End quote --- I also remember believing it was 23 bits + sign, although I can't find it at the moment. That would still give 8,388,608 possible values to cover a display that shows 20,000,000 counts. That is a little less than 2.4 displayable counts per possible binary output of the A2D converter. Some of those displayable values will be close to a binary value and some will not. If you add in a slight misalignment here or there and a relatively small sample size being averaged, it seems plausible that some values are more likely than others--especially if, as SilverSolder has proposed, the meter settles at one or two binary values. If this is how it works, then you would expect to see a more even distribution over a range like you are testing with the capacitor. No one particular displayable LSD is more likely over the whole range, even if specific full counts are less likely or impossible. I set my sample size to 212 and let it run, but the reading has drifted down 7uV overnight so I'll have to just keep watching. |
| SilverSolder:
--- Quote from: joeqsmith on January 29, 2021, 12:59:33 pm ---When I was reading the manual, I thought they stated that the fast R^2 converter was 24 bits including the sign. With that 10Gohm input impedance my little cap is not discharging very fast. After 15 hours its down to 7.4 volts. The LSD bins continue to look fairly even. --- End quote --- You would expect the bins to look balanced through a sweep - if not, there is definitely an alignment issue! - this test is really quite good, I'll try it and see what happens here... I believe the A/D converter works like this: The converter has 5 real bits which are used several times to "nibble away" at the voltage being measured, in 5 repeated measurements, where the remainder between each of the five readings is stored as an analog voltage on a capacitor. This is the "recirculating remainder" that they are talking about. You would think 5 readings of 5 bits add up to 25 bits, but in fact only the first reading is a 5 bit reading - the next 4 readings all overlap the previous 5 bit "nibble" by 1 bit, so they are effectively 4 bit readings. This is done right at the analog level, by the amplification chosen for the op amps that handle the remainder and feed it back. The reason they do that is to reduce noise, (EDIT: according to the manual actually I can't remember where I read that, or if I just thought that must be the reason). So, effectively, we get 5 bits plus 4x4 bit readings = 21 "real" bits for a complete cycle. The sign is handled by a separate circuit that flips the voltage reference to the opposite polarity. I suppose we can call that one extra bit, but the sign is a completely separate business from the recirculating remainder processing. The processing is equally happy working with positive or negative voltages, as long as the reference is flipped! When you look at how this thing actually works, you can't help being just a little bit awestruck that they actually managed to get this principle to work at all, let alone be accurate, since it is actually all being done by analog electronic circuits that are just "guided" by the CPU! :D [Edit:] Yes, the manual does say the result is "assembled into a 24 bit word describing the polarity and magnitude" - but that doesn't actually say how many of the bits are significant! :D |
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