Products > Test Equipment
Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
SilverSolder:
--- Quote from: dietert1 on January 29, 2021, 08:17:39 am ---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.
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LOL that's almost the same problem I made for myself by using a too-long screw to put the bottom chassis cover on. Lesson(s) learned! :D
--- Quote from: dietert1 on January 29, 2021, 08:17:39 am ---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
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That is very neat!
I have attempted a similar idea with 4 meters, but found that the difference between the "best" and "worst" performer was too great - the worst performing unit didn't really add any value to the best ones!
So, if nothing else, the exercise helped me identify which meter to work on next! :D
dietert1:
--- Quote from: SilverSolder on January 29, 2021, 04:03:58 pm ---..
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
...
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Yes a true miracle. Those high resolution DVMs from HP mainly used time division to realize the DAC, with even less bits. But i remember that for high resolution digital audio there are multi-bit sigma-delta DACs, that should be a mixture of both methods. So it's still around. Also the MSP430 MCUs that we use for our oximeters include segmented DACs: One 7 bit ladder and another 5 bit ladder to be muxed similar to a Kelvin-Varley Divider. The MCU has two of those and they can be software tuned for 19 bits accuracy. A stone remains a stone, independent of what marketing names it.
Regards, Dieter
guenthert:
--- Quote from: joeqsmith on January 29, 2021, 12:59:33 pm ---[..]
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.
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I haven't read the specifications of the 8505/6A, but generally those long(ish) scale DMMs specify an input resistance of >10GOhm (in some ranges, which should include 10VDC). That's only a nominal U/I ratio, not to be understood as plain resistor. There will be some bias current of the FETs at the input leaking out and the AZ circuit's capacitance will want to be fed, etc. . At 10V you should expect an average current of less than 1nA (it might be less than 100pA, but peak values might very well be considerably higher), hence the nominal >10GOhm.
The bias current means, that a low-leakage capacitance on the input will actually get charged (not necessarily towards +10V), which can be used to determine the average current.
bdunham7:
--- Quote from: guenthert on January 29, 2021, 06:05:07 pm --- The bias current means, that a low-leakage capacitance on the input will actually get charged (not necessarily towards +10V), which can be used to determine the average current.
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All true, but the numbers on the 8505/06A are lower. If you tweak it carefully, you can get bias currents below 1pA, specifications are <5pA adjusted, <30pA @ 1 year. So the effect on a capacitor of anything more than a few hundred pF will be pretty minimal on a properly adjusted meter of this type. I've no idea whether or how the U/I ratio changes in operation during measuring or over time, that would be another interesting question.
SilverSolder:
--- Quote from: guenthert on January 29, 2021, 06:05:07 pm ---
--- Quote from: joeqsmith on January 29, 2021, 12:59:33 pm ---[..]
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 haven't read the specifications of the 8505/6A, but generally those long(ish) scale DMMs specify an input resistance of >10GOhm (in some ranges, which should include 10VDC). That's only a nominal U/I ratio, not to be understood as plain resistor. There will be some bias current of the FETs at the input leaking out and the AZ circuit's capacitance will want to be fed, etc. . At 10V you should expect an average current of less than 1nA (it might be less than 100pA, but peak values might very well be considerably higher), hence the nominal >10GOhm.
The bias current means, that a low-leakage capacitance on the input will actually get charged (not necessarily towards +10V), which can be used to determine the average current.
--- End quote ---
The specification is max 5pA bias current. I measured mine to around 3pA with an electrometer. I've tried to look at the circuit diagram to figure out how they get it so low, but I'm not understanding it... analog black magic overload with stacked dual FETs etc.
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