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| Doing it RAW: Tapping Keithley 2001 ADC |
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| MegaVolt:
--- Quote from: openloop on August 02, 2020, 11:21:06 pm ---The ground issue was solved by a brilliant move - bring in the outside reference differentially (plus it sidesteps the thermal EMF thing). This trick is used in 2002 and DMM7510 (maybe others too). --- End quote --- What's the trick? |
| Zucca:
Thanks openloop and Kleinstein for your immense input. I have restored a K2001 (see here) and I want to learn from it. I still have to check some basics since I upgraded (probably with no real improvement) all the analog switches (i.e. DG411/DG404/DG211) to Maxim devices. Of course I lost the calibration, but in such old devices I did not lost too much. Anyway interesting readings: https://www.eevblog.com/forum/testgear/do-i-have-exceptional-keithley-2000-or-poor-keithley-2001/ and I still did not forget this Kleinstein's idea. but I need to set up a proper measurement system with some loggin data, and other projects kicked in... still work in progress... PS: Metrology thread already? |
| openloop:
--- Quote ---The reference reading with the 10.5 V has some nice features. --- End quote --- That's not really 10.5V. It is "10.5V" :D It's 6.9V * (R327+R334)/R327 And you don't know what the value of "(R327+R334)/R327" is. So you have 3 unknowns: gain, offset, "(R327+R334)/R327" Thus you need at least 3 measurements. Hence "zero" --- Quote ---From the sensitivity they could have as well directly read the 6.9 V reference in the 20 V range, getting around switching the gain. --- End quote --- The whole 20V range signal path looks like a last minute hack to me. The voltage does not go through the main "dual JFET" path, instead it's picked up off the guard rail, of all places :wtf: and it's going downhill from there. I suspect that the instrument was designed to have 10V range, but somebody (marketing?) decided that they want "20V!". Although, I must admit, unbroken span of +-20V is useful. --- Quote ---my best guess would be something like OPA189. --- End quote --- Indeed it is. ;D When I was replacing the opamp, I also shorted out L100. I recon it's not needed anymore, more of a liability. --- Quote ---So I would not blame the relatively high noise to the extra reference. --- End quote --- As I see it, the most of the noise out of K2001 is coming from the way firmware handling the drift. Looking inside the instument - everything and their grandma are drifting (including the extra reference). I kind of gave up on looking on other noise sources simply because they are of lower order. Looks like designers forsaked all drift mitigation attempts thinking that the clever architecture will allow them to mitigate it. --- Quote ---The 1.75 V is not from a PWM divider, but a charge pump / switched capacitor type. --- End quote --- While the chip used is indeed designed to switch capacitors, in this case it is used as a simple switch. It connects output PIN2,3 either to the 6.9V reference or to analog ground. And the duty cycle of REFCLK signal on the Cosc pin is 25%. Hence - PWM! :P |
| openloop:
--- Quote --- --- Quote --- The ground issue was solved by a brilliant move - bring in the outside reference differentially (plus it sidesteps the thermal EMF thing). This trick is used in 2002 and DMM7510 (maybe others too). --- End quote --- What's the trick? --- End quote --- Differential? Instead giving one signal and say "use this voltage between it and your ground (whatever it may be) as a measuring stick", it gives two wires and says "use voltage between these two and use it as a measuring stick" Thus you no longer need to worry about your ground being exactly at the same potential. Plus, voltage references usually live in hothouses, so getting the reference out with two wires made of the same material avoids having a thermocouple effect. |
| Kleinstein:
The 1.75 V ref. indeed is PWM :palm:. Still if the PWM clock in in sync with the ADC this is still a relatively low noise signal. However the LTC1050 adds a little to the noise. Directly loading the 6.9 ref. with the PWM switches is also no that great a practice. However chances are the ADC noise is still higher. For the 6.9 V part there is no filtering of the LM399 noise at least over the time of the AZ cycle. So the gain adjustment includes quite some of the higher frequency noise of the LM399. In this respect the 2 V signal with PWM and filtering may actually help, though they still get aliasing of the ref frequency from the input side. The 2001 is not made for low noise, it is more about high accuracy and good stability. It is an old design too. The noise may have been acceptable at the time. The 20 V range is a nice option, though it would be nice to also have access to the 10 V range directly to the ADC. The HW is there, just the SW does not no seem to support it - except in some Ohms modes I would guess. The 20 V full scale range is indeed odd, not sure if there is really need for it. At least the Datron1281 also has a 20 V range with the downsides of the extra divider. There would be even an alternative (and in many aspects better) way to get a 20 V range even with the ADC only supporting a +-10 V range, without relying an a stable divider. However this would requite a slightly different input amplification - though not necessary more complicated in some aspects even simpler. For the inverting gain of 0.5 one does need to know the individual resistor values, just the gain factor. The "10.5 V" reading gives this gain plus 1 times the 6,9 V, if one ignores the OP offsets and switch resistance. One would need the zero reading(s) because of the LT1007 offset, though not sure why so many. Using only the BSCOM signal from the AD548 is ODD: the main discrete JFET amplifier should be lower noise, though it could add more switching spikes at the input and more delay for the extra precharge phase. |
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