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| Doing it RAW: Tapping Keithley 2001 ADC |
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| Zucca:
After reading all of this and understood only 50% of it, I think changing the diode reference on the ADC board with a better one will not improve anything. This is what my spider sense are telling me. |
| Kleinstein:
I agree that changing the zener on the ADC board would not help. I expect it to be low noise already. The problem may be more with the software: filtering / averaging of all the readings of the AZ cycle is likely not the best. Many of the Keithley meters show avoidable noise on the 10 seconds time scale that is likely due to poor averaging. Chances are the 2001 is effected also, though I don't know for sure. The other SW issue is the 10 V range. With the AD548 the noise is quite high at some 1-2 Hz that are relevant for 10 PLC AZ. With 1 PLC one can start to see quantization noise. |
| Zucca:
I still not believe the prices of this unit right now: https://www.ebay-kleinanzeigen.de/s-anzeige/keithley-2001-multimeter/1476348108-168-4806 4200€? :horse:, last week there was one for 3000€ (but in good conditions... ) https://www.ebay.de/itm/Keithley-2001-Multimeter-with-leadset/383290498030?epid=1801719172&hash=item593de4dbee:g:dNcAAOSw7RteJpMG only 2699 USD. https://de.tek.com/tektronix-and-keithley-digital-multimeter/2001-series-7%C2%BD-digit-multimeter-scanning 6,930 € - 7,740 € new? eh?? |
| Mr. Scram:
That one has been on there for ages. I don't think this guy realises that kind of money buys a DMM7510. |
| openloop:
--- Quote ---Many of the Keithley meters show avoidable noise on the 10 seconds time scale that is likely due to poor averaging. Chances are the 2001 is effected also, though I don't know for sure. --- End quote --- Here's how I figured out what K2001 is doing with the averaging: I ran the instrument for several hours while collecting both raw readings and unfiltered, regular voltage readings over GPIB. Then I've mapped every raw reading (blue) to a corresponding "postprocessed" GPIB sourced voltage reading. Then I made a 3D plot with one axis for raw, one for regular and one axis for time. When I looked at the resulting scatter-plot their algorithm has become clear as day. Basically the plot consisted of short, about 1 minute long intervals, where readings aligned themselves into short but exact straight lines. That means that for every minute of measurement the gain and offset coefficients used to convert raw readings into voltage remained constant and as it's a linear calculation the dots on the plot fell into straight lines. Thus here's what's happening: for one minute they collect acal readings, about 35 of each (6.9V, 0V etc.). Then they average them and derive new Gain and Offset coefficients. These new gain and offset then immediately put into service and they'll remain active for the next minute, while the instrument collects the next batch of acal data. Rinse repeat. As a consequence of keeping gain and offset constant for entire minute, while ADC drifts every each way, readings start to fall into a characteristic ramp-like pattern (because used gain and offset coeffs no longer correspond to the reality of the drifted off ADC). The longer it goes, the further off base readings become. At the end of the minute, there's another jump (snapping back to a somewhat correct value) and a new ramp begins... And you know what's the stupidest thing is? Their digital filter even at maximum setting (100, 2 per second = 50 seconds) does not cover their own averaging AZ cycle (1 minute or so)! :palm: I don't know, maybe to get quiet readings one needs to set readings by timer to once per few seconds - that will allow stretching the aperture of the digital filter to cover at least a couple of AZ cycles... |
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