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Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)
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garrettm:
1995! That's a new record in my book. Looks like 872572 Rev. D was the final iteration of the DC Signal Conditioner in the 8505/6A series.

Here's an interesting read from Analog Devices on bootstrapping an input buffer like the newer DC Signal Conditioners use.

https://www.analog.com/en/analog-dialogue/raqs/raq-issue-168.html#

The DC Signal Conditioner is a pretty important module since both the Ohms and Current Shunt modules require it to scale their outputs before going on to the Active Filter and finally the Fast R2 ADC. For AC current on the 8505A, the Current Shunt will instead send its output to the AC Converter (RMS or AVG) before going to the Active Filter (bypassing the DC Signal Conditioner entirely).

The DC Signal Conditioner scales its input (from Vin/Ωsense or RT1) such that it becomes 20V on the output (RT6) at full-scale:

Range   Scaling
100mV:  *100
1V:     *10
10V:    *1
100V:   /64, *10  (effectively /6.4)
1kV:    /64, *1

Which is where the seemingly odd full-scale values for the 100V (128V) and 1kV (1200V) ranges comes from.

The 100V range could have been improved by using a second ratio tap on the RN1 network to use the 10V range (*1 scaling) rather than the 1V range (*10 scaling).

The Active Filter has a multiplexer, two 3-pole Bessel filters (SLOW and FAST) and a unity gain buffer. The filters can be bypassed and the multiplexer lets an external reference be connected for ratio measurements. The output of the active filter then goes on to the Fast R2 ADC.

As pointed out by SilverSolder, the Fast R2 ADC first detects the input polarity, adjusts the reference polarity, and then runs 5 iterations of the R2 routine. Each iteration generates 5 bits but overlaps the previous by 1 bit. So you have 5 for the first iteration and 4 for each subsequent iteration, yeilding a total of 22 bits, 21 for magnitude and 1 for sign. Pretty interesting design overall. I'm curious if it has any benefits over the multi-slope integrating method. Possibly speed?
garrettm:
I just noticed that the Active Filter module has a "ZERO ADJ" trim pot (R14). I've never thought to adjust this before. But after 27 years of use, I wouldn't mind checking to see if it's still in spec.

After looking around the manual, table 4-10 in the trouble shooting section appears to have the adjustment procedure:

1) Set the 8505/6A to DC Volts, 10V range and short the input.
2) Short RT6 (Active Filter input) to RT2 (Reference Common) using the ever elusive Bus Interconnect Monitor.
3) Adjust R14 for a display of 0.00000 ±2 counts.

I'm confused that Fluke didn't specifiy if the meter should be in cal mode with the software zero on or off or whether the filter should be on slow, fast or bypass. Maybe it doesn't matter? Without the Bus Interconnect Monitor, the question of where to access RT6 and RT2 from comes to mind. Looking at the circuit diagram for the DC Signal Condtioner, TP6/RC looks to be a Reference Common test point. Assuming RC means the same for the Active Filter, then TP1/RC is another Reference Common test point. Finally, RT6 can be accessed from TP8/OUT on the DC Signal Conditioner.

I'll give this a go in the next day or two and report back what I find. I assume R14 is for nulling the offset voltage of the unity gain buffer comprised of Q27 and U5, but I haven't examined the board yet to see if that is the case. Unfortunately the manual does not provide a circuit diagram for the Active Filter.
SilverSolder:
There are a couple of different versions of the Active Filter.  The main difference is that the later models have dual switching JFETs in parallel in some positions, examples in the attached diagram are Q34 and Q35.   Presumably, the older boards with a single JFET were marginal unless the JFET was a good one (may have had to be hand picked?).

I have had problems on two of these boards:  the voltage droops by 10uV when the filter is turned On.   I suspect a switching JFET somewhere, but haven't yet attacked this issue.



garrettm:
Okay, so I finally burned an entire day recalibrating my 8506A.

The output of the Active Filter was about 10uV higher than the output of the DC Signal Conditioner with the meter set to 10V range and input shorted with some copper wire. Ten microvolts is equivalent to 1 count at the ADC and is well within spec (i.e., ±2 counts or ±20uV). But being a perfectionist, I adjusted its output to match the DC Signal Conditioner's output (after hardware zeroing the 100mV range) to within 0.5uV (best I could do with the 1-turn trimmer and checking that the DMM4050 was zero corrected). Step 3) in the troubleshooting section for adjusting the Active Filter zero is pointless, as it does not account for offsets introduced by the ADC, which should be adjusted by R8 instead. So it's better to match the DC Signal Conditioner than zero the display. In this way, offsets introduced in the path from the DC Signal Conditioner to the output of the Active Filter are effectively eliminated. As a result, the ADC or Active Filter are not being used to remove offsets from other modules.

To speed up nulling Ib, I connected a Keithley 617 electrometer to the input and adjusted the bias pot for a reading of <1pA, shooting for ±0.5pA. I then connected the specified 1MΩ//0.22uF (with leads soldered into some banana plugs) to the input and placed a small concave metal sheet connected to chassis ground around the input, set sampling to 10 and checked for a reading of less than 10 counts on the display, which it was. Pleased with the result, I called it good. The manual asks for <30 counts (3pA), so I was well within adjustment.

Next I zeroed the ADC and performed the optional 10V ladder adjustment and ran through the normal 7-step ADC ladder adjustment three times before I was able to match my recently calibrated DMM4050 to within 2ppm (with one 4ppm outlier) on the table 4-5 linearity check. This is where much of my time was spent. A bit like the blind leading the blind, but I would rather the meters match measurements, even if their absolute values might not be correct.

Then it was back to the DC Signal conditioner for setting the 100mV, 1V and 100V full scale hardware adjustments (clearing the software gain corrections first, of course) with a round of software corrections for zero offset and gain for +/- values on each range. The separate software corrections for each polarity are very nice and help dial in the 1000V range since there is no hardware adjustment for it (though it barely needed it).

All this took about 5 hours with a separate 4 hour warm up! I would never tell anyone to perform a full hardware cal on one of these unless their linearity is out of spec. Hardware zero and software cal is how these should be calibrated as to do anything else is an entire day spent laboring over ppms while battling the effects of temperature changes. Thankfully I had a small heater I could use to keep the room within <1C of 23C.


I should point out that the manual makes the full DC / ADC calibration routine seem more complicated than it actually is and could be streamlined to:

0) Warm up: Power on UUT and wait a minimum of 4 hours for unit to fully stabilize. Keep room within 1C of 23C at all times.
1) Power supply adjustment: Remove top cover and set cal switch to on. Configure filter and sampling for appropriate values, then perform standard power supply adjustments and optional non-recurring PSU adjustment.
2) Hardware zero corrections: Short input and set range to 100mV. Disable software zero. Adjust zero offset and bias current on DC Signal Conditioner for 100mV range. Set range to 10V. Optionally check the Active Filter and adjust R14 so its output matches the DC Signal Conditioner’s output using a DMM with 0.1uV resolution. Adjust zero pot on the R2 ADC for a display of 0.0000000. Set new software zero for each range using the [Zero Vdc/Ω] key.
3) 10V hardware calibration / R2 ADC calibration: Set range to 10V and clear software gain corrections for 10V range. Perform optional 10V ladder adjustment.  Perform a two-part iterative cycle involving a 3-step ladder adj. (match ±10.10000V and remainder) and a 4-step ladder adj. (5V, 2.5V, 1.25V and 0.625V ladder) until linearity verification passes limits in table 4-5.
4) 100mV, 1V and 100V hardware calibration: Clear software gain corrections for 100mV range and store new software zero (if needed) then perfrom FS hardware gain adjustment of 100mV range. Repeat for 1V and 100V ranges.
5) Software calibration: Put top cover back on UUT. Apply software zero (if needed) and gain correction for +/- polarity of each range (10V, 100mV, 1V, 100V, 1kV, in this order for each polarity) at a value between 60% and 190% for 10V ranges and below or a maximum of 128V and 1200V, for the 100V and 1kV ranges. Calibration is now complete. Set cal switch to off.

It's a lot of work, but isn't as daunting as some people have made it out to be. And again, not needed unless linearity is out of tolerance or you have the time, equipment and patience to fiddle with chasing ppms. In general, steps 0-2 and 5 are all that's needed.


NOTES:

Since I often use the average mode during normal use, I performed the entire calibration with filter F enabled to account for any offsets it might add. Sampling was done between S9 and S11, depending on the step for dialing in sub ppm on the 10V range. The added delays between readings also helps to let the meter settle, which is needed for some of the ladder adjustments and obtaining a reliable software zero on the 100mV range.

With the electrometer already setup, I decided to check my other DMMs for comparison. The slightly outdated Tektronix DMM4050 measured about 16pA and the HP 3456A was 4pA with AZ on and about 1.6 with it off, though its value is affected by the meter's offset voltage and its reading quite jumpy when AZ was turned on.

The 8506A does not have an auto zero circuit like the HP 3456A, and is thus sensitive to temperature variations. Without auto zero on, the 3456A appears to perform worse than the 8506A during my fiddling around. So Fluke did make a valiant effort with an outdated approach to DMM design. It also has lower bias current and while there is some noise to it, it pales in comparison to the noise generated by the 3456A's AZ circuit.
joeqsmith:
I didn't find going through the manual to align it that much of a problem.  The manual seems well written and easy enough to follow.   Had the meter actually worked, it would have gone fairly smooth.   The problem I have is the lack of a way to produce the signals needed to align it so it's a poor man's setup at best.
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