Some old school instruments showing how it's done (HP 3325A and Fluke 8506a)

[garrettm]

If you like, I can take some decent pictures of this board to compare with yours and see if there are any differences.  Perhaps the performance was improved to the point that Fluke would just upgrade them as part of their service.

I would love to see what your board looks like. Mine has 872572 Rev. D written in marker on the PCB--which strangely does not match the number on the label (881722). Maybe it was replaced at some point? Seems odd they wouldn't match. If I'm reading the part numbers correctly, my board has date codes from 1994! But that's military contracts for you, whether or not Fluke wanted to keep making these, Uncle Sam forced 'em to anyways.

I wish I had a tripod to take the close ups and some way to correct for the chromatic dispersion or a better lens. My hands are not the steadiest things around, so there might be some slight blur to the zoomed in parts, particularly the resistor array.

[garrettm]

Here are a few more photos. I tried taking close ups of the ceramic resistor array, but my hands weren't very compliant while trying illuminate the area and hold the camera at the same time.

I added a couple of quick shots of the interior of the Thermal Converter, it also uses a bunch of custom Fluke ceramic resistor arrays.

[joeqsmith]

If you like, I can take some decent pictures of this board to compare with yours and see if there are any differences.  Perhaps the performance was improved to the point that Fluke would just upgrade them as part of their service.

I would love to see what your board looks like. Mine has 872572 Rev. D written in marker on the PCB--which strangely does not match the number on the label (881722). Maybe it was replaced at some point? Seems odd they wouldn't match. If I'm reading the part numbers correctly, my board has date codes from 1994! But that's military contracts for you, whether or not Fluke wanted to keep making these, Uncle Sam forced 'em to anyways.

Wouldn't surprise me if the bare board, populated board, tested board, final assembly would all have different part numbers.   

As you can see, mine is also Rev D, written with a marker.  Possibly 1995.  The 316 test stamp may be a date code.   

[SilverSolder]

I guess we can safely assume that @joeqsmith's board is among the last ones made?  -  so unlikely that there are any revisions newer than Rev. D?

It is really cool with a modular construction like this, they can keep improving and revising different parts over a long production run and still keep everything working together no matter how you swap stuff, pretty much.

Of course, that kind of philosophy would be strictly forbidden nowadays...  let's just launch facelifted models every year, nothing compatible with anything else!

[joeqsmith]

A few closer pictures of the resistor network.    I wanted to try and show the glass.  The rest are looking through the glass.   I wonder what just this one part cost as I assume that 735118 is a Fluke part number.   

[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.

[SilverSolder]

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.

That sounds like a long time spent with the tongue at exactly the right angle!

Very cool insight with matching the Active Filter output to the DC Signal Conditioner, so both can be eliminated - I will have to try that.

Did you by any chance look at how much offset your Active Filter adds (i.e. in your case, by switching it off)?  -  On a good filter module, I can't tell any difference at all.  On a bad one, I've seen ~10uV on a 10V input.

One section in the manual talks about the software zero affecting all ranges above it.  I've never completely grokked how that actually works...  when you clear all the zeros, are they effectively set to "NULL" and take their value from the next lower range?  What happens if you set a low range, skip a range or two, and set the higher one... will the middle range(s) inherit from the lowest one, and the upper ranges inherit from the higher one?  ... will have to experiment with this...

[garrettm]

That sounds like a long time spent with the tongue at exactly the right angle!

Very cool insight with matching the Active Filter output to the DC Signal Conditioner, so both can be eliminated - I will have to try that.

Did you by any chance look at how much offset your Active Filter adds (i.e. in your case, by switching it off)?  -  On a good filter module, I can't tell any difference at all.  On a bad one, I've seen ~10uV on a 10V input.

One section in the manual talks about the software zero affecting all ranges above it.  I've never completely grokked how that actually works...  when you clear all the zeros, are they effectively set to "NULL" and take their value from the next lower range?  What happens if you set a low range, skip a range or two, and set the higher one... will the middle range(s) inherit from the lowest one, and the upper ranges inherit from the higher one?  ... will have to experiment with this...

So I just ran some tests.

After playing around with the temporary zero corrections, I think your understanding is correct. Zeroing a lower range does affect the ranges above it but not those below it. From what I can tell, zeroing a lower range clears the stored zero corrections for the ranges above it. Furthermore, temporary zero corrections are stored separately for each range. So to properly apply temporary zero corrections, one would start on the 100mV range and work up to the 1kV range. I don't know why Fluke decided to do it this way, but it is nice that each range can be fully zero corrected. Despite the weird interaction between ranges, the 8505/6As method is still better than the DMM4050, which does not allow for retaining separate zero corrections for each range. And another annoying thing with the DMM4050 is that if you use the stats mode or any analyze functions the zero correction is removed...

For the next test, I should point out that the filter has 5 modes of operation:

|-----------Filter Modes----------|
| CMD Mode # Type      Tout   LED |
| F   blank  slow      none   on  |
| F0  0      fast      none   off |
| F1  1      bypassed  none   off |
| F2  2      slow      550ms  on  |
| F3  3      fast      50ms   off |
|---------------------------------|
NOTE: Pressing [Filter] changes its value from F0 to F, and back.
NOTE: To select a specific mode use key sequence [Store]-->[numeric entry]-->[Filter], a blank numeric field yields filter F.
NOTE: When in AVG Mode, any filter setting other than F or F2 will forcibly exit AVG Mode and set sampling to S7.
NOTE: When 8506A is in AC Volts, only F0 or F (for inputs <40Hz) are allowed.

Modes F and F2 as well as F0 and F3 are basically the same, so we really only need to test 3 different setting. Since average mode will exit if an incompatible filter setting is applied, we lose the extra resolution that the 10V range could provide to delineate differences in zero offsets. However, we can get that extra digit on the 10V range by:

1) Set filter to F1, sampling to S10 (4.3 second average).
2) Let meter settle and perform temporary software zero.
3) Enter average mode (uses default filter F) and note the difference.
4) Repeat steps 1-3 for F0.

In doing the above, I saw no difference in offset between filter modes, so I assume my filter module is working okay. I think this could be a pretty simple way to test if a meter has a bad filter module. Though, the meter should be warmed up for at least 2 hours before doing this to avoid false positives.

[garrettm]

I ran a few more tests tonight to see if I was missing any codes on the 7.5 digit readout. I don't really have the best set up to do this as my Valhalla 2701C is still being repaired, but my Advantest R6144 does have microvolt resolution on its lowest range.

Shown in the photos is the Fluke 8506A and my HP 3456A connected to the Advantest R6144 with low thermal connections. I zeroed/nulled both meters with the R6144 set to 0.000mV and then steped from 1uV to 10uV with the 8506A on the 10V range and averaging enabled while the 3456A was on the 100mV range with 100 NPLC integration time.

While slow, the 8506A was able to accurately resolve each microvolt step as verified by the reading shown on the 3456A. I'm not sure if there is a better test to show missing codes, or if my methodology is in error, but each code is present. It's possible that if a signal drifted too quickly the meter might not be able to resolve each intermediate change in LSD and appear to have missing codes. I didn't think to time how long it took to resolve each digit, but it's not very fast.

[bdunham7]

I didn't see any missing codes using that method either.  Where I saw them was a long-term observation of a 10V source. There was a few PPM of noise and drift, but I would consistently fail to observe certain values.

[SilverSolder]

[...] After playing around with the temporary zero corrections, I think your understanding is correct. Zeroing a lower range does affect the ranges above it but not those below it. From what I can tell, zeroing a lower range clears the stored zero corrections for the ranges above it but does not affect the ranges below it. Furthermore, temporary zero corrections are stored separately for each range. So to properly apply temporary zero corrections, one would start on the 100mV range and work up to the 1kV range. I don't know why Fluke decided to do it this way, but it is nice that each range can be fully zero corrected. [...]

Yes it is indeed a little strange.  Perhaps setting a zero at a low range is, in principle, supposed to get the zero for the higher ranges "good enough for Australia" as well, but that assumes there is no range-dependent offset (an assumption which does not appear to hold!).

In practice, since the meter does not have "Auto Zero" and things can easily move with a slight temperature change, it seems best to just set the zero every time you switch range anyway - so the "weirdness" isn't really an issue in practice and there is no need to zero all the ranges except during software calibration - where the takeaway is, start zeroing the lowest range and work your way up, to avoid disappointment! 

For the next test, I should point out that the filter has 5 modes of operation:

|-----------Filter Modes----------|
| CMD Mode # Type      Tout   LED |
| F   blank  slow      none   on  |
| F0  0      fast      none   off |
| F1  1      bypassed  none   off |
| F2  2      slow      550ms  on  |
| F3  3      fast      50ms   off |
|---------------------------------|
NOTE: Pressing [Filter] changes its value from F0 to F, and back.
NOTE: To select a specific mode use key sequence [Store]-->[numeric entry]-->[Filter], a blank numeric field yields filter F.
NOTE: When in AVG Mode, any filter setting other than F or F2 will forcibly exit AVG Mode and set sampling to S7.
NOTE: When 8506A is in AC Volts, only F0 or F (for inputs <40Hz) are allowed.

The way the filter settings work, looks like a lot of afterthought has gone into the design! - or perhaps more charitably, improvements were added while retaining compatibility with the original commands.

I don't fully understand the 50ms / 550ms "settling delay" property of the F2, F3 filters - is the start of the sampling of the input literally delayed by that amount of time after being triggered?

Modes F and F2 as well as F0 and F3 are basically the same, so we really only need to test 3 different setting. Since average mode will exit if an incompatible filter setting is applied, we lose the extra resolution that the 10V range could provide to delineate differences in zero offsets. However, we can get that extra digit on the 10V range by:

1) Set filter to F1, sampling to S10 (4.3 second average).
2) Let meter settle and perform temporary software zero.
3) Enter average mode (uses default filter F) and note the difference.
4) Repeat steps 1-3 for F0.

In doing the above, I saw no difference in offset between filter modes, so I assume my filter module is working okay. I think this could be a pretty simple way to test if a meter has a bad filter module. Though, the meter should be warmed up for at least 2 hours before doing this to avoid false positives.

Nice workaround, I'll see if I can characterize my problem boards this way. 

Perhaps it is just a hardware offset adjustment that is the problem. 

[joeqsmith]

Outside of a short note about the there being a fast (Y) and slow (Z) filter and them being a 3-pole Bessel with different cutoffs, they seem to provide very little detail about them.  Maybe the schematic IS the document.   

With Dave's UEI meter, I would feed a chirp into it and record the filters response.   Seems like you could do something like this to test out the hardware filters.

[SilverSolder]

Outside of a short note about the there being a fast (Y) and slow (Z) filter and them being a 3-pole Bessel with different cutoffs, they seem to provide very little detail about them.  Maybe the schematic IS the document.   

With Dave's UEI meter, I would feed a chirp into it and record the filters response.   Seems like you could do something like this to test out the hardware filters.

I ran the filters from the diagram in LTSpice and came up with the figures in red, in the Active Filter circuit diagram I posted earlier  (i.e. 7.5Hz and 75Hz, 18dB/octave for the slow and fast sections respectively).

I think those numbers are unrelated to the 50ms and 550ms "timeouts" as they are called in the manual.  My theory is that when one of the "timeouts" is activated, the meter simply waits that number of ms after a trigger before performing the reading (to guarantee the filter has settled).  Should be easy to verify on GPIB (still not set up here!)

[garrettm]

I didn't see any missing codes using that method either.  Where I saw them was a long-term observation of a 10V source. There was a few PPM of noise and drift, but I would consistently fail to observe certain values.

When I get the GPIB figured out I'll try observing my Advantest R6144 at 10V to see if mine does the same thing. I also want to try superimposing a ramp or noise signal on the output of the R6144 with a signal generator and compare the results.

[SilverSolder]

I didn't see any missing codes using that method either.  Where I saw them was a long-term observation of a 10V source. There was a few PPM of noise and drift, but I would consistently fail to observe certain values.

When I get the GPIB figured out I'll try observing my Advantest R6144 at 10V to see if mine does the same thing. I also want to try superimposing a ramp or noise signal on the output of the R6144 with a signal generator and compare the results.

Perhaps just turning off the Filter will have the desired (noise) effect?

The filter will suppress any signal over 7.5Hz or 75Hz, respectively, so it is unclear how much effect adding a signal would have unless sample periods get very long?

The noise may need to be added later in the chain, perhaps in the R2 converter itself...

[garrettm]

[...]
Nice workaround, I'll see if I can characterize my problem boards this way. 

Perhaps it is just a hardware offset adjustment that is the problem.

Hopefully all you'll need is to play with some trimmers, but if not, I have a spare Active Filter module (716316) on hand, though shipping might be more than its worth.

If needed, I could unsolder the j-fets and mail a few off. I recently found out my Current Shunt module has weird offsets that come and go, so I think it needs a few j-fets replaced too. Parting out the spare filter might be able to fix both our problems.

I also have a spare GPIB controller if anyone wants it.

[SilverSolder]

[...]
Nice workaround, I'll see if I can characterize my problem boards this way. 

Perhaps it is just a hardware offset adjustment that is the problem.

Hopefully all you'll need is to play with some trimmers, but if not, I have a spare Active Filter module (716316) on hand, though shipping might be more than its worth.

If needed, I could unsolder the j-fets and mail a few off. I recently found out my Current Shunt module has weird offsets that come and go, so I think it needs a few j-fets replaced too. Parting out the spare filter might be able to fix both our problems.

I also have a spare GPIB controller if anyone wants it.

Thank you for your kind offer, I'll keep that in mind if I lose traction completely with this one!

[garrettm]

Perhaps just turning off the Filter will have the desired (noise) effect?

The filter will suppress any signal over 7.5Hz or 75Hz, respectively, so it is unclear how much effect adding a signal would have unless sample periods get very long?

The noise may need to be added later in the chain, perhaps in the R2 converter itself...

Good point.

One interesting feature of the R2 ADC design is that each reading from the ADC is 6.5 digits. So with S0 sampling, that’s 250 6.5 digit readings per second. To see if there really are missing codes and how the filters might affect the results, it could be worthwhile to do the averaging remotely on a PC than at the 8505/6A. Simply wait for 1024 samples before computing the mean and place that value into a new time series (which would be equivalent to an average mode reading direct from the DMM). This way we eliminate any limitations with the precision of the internal data types and can quantify the noise of the ADC from the raw samples. This also lets us compute 7.5 digits with any of the three filter settings.

Unlike integrating ADCs, it makes more sense to use the fastest sample rate possible from a R2 ADC and perform the appropriate conditioning at the PC, rather than on the DMM. At least for these ancient boat anchors. With an auto zero circuit and more advanced controller and display, the 8506A could have been a much more impressive instrument. The "analyze" features on the DMM4050 (trend plot, stats and histogram) are be perfectly suited for the 8505/6A and its high speed, high resolution sampling. And with finer sample size control, speed vs noise could be better optimized for a given measurement. Then there is the possibility of using digital filtering to avoid the need for analog filters.

[bdunham7]

With an auto zero circuit and more advanced controller and display, the 8506A could have been a much more impressive instrument. The "analyze" features on the DMM4050 (trend plot, stats and histogram) are be perfectly suited for the 8505/6A and its high speed, high resolution sampling. And with finer sample size control, speed vs noise could be better optimized for a given measurement. Then there is the possibility of using digital filtering to avoid the need for analog filters.

Although true, the 'advanced controller' would have been tough to come by when the 850x was being developed.  There's a lot of performance difference between an 8080 and the twin Cyclones of the 8446A/DMM4050--over 10,000 times!  Averaging samples was about as sophisticated a digital filter as was practical.

[joeqsmith]

Outside of a short note about the there being a fast (Y) and slow (Z) filter and them being a 3-pole Bessel with different cutoffs, they seem to provide very little detail about them.  Maybe the schematic IS the document.   

With Dave's UEI meter, I would feed a chirp into it and record the filters response.   Seems like you could do something like this to test out the hardware filters.

I ran the filters from the diagram in LTSpice and came up with the figures in red, in the Active Filter circuit diagram I posted earlier  (i.e. 7.5Hz and 75Hz, 18dB/octave for the slow and fast sections respectively).

I think those numbers are unrelated to the 50ms and 550ms "timeouts" as they are called in the manual.  My theory is that when one of the "timeouts" is activated, the meter simply waits that number of ms after a trigger before performing the reading (to guarantee the filter has settled).  Should be easy to verify on GPIB (still not set up here!)

Odd they wouldn't have documented the filters better and that you had to run SPICE to sort it out.     

Log sweep from 100mHz to 10Hz, attempting to collect the data as fast as the meter will allow it (about 20Hz).  Showing the rolloff with the slow filter enabled.   
 

[joeqsmith]

Log Sweep 1Hz to 100Hz with fast filter enabled.   Obviously aliasing on the PC side but we can still see the roll off.