Keithley 2000 TRMS converter noise - Page 1
Forum section: electronics / testgear

author: sync
author_rank: Frequent Contributor
timestamp: February 21, 2014, 10:56:56 pm
content: HiI repaired a dead Keithley 2000. The power supply caps leaked and eaten PCB traces. It's working now again.I would like to know the typical TRMS converter noise. It would be nice if someone with a Keithley 2000/2015 could measure it. Set the meter to ACV, 5.5 digits and short the input. The reading is the TRMS converter noise. I get 121 counts. The manual states 100 counts typically.

author: ratsept
author_rank: Newbie
timestamp: February 21, 2014, 11:31:57 pm
content: Hi,I just tried my 2015 THD and I'm getting 127 counts. Hope it helps.

author: sync
author_rank: Frequent Contributor
timestamp: February 22, 2014, 01:40:16 pm
content: Yes, it helped. The TRMS noise of my unit didn't degraded.Thanks!

author: TiN
author_rank: Super Contributor
timestamp: February 22, 2014, 03:10:12 pm
content: 5.5digit , 1 NPLC, REL + Auto filter

author: quarks
author_rank: Frequent Contributor
timestamp: February 22, 2014, 03:33:41 pm
content: Quote from: sync on February 21, 2014, 10:56:56 pmI have never really checked this on my Keithley.Therefore out of interest I just ckecked my 2015 and it is around 0.13mV and the Tek DMM 4050 is 0.04mV.Is this an indication that the 4050 is 3x better in this respect?Quote from: sync on February 21, 2014, 10:56:56 pm@sync, where in the Manual can I find this "100 Count" information?Quote from: TiN on February 22, 2014, 03:10:12 pm@TiN, I would be interested to see your values without "REL", if possible.thanksquarks

author: sync
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:03:02 pm
content: Keithley 2000 user manual, Page 2-19:QuoteRegarding if the 4050 is 3x better. I don't know. Maybe they use a less noisy analog TRMS converter or a different TRMS conversion method.@TiN: Many thanks for hosting the K2000 schematic. It helped a lot during repair.

author: quarks
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:12:18 pm
content: Quote from: sync on February 22, 2014, 04:03:02 pmthanks syncit is the same in the 2015 Manual (see att.)

author: quarks
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:24:53 pm
content: because I made my first test in 100mV range, I now repeated it in 1V rangeKeithley 2015 is now 1.27mVTEK 4050 is 0,33mV

author: TiN
author_rank: Super Contributor
timestamp: February 22, 2014, 04:32:32 pm
content: Time to open KI fanclub?Btw, this is after AC CAL (internal AC calibration procedure)KI2002:200mVAC rangeInputs shorted0.052xxx mVAC RMS10 NPLC, 8.5d, no REL200mVAC rangeInputs open6.559xxx mVAC RMS10 NPLC, 8.5d, no REL2 VAC rangeInputs shorted0.000142xx mVAC RMS10 NPLC, 8.5d, no REL2 VAC rangeInputs open0.00657xxx mVAC RMS10 NPLC, 8.5d, no REL20 VAC rangeInputs shorted00.0032xxx mVAC RMS10 NPLC, 8.5d, no REL20 VAC rangeInputs open00.0084xxx mVAC RMS10 NPLC, 8.5d, no REL200 VAC rangeInputs shorted000.010xxx mVAC RMS10 NPLC, 8.5d, no REL200 VAC rangeInputs open000.0146xx mVAC RMS10 NPLC, 8.5d, no REL750 VAC rangeInputs shorted0000.048xx mVAC RMS10 NPLC, 8.5d, no REL750 VAC rangeInputs open0000.051xx mVAC RMS10 NPLC, 8.5d, no RELSorry, 2001 off now, messing with NVRAMs in it ...

author: robrenz
author_rank: Super Contributor
timestamp: February 22, 2014, 04:32:49 pm
content: What I found inthis old threadimplies the 8846A AC performance may be better than average

author: quarks
author_rank: Frequent Contributor
timestamp: February 22, 2014, 04:50:41 pm
content: @TiN, thanks for the numbers.but can you please recheck beginnen from 2V range your values are probably VAC and not mVAC@robrenz, yes once again the Fluke8846A/TEK DMM4050 show amazing good results.Just to see what my 8.5 Digits meters show with this quick test, I fired them up.Whatever this really means, the little 4050 is second best!!!

author: casinada
author_rank: Frequent Contributor
timestamp: February 22, 2014, 05:48:28 pm
content: On my 2015 it shows:100mV range: 000.0084 mVAC1V range: 0.00074 VAC10V range: 00.0073 VAC100V range: 000.073 VAC750V range: 0000.71 VAC

author: sync
author_rank: Frequent Contributor
timestamp: February 22, 2014, 05:57:03 pm
content: Quote from: casinada on February 22, 2014, 05:48:28 pmInteresting. That is considerably less than the 120-130 counts we had so far. Is this a newer unit?

author: quarks
author_rank: Frequent Contributor
timestamp: February 22, 2014, 06:03:26 pm
content: Quote from: casinada on February 22, 2014, 05:48:28 pmThat is interesting and is well below the 100 number from the manual(only the 000.0084 mVAC should probably be 000.084 mVAC).I wonder if the test results can be improved somehow.Have you observed if the "Offset" changed over time?I hope some more members do this test, and Report their values.

author: dr.diesel
author_rank: Super Contributor
timestamp: February 22, 2014, 06:09:13 pm
content: Quote from: quarks on February 22, 2014, 06:03:26 pmK2010, eta: Monday afternoon.

author: mimmus78
author_rank: Unknown
timestamp: February 23, 2014, 12:19:31 am
content: Mine numbers:100mv 000,0321V 0,0002810V 00,0028100V 000,027750V 0000,26so I'm the winner of K2000 contest?

author: quarks
author_rank: Frequent Contributor
timestamp: February 23, 2014, 08:14:51 am
content: Quote from: mimmus78 on February 23, 2014, 12:19:31 amThat looks really good.Can you give us more details of your meter and setup?Here are links to some more info about this topic:https://www.eevblog.com/forum/testgear/dmm-measuring-ac-volts/msg233469/#msg233469http://www.ni.com/white-paper/2966/en/#toc6

author: robrenz
author_rank: Super Contributor
timestamp: February 23, 2014, 03:23:34 pm
content: Numbers from my trusty 8846A(yes, I kiss my meter)Shorted inputs,notzeroed out, high resolution, stat mode of 10 readings. No typo's possible with screen shots100mV1V10V100V1000V

author: dr.diesel
author_rank: Super Contributor
timestamp: February 23, 2014, 04:43:09 pm
content: robrenz's weaker brother, the DMM4020, not zeroed, slow rate:Code:200mV0.000mV2V0.00024V20V0.0018V200V0.011V750V0.04V

author: quarks
author_rank: Frequent Contributor
timestamp: February 23, 2014, 04:58:06 pm
content: Quote from: robrenz on February 23, 2014, 03:23:34 pmwow, that is 3x lower than my own measurements

author: KJDS
author_rank: Super Contributor
timestamp: February 23, 2014, 05:17:29 pm
content: I just measured three 2015THD on the 1V rangemeasured counts were 171, 80, 71

author: sync
author_rank: Frequent Contributor
timestamp: February 23, 2014, 06:31:34 pm
content: Robrenz, these numbers are very low. Do you know how good it measures at the low end of the scale? My 34401A shows also very low noise with shorted input. Less that 6ppm of the range. But the measurements drop off dramatically under 0.4% of the range, 0.5% on the 100mV range. IMHO these low noise readings are faked on the 34401A. Geller Labs have observed this too (http://www.gellerlabs.com/34401A%20AC%20zero.htm).

author: dr.diesel
author_rank: Super Contributor
timestamp: February 23, 2014, 06:42:17 pm
content: Code:HMC8012, ADC=Slow, AC Filter=Med, Not zeroedRangeMeanMinMaxStd DevPk to Pk# Measures (1000=~3min20sec)400mV0.062mV0.058mV0.065mV0.001mV0.007mV10004V0.00072V0.00070V0.00073V0.00001V0.00004V100040V0.0080V0.0078V0.0081V0.0000V0.0003V1000400V0.080V0.077V0.082V0.001V0.005V1000750V0.06V0.05V0.06V0.00V0.01V1000

author: robrenz
author_rank: Super Contributor
timestamp: February 23, 2014, 08:57:06 pm
content: Quote from: sync on February 23, 2014, 06:31:34 pmI addressed this in the link I posted on reply #9 of this thread. Itshere again for convienience

author: sync
author_rank: Frequent Contributor
timestamp: February 23, 2014, 09:46:38 pm
content: Quote from: robrenz on February 23, 2014, 08:57:06 pmSorry, I missed that. Wow, the 8846A is impressive! I subtracted the 12.5uV noise from your measurements.VMEAS= sqrt(VREADING² - VNOISE²).VINVMEAS20uV 19.1uV30uV 29.7uV40uV 40.7uV50uV 50.7uV60uV 61.3uV70uV 74.5uV80uV 82.3uV90uV 92.6uV100uV 102.9uV

author: robrenz
author_rank: Super Contributor
timestamp: February 23, 2014, 09:55:22 pm
content: Quote from: quarks on February 23, 2014, 04:58:06 pmWhat does the better Fluke measure?

author: robrenz
author_rank: Super Contributor
timestamp: February 23, 2014, 10:02:14 pm
content: Quote from: sync on February 23, 2014, 09:46:38 pmI think the 8846A is a very under appreciated piece of test equipment. I did a lot of manual/specs study before concluding it was the best meter on paper. I have seen a lot of things on the forum that indicate I probably made a good choice.

author: quarks
author_rank: Frequent Contributor
timestamp: February 24, 2014, 10:28:24 am
content: Quote from: robrenz on February 23, 2014, 09:55:22 pmin the default "Transfer"-Mode it is 0 in all ranges, if I turn this "Special" mode off, it is very close to your numbers.

author: robrenz
author_rank: Super Contributor
timestamp: February 24, 2014, 12:21:46 pm
content: Thanks quarks, that really is amazing. I also noticed in the Geller labs info that I linked that his 3458A was 22 to 25 µV. It may be that I was just fortunate to get a unit that was on the center of the bell curve on this.

author: dr.diesel
author_rank: Super Contributor
timestamp: February 24, 2014, 12:35:51 pm
content: Quote from: robrenz on February 23, 2014, 10:02:14 pmQuote from: robrenz on February 24, 2014, 12:21:46 pmA few more samples would be helpful (an easy test and i'm quite sure you're not the only member with one), but I'm betting your first statement above is dead on.8846A/DMM4050, anyone else?

author: quarks
author_rank: Frequent Contributor
timestamp: February 24, 2014, 12:39:16 pm
content: here is what I have collected so far

author: quarks
author_rank: Frequent Contributor
timestamp: February 24, 2014, 12:45:34 pm
content: Quote from: robrenz on February 24, 2014, 12:21:46 pmyes, be happy this will be very hard to beatBTW I tested trendplot (see att. with no warmup). It starts low and then goes up over time. Can you check this also?

author: robrenz
author_rank: Super Contributor
timestamp: February 24, 2014, 01:44:38 pm
content: I will test that but I have to let it cool off.BTW I am on of those that only turns on my meter when necessary. It probably has less than 3 weeks of on time in its several years of life so far.

author: TiN
author_rank: Super Contributor
timestamp: February 24, 2014, 01:56:24 pm
content: I'd suggest maybe test with some low-speed sine-wave?Let's say 1kHz sine 1V RMS. Something obtainable with common ARB/siggen.As we are more measuring zero offsets and such, rather than RMS noise like thread name..I have 33120A and R&S SMT03 here to try.

author: robrenz
author_rank: Super Contributor
timestamp: February 24, 2014, 05:03:26 pm
content: 8846A off for 2 hours. Turn on with no short while initializing. Put short in and set AC 100mV range. Trend plot.

author: sync
author_rank: Frequent Contributor
timestamp: February 24, 2014, 05:10:20 pm
content: Quote from: TiN on February 24, 2014, 01:56:24 pmWhat do you what to test exactly?QuoteYes, offset is a more adequate term. It's noise + other things.

author: HKJ
author_rank: Super Contributor
timestamp: February 24, 2014, 05:13:19 pm
content: Here is data from two 8846AThe first one has run nearly 24/7 for two years, the other one not very much:Agilent 34461A and Rigol DM3068 forces the value to 0

author: sync
author_rank: Frequent Contributor
timestamp: February 24, 2014, 05:28:58 pm
content: The 34461A doesn't use an analog TRMS converter anymore. It samples the input and do the TRMS calculation in software. It would be interesting how good it is on the bottom of the scale.

author: dr.diesel
author_rank: Super Contributor
timestamp: February 24, 2014, 06:16:24 pm
content: Quote from: quarks on February 24, 2014, 12:39:16 pmquarks, I have verified and rechecked,nota typo on either.

author: HKJ
author_rank: Super Contributor
timestamp: February 24, 2014, 06:48:48 pm
content: A trend plot from one of the meters:

author: dr.diesel
author_rank: Super Contributor
timestamp: February 24, 2014, 06:50:18 pm
content: Code:K2010, Rate=Slow, Filter=10 Readings, Not zeroed (Last Cal=2001)RangeValue100mV0.038mV1V0.00036V10V0.0036V100V0.035V750V0.34V

author: quarks
author_rank: Frequent Contributor
timestamp: February 24, 2014, 07:36:08 pm
content: Quote from: dr.diesel on February 24, 2014, 06:16:24 pmQuote from: dr.diesel on February 24, 2014, 06:50:18 pmthanks I have also updated with new data

author: mimmus78
author_rank: Unknown
timestamp: February 24, 2014, 08:43:16 pm
content: @quarksI set it to slow mode, no filter, no relative mode and shorted with the cheapest alligator leads you can found on ebay.I repaired this meter some months ago (replaced the ADC) and I think it is very low noise one (at least at DCV).I can run it at medium speed and have same readings as it was set to slow speed. Only time to time I can see flips on the last digit (don't know if this behavior is usual).

author: casinada
author_rank: Frequent Contributor
timestamp: February 24, 2014, 09:22:55 pm
content: Sure, I added one extra zeroMy 2015 is probably year 2004 vintage, probably 2005 as some chips say 0440.After 15 minutes it settles in 000.082 mVAC

author: electronic_eel
author_rank: Regular Contributor
timestamp: February 24, 2014, 09:34:19 pm
content: Keithley 2000, last cal unknown:000.047mVAC

author: flolic
author_rank: Frequent Contributor
timestamp: February 24, 2014, 11:01:10 pm
content: My K2000:100mV 0.057mV1V 0.00057V10V 0.0056V100V 0.055V750V 0.54V

author: quarks
author_rank: Frequent Contributor
timestamp: February 25, 2014, 02:42:22 pm
content: updated all and arranged in groups

author: sync
author_rank: Frequent Contributor
timestamp: February 25, 2014, 03:13:53 pm
content: I measured my Keithley 2000 again. I got sightly higher values. And I found that the offset is lower using fast reading rate.range medium, slow fast100mV 0.128 0.0901V 0.00127 0.0009010V 0.0127 0.0090100V 0.127 0.089750V 1.25 0.88

author: quarks
author_rank: Frequent Contributor
timestamp: February 25, 2014, 03:38:54 pm
content: ok revised again and now made a graph (normalized the numbers to Digits)all the flat lines look like what I expected (starting a bit higher at 100mV and then more or less flat),with the others I do not understand what can have caused the zigsag, if there are no typosmaybe someone has an Explanation

author: dr.diesel
author_rank: Super Contributor
timestamp: February 25, 2014, 03:57:00 pm
content: In the Hameg case, I bet the meter is just BSing the value, perhaps the 4020 as well. The Hameg reads 0.06V while shorted and open circuit in the 750V range.

author: mimmus78
author_rank: Unknown
timestamp: February 25, 2014, 04:04:44 pm
content: K2000 and K2001 have the same TRMS TO DC CONVERTER (AD637JR) it should be interesting to check how they both compares.I will test my K2001 this evening.

author: sync
author_rank: Frequent Contributor
timestamp: February 25, 2014, 04:10:00 pm
content: Quote from: quarks on February 25, 2014, 03:38:54 pmThe two highest AC ranges on the HMC 8012 are 400V and 750V. That's not 1:10. Maybe this is the reason that the line is not flat in your graph. Perhaps it's better to do it as offset/FS.

author: TiN
author_rank: Super Contributor
timestamp: February 25, 2014, 05:09:00 pm
content: 2002 is using same AD637JR too, but have more compensation stuff and calibration constants.On hardware level all layout and parts seem to be same , in terms of AC processing circuitry.KI2001 #7 (Firmware A08):No AC-CAL, shorted inputs.200mVAC range0.153xx mVAC RMS10 NPLC, 7.5d, no REL2 VAC range0.000110x VAC RMS10 NPLC, 7.5d, no REL20 VAC range00.0183xx VAC RMS10 NPLC, 7.5d, no REL200 VAC range000.0022x VAC RMS10 NPLC, 7.5d, no REL750 VAC range0000.010x VAC RMS10 NPLC, 7.5d, no RELAC-CAL done200mVAC range0.116xx mVAC RMS10 NPLC, 7.5d, no REL2 VAC range0.000183x VAC RMS10 NPLC, 7.5d, no REL20 VAC range00.0181xx VAC RMS10 NPLC, 7.5d, no REL200 VAC range000.0025x VAC RMS10 NPLC, 7.5d, no REL750 VAC range0000.010x VAC RMS10 NPLC, 7.5d, no RELAC-CAL done with inputs shorted (but procedure requires inputs open during this normally)200mVAC range0.075xx mVAC RMS10 NPLC, 7.5d, no REL2 VAC range0.000202x VAC RMS10 NPLC, 7.5d, no REL20 VAC range00.0180xx VAC RMS10 NPLC, 7.5d, no REL200 VAC range000.0010x VAC RMS10 NPLC, 7.5d, no REL750 VAC range0000.004x VAC RMS10 NPLC, 7.5d, no RELSo it's all over the place..Values change even when just toggle function, or change front/rear terminals.

author: quarks
author_rank: Frequent Contributor
timestamp: February 25, 2014, 05:26:27 pm
content: Quote from: sync on February 25, 2014, 03:13:53 pmI have also done the "fast" reading and it is also much lower100mV 0.0811V 0.0007510V 0.0072100V 0.074750V 0.73I also found out, that the TEK 4050 numbers are significantly lower (around 25) when I turn off the 2015THD underneath it

author: dr.diesel
author_rank: Super Contributor
timestamp: February 25, 2014, 05:45:22 pm
content: lol, Fast rate mine are higher!Code:K2010, Filter=10 Readings, Not zeroed (Last Cal=2001)RangeValue (Rate=Slow)Value (Rate=Fast)100mV0.038mV0.057mV1V0.00036V0.00058V10V0.0036V0.0057V100V0.035V0.057V750V0.34V0.55VEdit: Correct numbers this time,, (one extra zero on the Rate=Fast 100mV value)

author: mimmus78
author_rank: Unknown
timestamp: February 25, 2014, 09:17:25 pm
content: An this is another K2001AC-CAL done (not shorted) 10 NPLC, 7.5d, no REL200mVAC range 0.118x mVAC RMS2 VAC range 0.000093x VAC RMS20 VAC range 00.01558x VAC RMS200 VAC range 000.006x VAC RMS750 VAC range 0000.028x VAC RMS

author: casinada
author_rank: Frequent Contributor
timestamp: February 25, 2014, 11:11:51 pm
content: And another 2001 Firmware B09 A02Reset the unit to defaults:000.157 mVAC 200mVAC Range0.00024 VAC 2VAC Range00.0193 VAC 20VAC Range000.024 VAC 200VAC Range0000.12 VAC 750VAC Range

author: quarks
author_rank: Frequent Contributor
timestamp: February 26, 2014, 11:02:32 am
content: added the new numbers, but it is now more confusing, therefore I arranged two Graphs

author: TiN
author_rank: Super Contributor
timestamp: February 26, 2014, 11:07:34 am
content: If you use Excel, post a source table file, and let people feedle data themselves and make graphsOr we can make some test points (not zero) for measurements and make a simple online database.

author: dr.diesel
author_rank: Super Contributor
timestamp: February 26, 2014, 11:34:34 am
content: Most likely the values on my cheaper 5-1/2 meters are just fudged. Maybe I'll build up a 0.01VAC to say 1VAC test circuit and sweep both the 4020 and 8012 at the 750V range.

author: quarks
author_rank: Frequent Contributor
timestamp: February 26, 2014, 11:34:46 am
content: here is xls file

author: dr.diesel
author_rank: Super Contributor
timestamp: February 26, 2014, 02:09:45 pm
content: No surprises here really, perfectly repeatable:Code:DMM4020 RangeDMM4020 ReadingHMC8012 Reading (750V Range)200mV10.0mV0.05V200mV20.0mV0.04V200mV30.0mV0.03V200mV40.0mV0.02V200mV50.0mV0.01V200mV60.0mV0.00V200mV70.0mV0.01V200mV80.0mV0.02V200mV90.0mV0.03V200mV100.0mV0.04V200mV110.0mV0.05V200mV120.0mV0.06V200mV130.0mV0.07V200mV140.0mV0.08V200mV150.0mV0.09V200mV160.0mV0.10V200mV170.0mV0.11V200mV180.0mV0.12V200mV190.0mV0.13V2V200.0mV0.14V2V300mV0.24v2V400mV0.34V2V500mV0.44V2V600mV0.54V2V700mV0.64V2V800mV0.74V2V900mV0.84V2V1.0V0.94V2V2.0V1.95V20V3.0V2.94V20V4.0V3.95V20V5.0V4.95V20V8.5V8.49V

author: G0HZU
author_rank: Super Contributor
timestamp: February 26, 2014, 05:24:08 pm
content: My 30 year old analogue AC Vrms meter from Racal UK shows about 8uV residual noise on the 100uV range if that helps as a comparison?

author: casinada
author_rank: Frequent Contributor
timestamp: February 26, 2014, 05:43:17 pm
content: The 2001 and 2002 show similar curves, may be they aged the same way? they behave different from the 2000/2015. Is that something that can be corrected?

author: G0HZU
author_rank: Super Contributor
timestamp: February 26, 2014, 05:57:16 pm
content: The old Racal meter performs as below when terminated in a 50 ohm load and fed from a Marconi 2024 sig gen:10kHz 28.2uV (-78dBm) reads 32uV on the Racal100kHz 28.2uV (-78dBm) reads 32uV on the Racal1MHz 28.2uV (-78dBm) reads 31uV on the Racal5MHz 28.2uV (-78dBm) reads 31uV on the Racal15MHz 28.2uV (-78dBm) reads 33uV on the Racal20MHz 28.2uV (-78dBm) reads 33uV on the RacalIf I up the level to 100uV then it reads pretty close to the sig gen at all frequencies. It is supposed to work up to 20MHz but I rarely use it above 10MHz for reading Vrms.

author: cyr
author_rank: Frequent Contributor
timestamp: February 26, 2014, 08:51:33 pm
content: Here are the counts from my Keithley brothers at default 5.5 digit setting.2015: 100mV 61 counts, 1V 36 counts, 10V 31 counts, 100V 30 counts, 750V 26 counts.2001: 200mV 149 counts, 2V 24 counts, 20V 195 counts, 200V 21 counts, 750V 10 counts.The zigzag pattern on the 2001 is interesting, but overall I'm not sure if this data is in any way useful. A better test would be to see what level of input signal can actually be measured within a certain % maximum error. I don't have a suitable source to check...

author: mimmus78
author_rank: Unknown
timestamp: February 26, 2014, 10:35:08 pm
content: If K2000 K2001 and K2002 have the same AD637JR this means that there is (was) not better off-the-shelf TRUE RMS ADC ?

author: quarks
author_rank: Frequent Contributor
timestamp: February 27, 2014, 07:18:23 am
content: Quote from: cyr on February 26, 2014, 08:51:33 pmall of the reported numbers are nothing to worry about, if you do the math, but I just wonder if there is an explanation

author: jpb
author_rank: Super Contributor
timestamp: June 02, 2014, 10:52:20 pm
content: Having just acquired a K2000 (the one TopLoser was selling) I thought I'd try this test.I get 59 counts on all ranges except the bottom one where it is a bit lower (but rapidly changing).On the 1V range it is changing between 58 and 59.So somewhere in the middle of the range.I don't know the age of the meter but it is 2011 or later since the label on the front has "Keithley a Tektronix Company" and the sale was announced in late September 2010.The calibration is May this year. The meter was made in China.

author: macboy
author_rank: Super Contributor
timestamp: June 03, 2014, 12:16:22 am
content: Quote from: mimmus78 on February 26, 2014, 10:35:08 pmOff-the-shelf, it is just about as good as it gets, and has been for decades.Better can be done discretely. I have a HP audio distortion analyser that has several AD637 in it, but not to do the actual readings. They only to do the auto-ranging and other side functions that are not directly part of the measurement. The measurements are done with a discrete RMS converter that is presumably better.

author: ve7xen
author_rank: Super Contributor
timestamp: June 03, 2014, 05:42:46 am
content: Another Keithley 2000 data point for you. Manufacture date late 1994 early 1995 per datecodes. Last calibration unknown. Slow reading, 10x filter. Very crappy clip leads used as a short.100mV = 46uV1V = 420 uV10V = 4.3mV100V = 42mV750V = 400mV

author: bdivi
author_rank: Regular Contributor
timestamp: June 03, 2014, 08:11:50 am
content: Keithley 2015 here000.063 mVAC 100mVAC0.00039 VAC 1VAC00.0036 VAC 10VAC000.037 VAC 100VAC0000.34 VAC 750VACStrangely with MED or SLOW rate the offset counts go above 200 on all ranges

author: jpb
author_rank: Super Contributor
timestamp: June 03, 2014, 10:03:43 am
content: After posting I had another look at my 100mV range and it is surprisingly low compared to the other ranges, unlike most results published here.It rapidly changes (I'd not got much filtering on) but is in the range 8-20 whereas all other ranges are 58-59. Slightly curious. I was using quite a long lead as a short so perhaps at the lowest level it is picking up something (though I'd expect that to increase the noise not decrease it).

author: jpb
author_rank: Super Contributor
timestamp: June 03, 2014, 09:11:35 pm
content: This is a bit weird.I repeated the experiments this evening. Now the value is much larger at 149 counts with the lowest range being up to 160 counts.It seems a big increase from 59 up to 149 from one day to the next.Is this sort of variation normal? The temperature is less than 0.5 degrees different. Humidity is slightly higher (60% vs 55%).Do others have more consistency in their readings?

author: rf-design
author_rank: Regular Contributor
timestamp: June 04, 2014, 01:05:37 pm
content: Quote from: macboy on June 03, 2014, 12:16:22 amCould you state which kind of discrete RMS converter you use? What is the advantage over a fab trimmed bipolar ADI part?

author: jpb
author_rank: Super Contributor
timestamp: June 04, 2014, 02:36:08 pm
content: Quote from: cyr on February 26, 2014, 08:51:33 pmMy 2000 was calibrated just last month so I had a look at the AC results and they are both below and above the calibration standards depending on the frequency as well as magnitude so the rms noise is hidden by calibration constants presumably. The values are all well within spec so unless my meter has suddenly changed, having a relatively high (on my second measurement) count of 149 is hopefully not a problem.For example, the lowest level calibration standard was 10mV at 1kHz and my meter read this as 9.9993mV. At 100mV on the same range (100mV range) it read this at 99.9972mV for the 10kHz signal but at 100.0072mV for the 50kHz signal so it is difficult to deduce anything.

author: jpb
author_rank: Super Contributor
timestamp: June 04, 2014, 06:39:46 pm
content: The mystery of getting different values comes down to measurement rate (as per one of the posts above)Range Fast Medium Slow100mV ~20 162 1621V 40 154 15510V 39 155 155100V 39 154 155So it seems you get less noise at the faster rate (or at least less offset)

author: robrenz
author_rank: Super Contributor
timestamp: June 04, 2014, 07:19:11 pm
content: My previous postherewas with the 3Hz filter on. These are with the 200Hz filter on and are significantly lower.Edit: Very uniform 34% +/-1% reduction from the 200Hz filter. The 8846A high med and low resolution modes in AC only change the normal display resolution and do not affect the stat mode resolution or readings at all.100mV range was 12.48081µ with 3Hz filter1V range was 106.8218µ with 3Hz filter10 range was 998.5290µ with 3Hz filter100V range was 11.89725m with 3Hz filter1000V range was 110.9135m with 3Hz filter

author: macboy
author_rank: Super Contributor
timestamp: June 04, 2014, 07:52:39 pm
content: Quote from: rf-design on June 04, 2014, 01:05:37 pmYou can find the circuit and a very good description of its operation in the 8903B service manual, available at Agilent's website.

author: rf-design
author_rank: Regular Contributor
timestamp: June 05, 2014, 07:50:37 am
content: Quote from: macboy on June 04, 2014, 07:52:39 pmSorry, could not find the difference. I assume that HP is using similar matched quad NPN. Otherwise the performance lacks because of mismatch and non-thermal tracking. I think that the AD637 could be much better because it could use internal trimming methods to improve over an already matched quad.Could you state what is the specific advantage of the HP8903B rms detector over ADI637?