what does your 3456a read with shorted inputs on AC mode? - Page 1
Forum section: electronics / general

author: imaradiostar
author_rank: Contributor
timestamp: September 14, 2019, 04:37:31 am
content: I was surprised to see my recently repaired (PS recapped) 3456a reads around 800uV of noise with the input shorted on ACV mode with autorange on. I looked at the manual and even thought it specifies a higher count error on lower ACV ranges, I was surprised to see that much noise when shorted.I'm thinking there is more work to be done!What does yours do? Mine was set to 100 cycle intg and 6 digits.Thanks!Jamie

author: bob91343
author_rank: Super Contributor
timestamp: September 14, 2019, 04:42:23 am
content: About 500 microvolts.

author: imaradiostar
author_rank: Contributor
timestamp: September 14, 2019, 04:43:24 am
content: So maybe mine isn't so far off?Jamie

author: Kleinstein
author_rank: Super Contributor
timestamp: September 14, 2019, 09:25:22 am
content: AFAIK the lowest AC range is 1 V on the 3456. The AC conversion is done analog and as usual not good at low levels (e.g. below 0.1-1% of the full scale). This is kind of a immanent limitation to the old style analog RMS circuit. There may be some adjustments during calibration to to effect the reading with a short, but there still is a problem in the low range. It starts with the rectification not working well at very low voltage. At some point there would be just an offset that swamps the AC part. So even if the zero is adjusted, chances are very low values are not real.Some modern meters have lower AC ranges (e.g. 10 mV) for this reason.

author: dietert1
author_rank: Super Contributor
timestamp: September 14, 2019, 09:32:31 am
content: Low level AC measurements may also depend on the guard usage. In audio you would call it "shield".Regards, Dieter

author: borghese
author_rank: Regular Contributor
timestamp: September 14, 2019, 10:11:18 am
content: The mine is about 330 microvolt

author: alm
author_rank: Super Contributor
timestamp: September 14, 2019, 04:33:33 pm
content: This may not be noise, but non-linearity of the AC-to-DC converter near the bottom of the scale. In the specifications, best 24h accuracy for ACV is 0.05% + 550 counts for > 1% of full scale. On the 1.0 V scale, this would be a 12 mV signal. An uncorrelated 800 µV offset (noise or otherwise) would contribute \$12 mV - \sqrt{(12 mV)^2 + (800 µV)^2} = 27 µV\$ error to the measurement. Well within the 550 µV offset error allowed by the specification.Keithley used to have a nice FAQ describing this, but it disappeared a long time ago when their content migrated to the Tektronix website.Fluke (first question)andKeysight (page 7)refer to this, but without any details.

author: MarkL
author_rank: Unknown
timestamp: September 14, 2019, 06:23:56 pm
content: 384uV here.

author: Edwin G. Pettis
author_rank: Frequent Contributor
timestamp: September 15, 2019, 04:39:28 am
content: My 3456A is reading between 250uV and 255uV on the lowest scale, it also requires a few seconds to stabilize, set at 10 readings/integration and 6 digits. Same reading at 100 readings/integration but digit fluctuation drops to 1-2 digits instead of 5. (over 35 years old)

author: guenthert
author_rank: Frequent Contributor
timestamp: September 15, 2019, 04:57:30 pm
content: I don't have access to it right now, but iirc it was around 560uV, well within specifications (well, it's specified only for signals > 1% of full scale).

author: TWMIV
author_rank: Contributor
timestamp: September 15, 2019, 09:43:05 pm
content: I get 214uV and 154uV on mine. The 214uV is from the newer (rubber buttons) of the two while the older one (plastic buttons) reads 154uV.

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 26, 2019, 09:28:24 am
content: 209 to 211µV on the one sitting besides me. Newer Version with rubber buttons.

author: Alex Nikitin
author_rank: Super Contributor
timestamp: September 26, 2019, 03:31:23 pm
content: ~350uV here.CheersAlex

author: Ysjoelfir
author_rank: Frequent Contributor
timestamp: September 26, 2019, 03:44:59 pm
content: Right before switching it off for the night and leaving work mine reached ~310µV.

author: wn1fju
author_rank: Frequent Contributor
timestamp: September 26, 2019, 03:45:17 pm
content: 154 uV for 3456A version 2201A.

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 26, 2019, 04:40:24 pm
content: Maybe a dumb question but has anyone noticed their 3456A doesn't seem to settle when the inputs are floating and moves around in value but when testing actual DCV it's spot on accurate? Is this normal for these or is this some symptom of old aged out electrolytics? I know a little of this normal even for the 34401A. I tested it against two voltage references and it's pretty much spot on. I was happy to see my brand new DMMCheckPlus came and my 34401A shows the 5VDC as 5.00000 on the button. I also have a Calibrators Inc. DVC-350A. It's only 4 digit but also shows spot on 5VDC on my 34401A which now with the DMMCheckPlus makes be feel pretty good about my current calibration on my MM's. My 3456A is close with being off by .00001 or .00002 on the last digit when testing 5VDC. Now I gotta short the inputs and check the AC after reading this thread o.O! I'll check it tonight. I was considering buying another voltage reference but now that everything seems pretty much in sync I'm thinking maybe a differential voltage meter might be more fun fix up and get working right.Bill

author: CatalinaWOW
author_rank: Super Contributor
timestamp: September 26, 2019, 07:21:16 pm
content: 365 uV on mine, which has been on for weeks. Same with and without guard, front and rear terminals. But went up to stable 1400 on higher ranges. Back to 365 when on auto or manually set for lower volt ranges.I haven't looked into the reason for the range dependency.

author: alm
author_rank: Super Contributor
timestamp: September 26, 2019, 07:51:03 pm
content: Quote from: notfaded1 on September 26, 2019, 04:40:24 pmCompletely normal for bench meters. The input impedance at ranges up to 10 V is > 10 GOhm, in parallel with some amount of capacitance. It takes only a tiny amount of induced current (~ 1nA) to charge the input capacitance up to over 10 V.

author: notfaded1
author_rank: Frequent Contributor
timestamp: September 27, 2019, 01:32:45 am
content: 237uV on mine I just checked tonight. It seems obviously much more stable after leaving it on an hour or so and I waited to do the short.Bill

author: KK6IL
author_rank: Regular Contributor
timestamp: September 29, 2019, 06:53:06 am
content: 265-270 uV with 100 cycles.