Trouble Measuring Small DCV -Agilent 34411A-Keithley 6500DMM-Siglent SDM3068X - Page 1
Forum section: electronics / testgear

author: strahd_von_zarovich
author_rank: Contributor
timestamp: July 17, 2025, 11:21:42 am
content: Hi everyone,I’m currently working with a highly sensitive analog system and trying to measure very small DC voltage changes — in the range of 100 nV to 1 µV.I have three different 6.5-digit multimeters:Agilent 34411ASiglent SDM3068XKeithley DMM6500Here’s what I’ve experienced:The Agilent 34411A gives me consistent and repeatable results when measuring these tiny changes.Expecting better performance, I purchased the Siglent SDM3068X, since most of its listed specifications are superior to those of the Agilent. However, I noticed that the measurements fluctuate significantly, and I couldn't get consistent readings.I then bought the Keithley DMM6500, hoping it would resolve the issue. The noise level is indeed lower compared to the Siglent, but I still can’t reliably measure the small voltage variations as I can with the Agilent.For all devices:I’m using the same test setup and connectionsI’ve set the highest NPLC settingsThe signal source and conditions remain constantThis situation has made me wonder — could the Agilent 34411A have a superior analog front-end that handles noise better, especially for very low-level DC measurements?Has anyone encountered similar behavior when comparing these models?Is there something I'm overlooking in the configuration or setup?Any insights or suggestions would be greatly appreciated.Thanks in advance!

author: Martin72
author_rank: Super Contributor
timestamp: July 17, 2025, 12:19:36 pm
content: What exactly are you measuring, i.e., in which circuit?Pictures of the setup would be helpful.A short clip to visualize what is bothering you would be even better.ThenYou mention minor changes in the range of 1µV to 100nV, which is quite significant.But how quickly do they change?Or are these deliberately induced changes, a sensor that reacts to something, or something similar?As you can see, a detailed description is helpful if you want specific assistance.

author: wraper
author_rank: Unknown
timestamp: July 17, 2025, 12:26:48 pm
content: did you measure with a shielded cable?

author: tszaboo
author_rank: Super Contributor
timestamp: July 17, 2025, 12:56:26 pm
content: Quote from: strahd_von_zarovich on July 17, 2025, 11:21:42 amThermal EMF is probably your biggest source of error and noise.Shield the entire thing from air currents. Place it in a styrofoam box or something like that. Also shield the multimeter's connection.And you might need to buy special low EMF connectors for that. Pomona.I would rather place an instrumentation amplifier with a lot of gain right next to what you are measuring. Like INA128 or something better, zero it out and solder it directly on your voltage with a hundred gain.I don't think the DMMs make too much difference. I mean the used copper alloy in their terminal will matter more than the circuit inside.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 17, 2025, 06:46:54 pm
content: A separate DC amplifier is definitely an option to consider. Compared to a DMM it may get away with less connectors and protection and this way avoid some of the extra LF noise. The best choice depends on the signal source. This may now be something around an AZ OP-amp.

author: lugaw
author_rank: Regular Contributor
timestamp: July 18, 2025, 09:23:21 am
content: What I notice about the Agilent meters is they are very good at averaging the measurement before displaying it and does auto zero so you feel like you are seeing a smoother reading. The Keithley does not auto zero and just display the reading as fast at it can making the display more busy compared to Agilent. If you just dont look at the changing numbers and just look at the numbers not changing you will see they show the same reading.

author: KungFuJosh
author_rank: Super Contributor
timestamp: July 18, 2025, 04:35:21 pm
content: Quote from: lugaw on July 18, 2025, 09:23:21 amAll of his meters listed can Auto Zero, it's just a question if he has that enabled or not. AZ, PLCs, etc. can all be set.One other thing to consider is that his 34411A has obviously had the most aging, so assuming all settings equal, it might appear more stable anyway in the short term.

author: Kleinstein
author_rank: Super Contributor
timestamp: July 18, 2025, 05:08:55 pm
content: The input stages of the KS34411, DMM6500 and SDM3065 are quite different and also the way AZ mode is implemented can be different.For the KS meters it is usually the simple way with switching very close to the input, with a precharge phase to reduce the switching spike. The amplifier should be a low noise FET based OP-amp. The AZ sequence to be the basic alternation between the input and ground. So reading the input half the time and the zero for the other half.For the SDM3065 we have less information. The initial amplifier / buffer seems to be an AD8622 low bias BJT based OP-amp and AZ switching is likely also in front. It is a bit unclear if there is a pre-charge phase. Another compliction can be from the low side, that may not be at circuit ground - at last it is not for the 20 V range. So low side part may also contribute with some thermal EMF error at least. Best case the AZ switching would swap positive an negative side and this way get near 100% sampling for the inpu.For the DMM6500 I have even less information. It looks a bit like the input could be similar to the older Keithley 2000 (and somewhat similar to K2001,2002): with an AZ OP-amp as buffer at the front and AZ switching only after this buffer. This avoid the switching spikes at the input, but has a little more input circuitry that could add thermal EMF errors. The Keithley DMMs are known to have some addional not well understood LF noise (e.g. 0.1 Hz range). It could be in part from the more complex AZ loop with readings for the input, zero and a reference voltage at more than a simple 1:1 sequence. So there seem to be some averaging for the zero and ref. readings to not loose more than 50% of the active time. This defeats the LF noise suppression of the AZ loop at least to some degree.I don't think the aging part would be relevant. With the small voltage it would be more the offset drift and very low frequency noise from the amplifier.Aging would be even slower than the usual experiments.

author: GigaJoe
author_rank: Frequent Contributor
timestamp: July 18, 2025, 06:58:31 pm
content: it kinda on edge of a floor noise for all of them ....would it be ok considering to build heavy shielded, battery powered preamplifier to let say 1:100 or more , bumping signal above the noise level ?

author: Doctorandus_P
author_rank: Super Contributor
timestamp: July 19, 2025, 11:03:56 am
content: So you have small voltage changes, but what is the DC voltage? Is that low (100mV?) or is it 1000V?Measuring a 100nV change on a 1000V bus would be quite a challenge.

author: Martin72
author_rank: Super Contributor
timestamp: July 19, 2025, 12:10:16 pm
content: https://www.eevblog.com/forum/testgear/trouble-measuring-small-dcv-agilent-34411a-keithley-6500dmm-siglent-sdm3068x/msg5981335/#msg5981335I would still really like to know; everything else always degenerates into more or less meaningful guessing games.

author: mawyatt
author_rank: Super Contributor
timestamp: July 19, 2025, 02:23:12 pm
content: Something we did back in the 70s with Miles Signal Processor (MSP to replace MAID) to measure very low frequency (under 1Hz) disturbances at nanovolt levels in the presence of much larger interference, was push an integrator right up to the front end. This was followed by multiple notch filters to remove the Mains frequency and harmonics, then high order active LPF for overall band selection followed with other filters for specific bands. If the result needed "normalization" we used a differentiator to cancel the integration effects after all the post integration and notch filtering.Pushing an integrator towards the signal source may help before looking at the signals of interest with a DMM or other means. The notch filtering likely isn't necessary with a DMM measurement since they have excellent Mains rejection.Anyway, hope some may find this useful.Best

author: Roehrenonkel
author_rank: Frequent Contributor
timestamp: July 19, 2025, 06:32:36 pm
content: Hi,get a HP 3457A.Has 10nV Resolution in the 30mV-range.Good luck