New (old) HP 3478A with a very floaty volts range - Page 1
Forum section: electronics / testgear

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 03:08:11 am
content: I just picked up a used HP3478A on Ebay, wanting a bit more serious of a bench meter (I've got a few meters now, but by far the best is an Agilent 4.5 digit - good, but not a bench meter). I managed to pick it up for like $80 shipped I think, after seeing the model name in an EEVblog post and searching around.In general, it seems reasonably accurate. Assuming my Agilent is dead-on-balls accurate, the measurement below is .024% out. It's well outside the .004%+3 accuracy quoted for the bench meter on the 30v range, suggesting it's proooooobably out of spec, but it's within the .025%+5 spec for the Agilent, so I can only assume they're both in spec for want of a better reference.My real question is this: when I open the measurement totally on the Agilent, it settles to a reading of 0, perhaps plus or minus a count, on and off. If I open the measurement on the HP, it floats wildly! It's been sitting on my bench for a while now and is reading .775xx ± .002ish, drifting around in there. In the image below, it reads .39468, because I was too impatient to let it settle for the picture.So what's up with this? If I read the input impedance with my agilent, it's out of range, and indeed the spec for the HP says the meter has 10Meg input impedance for the 30V and 300V input ranges, but >10Gig for the lower ranges. Oddly, I can't seem to set it any lower on the lower ranges, like I believe you can for most high-impedance-capable meters (someone tell me if I'm wrong and there's a setting for that! I haven't found any in the manual, but I'm not all the way through it either). Am I really just seeing the effect of ultra-high input impedance, perhaps measuring the E field between those two terminals on the front panel, or some internal field due to a power supply? Or is there something else going on here?When I put a 10meg resistor in a dual banana plug passthrough, it settles down to -0.07mV ± 0.05ish. This is for SURE out of spec (.025% +40 - it could be 40ct out and it's 740ct out), but again the miscalibration confuses me less than the huge drift. A dead short results in a measurement of 0.0010±3ct or so, which is within spec.

author: EEVblog
author_rank: Unknown
timestamp: March 24, 2014, 03:10:11 am
content: That's normal, just the high input impedance of the low voltages ranges.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 03:14:40 am
content: Quote from: EEVblog on March 24, 2014, 03:10:11 amAny explanation of why the 10meg resistor consistently measures -.0X mV where the dead short is actually within tolerance of a perfect zero?

author: cyr
author_rank: Frequent Contributor
timestamp: March 24, 2014, 06:35:08 am
content: 0.07mV over 10Meg is 7 picoamps. Not sure what the input leakage current spec is for the meter, but that doesn't sound unreasonable to me.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 06:37:04 am
content: Interesting thought, hadn't considered that, but that would imply there was something to SOURCE 7pA. Surely, the input will not leak current OUT, much less from ground to positive in (in order to get the negative reading)...

author: Vgkid
author_rank: Super Contributor
timestamp: March 24, 2014, 06:47:28 am
content: Some current leakage always happens, that 7pA is almost 3x less leakage then my bench DMM.

author: EEVblog
author_rank: Unknown
timestamp: March 24, 2014, 06:49:50 am
content: Quote from: alexwhittemore on March 24, 2014, 06:37:04 amExternal stuff around you can easily induce 7pA into the antenna test leads.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 07:03:02 am
content: No test leads, we're talking about a stub. See pics.

author: cyr
author_rank: Frequent Contributor
timestamp: March 24, 2014, 07:04:23 am
content: Quote from: alexwhittemore on March 24, 2014, 06:37:04 amWhy not? The leakage is within the components of the input stage and can go towards any potential within the supply voltages. In your case probably about .775V since that's where the meter wants to settle.Try connecting a (low leakage) capacitor over the input and watch it slowly get charged up... My keithleys usually like to hang around at about 2V.

author: sync
author_rank: Frequent Contributor
timestamp: March 24, 2014, 12:30:03 pm
content: 7pA input bias current is not bad. For example the HP 34401A is specified <30pA.

author: EEVblog
author_rank: Unknown
timestamp: March 24, 2014, 01:11:05 pm
content: Quote from: alexwhittemore on March 24, 2014, 07:03:02 amStill more than enough.Start here:http://www.keithley.com/knowledgecenter/knowledgecenter_pdf/LowLevMsHandbk.pdf

author: macboy
author_rank: Super Contributor
timestamp: March 24, 2014, 01:15:34 pm
content: Quote from: alexwhittemore on March 24, 2014, 06:37:04 amThe meter has an extremely high input impedance in the lower voltage ranges, in order to prevent loading the circuit under test. Even a typical 10 M? meter across certain circuits can have substantial effect. This can't be done for the higher voltage ranges since a voltage divider is implicitly required for those.In order to provide the high input impedance, the input signal is basically directly connected to a FET gate, either a discrete FET or the + input of a FET op-amp. Even though the input impedance is extremely high (10's or 100's of G?), there is some leakage current at that FET gate. Higher end meters will use components 'selected' for low parasitic parameters in for critical jobs, but the 3478A was only a mid-range instrument and may not receive such treatment. Note also that there can be leakage currents flowing across the bare surface of the PCB to/from the ultra-high impedance input signal traces. The engineers try to mitigate this with guard traces, but there is only so much that can be done to prevent disturbing that signal. Again, the highest end meters use techniques such as point-to-point construction of critical components between ceramic or teflon stand-offs so that there is no PCB to conduct those miniscule leakage currents.

author: Dongulus
author_rank: Regular Contributor
timestamp: March 24, 2014, 03:50:27 pm
content: Good score. I bought a HP3478A about a year ago and encountered the same phenomenon and I was similarly concerned. I later found out that this behavior is normal, but it's nice to get some explanation.On an unrelated note, I wanted to ask if you have checked the voltage of the calibration memory battery. If the battery loses charge, you lose the calibration data and paying for re-calibrating will cost more than the multimeter is worth. The threshold for the battery voltage seems to be nominally 2 V, but you would want it well above that.Hopefully you won't need to, but the battery can be replaced. It's a tricky processes and you have to be careful to keep power on the SRAM during the process or else. Here's some information on how it's done:https://www.eevblog.com/forum/beginners/changing-battery-in-a-hp-3478a-can-i-use-a-3v-lithium/

author: Neganur
author_rank: Unknown
timestamp: March 24, 2014, 03:56:09 pm
content: That thing has actually 10 GOhm while in 30 mV, 300 mV and 3 V ranges. Just short the volt inputs if it irritates you.Both of mine show numbers all over the place.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 04:05:12 pm
content: Quote from: Dongulus on March 24, 2014, 03:50:27 pmHelpful tip, thanks! Thing is, this meter is very likely around an order of magnitude out of spec anyway. Even more unfortunate, I just cal'd a (slightly) out of spec Fluke 83 and it was stupid easy - one resistor adjustment. Of course, there's not a ton to go wrong when you're talking about a 3.5 digit meter. Sadly, the 3478 is all digital, which means you need to cal each range independently (I don't have an arbitrary voltage reference) and you need to cal it with the exact full scale for that range (30mv, 300mv, 3V, 30v, 300v), you can't use something close-enough and call it a day (if I try to use a 2V reference to cal the 3V range, for example, it just won't play ball, I guess even though you can manually input the 'known', it's still looking for something close enough to the 3V reference).Seems like a reasonable answer would be to make a precision, say, 10v reference and use a precision decade divider to get the voltage references, but alas, I don't have a divider either, and they tend to be around as expensive as a proper calibration on the cheap end.Maybe I just need to make friends with someone at a cal lab.

author: Neganur
author_rank: Unknown
timestamp: March 24, 2014, 04:12:33 pm
content: We have suitable Fluke calibrators in the uni (the fellas from the power electronics use them to verify their gear). Maybe you could ask around if there's similar stuff where you are.IIRC the voltage range is the most important one, since the current and resistance measurements depends on it. And yeah if you abort at 300 V due to lack of a suitable source, it'll fail the entire cal attempt.

author: sync
author_rank: Frequent Contributor
timestamp: March 24, 2014, 05:00:43 pm
content: Quote from: alexwhittemore on March 24, 2014, 04:05:12 pmWhy do you think that?The 3478A has 0.007% + 4 counts (30V range, 1 year). Don't use the 24h specification. Assuming both meters from your example are within spec:3478A: 4.9965V .. 4.9979VU1253B: 4.9967V .. 5.0001VWhere is the problem?Also show both meter in the used range exactly 0V when you short the input?Did you the one hour warm-up before the measurement?

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 05:29:13 pm
content: Quote from: sync on March 24, 2014, 05:00:43 pmExactly right - I can't say for sure that the HP is OUT of spec because the degree to which they disagree is within the overall system error budget (specifically, if I were to assume the HP was spot-on, the Agilent would be .024% out, which is within ITS spec). That said, when I add in a third reference, a REF3333 (.15% accurate 3.3v reference), the Agilent and REF3333 disagree by something like .006%. It seems likely that they're probably both performing better-than-spec whereas the old and crusty HP is out - it's less believable that they BOTH (Agilent+REF3333) happen to be on the edge of their accuracy envelopes yet agree so perfectly. Though obviously not impossible.QuoteThe Agilent shows dead-on zero whether I short the input or leave it open (though again, 10meg impedance). The HP floats like a beast open, floats a bunch with a 10meg resistor, and is within spec but not spot on (15ct ±5ct off, 40ct stated accuracy) with a dead short.

author: sync
author_rank: Frequent Contributor
timestamp: March 24, 2014, 05:47:01 pm
content: Quote from: alexwhittemore on March 24, 2014, 05:29:13 pmSo the 3478A shows about 0.0015V (30V range) when shorted. Positive or negative?

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 05:55:40 pm
content: Quote from: sync on March 24, 2014, 05:47:01 pmWhen shorted, +.0010 to +.0020, doesn't really settle down in that range. When 10meg, -.07 ± .01 or so. Both of those are on 30mV range.the 30V range is dead-on zero, occasionally it'll go up or down a count. That's regardless of open, 10meg resistor, or dead short.

author: sync
author_rank: Frequent Contributor
timestamp: March 24, 2014, 06:23:24 pm
content: Quote from: alexwhittemore on March 24, 2014, 05:55:40 pmThat's 1 - 2uV. It's normal. It's in the range of thermal EMF errors. A small temperature difference at dissimilar metals (test lead connection) can cause this.QuoteGood. It doesn't have an offset error in this range.But really you can't say the 3478A is way out of spec testing it with a U1253B and REF3333. They are both not good enough. I have also a reference IC with is spot on with one of my DMMs. BUT they don't agree with my other references and DMMs. It's just a coincidence that they are spot on to each other.

author: cyr
author_rank: Frequent Contributor
timestamp: March 24, 2014, 06:35:01 pm
content: For comparison, here is my Keithley 2015 (6.5 digits / 1000000 counts, 50 ppm + 35 counts on 100mV range) and 2001 (7.5 digits / 20000000 counts, 37 ppm + 120 counts on 200mV range).The photos show a 1M resistor on the input, a dead short, and floating inputs.What you are seeing is perfectly normal AFAIK.

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 07:44:45 pm
content: Quote from: Neganur on March 24, 2014, 04:12:33 pmActually, I verified earlier this isn't quite true. To cal any range, you turn it to cal mode, manually range to what you want to cal, short, hit the cal button and hit the trigger button to verify, then hook up your source voltage, hit the cal button, adjust if necessary then hit trigger button to verify. It turns out that since it calibrates each range independently and keeps a digital table of offsets, the calibration for 3V doesn't have any bearing on the calibration for 30V, and you can choose to cal 3V and not 30V if you want to (or any subset, as testing shows necessary). However, though you can cal the 3V range with an inaccurate but precisely known 3V reference by manually adjusting the known input value up or down a count at a time, if the input is more than I guess some arbitrary percentage away from the 3x10^x volts it expects, it just won't let you. Which of course makes sense, it'd be a poor choice to, say, calibrate the 300v range with a 3V input, since any scale error down at 3V would magnify two orders up at full scale.But anyway, one CAN make a precision 30, 3, .3, .03V generator and use that to cal all ranges EXCEPT 300V. Which I think I'll probably make a project of, seems to me I can probably make a .01% or better 4-range generator for under $100, and of course, even if I totally cock it up, good learning experience

author: cellularmitosis
author_rank: Unknown
timestamp: March 24, 2014, 11:26:26 pm
content: Quote from: alexwhittemore on March 24, 2014, 07:44:45 pmThat is tremendously useful info! Thanks!I've been thinking about doing something similar (making a DIY 3478A "calibrator"), and possibly even basing it off of the Gellar Labs SVR or the voltagestandard.com units, perhaps using an LTC1043 to divide or multiply the reference reasonably precisely. If you start working on this, be sure to start a thread, I'd love to help out! I just took the first Contextual Electronics course, so I now know how to make a basic PCB design

author: cellularmitosis
author_rank: Unknown
timestamp: March 24, 2014, 11:27:45 pm
content: see alsohttps://www.eevblog.com/forum/testgear/diy-calibrating-a-hp-3478a/

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 24, 2014, 11:45:57 pm
content: Quote from: cellularmitosis on March 24, 2014, 11:26:26 pmCool! I didn't know about the Gellar standard, but yeah, if I get around to it I'll definitely make a thread! I was thinking about designing it with optional most significant digits too, so decades plus a switch for 2/3/4/5/6, but I'm guessing if I follow through on that idea the design will balloon out of practicality pretty quickly.

author: Neganur
author_rank: Unknown
timestamp: March 25, 2014, 02:32:22 am
content: Quote from: alexwhittemore on March 24, 2014, 07:44:45 pmyeah, but don't abort the cal

author: bingo600
author_rank: Super Contributor
timestamp: March 25, 2014, 09:06:31 pm
content: Quote from: alexwhittemore on March 24, 2014, 04:05:12 pmI can recommend thatGot my 3478A,3457A and 34401A cal'ed for 142 us$ , last Sep./Bingo

author: alexwhittemore
author_rank: Frequent Contributor
timestamp: March 25, 2014, 10:09:27 pm
content: Quote from: bingo600 on March 25, 2014, 09:06:31 pmDamnnnnnnn. Do want

author: mmagin
author_rank: Frequent Contributor
timestamp: May 14, 2016, 05:10:00 pm
content: Me too. It's harmless but I've always been curious about this. I have a perfectly functioning 3478A, and I've verified that it is a very real voltage with my antique Keithley 616 connected to it -- the reading matches.What's funny is that it creeps up to a little over 3 volts, then it sees that it's overrange and it starts toggling between ranges -- and the 616 observes it going between 3 and 0 volts repeatedly. It doesn't need the longer period of time to recharge to 3 volts as when you'd shorted the input. The whole thing is insensitive to the autozero setting too.If I'm reading the schematic correctly, it looks like there's a 470 pF capacitor in the input circuit which is only switched in for the lower ranges. Where exactly the charge is actually coming from, I don't yet know.(Edit: and putting the 616 in the 10^-11 amps range, I get about oh, +2 pA out of the 3478A with auto-zero off. With Auto-zero on, it's more like +15 pA)