6.5 digit meter's HiZ input DC Volts sourcing current? - Page 1
Forum section: electronics / testgear

author: AlfBaz
author_rank: Super Contributor
timestamp: September 05, 2013, 03:44:57 am
content: Was just watching Dave's battery video and didn't want to derail an interesting discussion, so I thought I'd post another threadI have an HP3457A 6.5 digit meter that can provide 10GOhm input impedance on 3V and lower ranges. Above these ranges it jumps to 10MOhmIf I set it to 10MOhm input impedance and place abrand new, never charged10nF cap across the meter on DC volts it doesn't move (aside from noise) When I switch to 10GOhm it seems to start charging the cap at a very constant rate. Using I = C x dV/dt I get about 2.56pAI don't know if this is something peculiar to the HP3457A, just mine or it happens with all meters.Curiously I thought opamp input bias/leakage current flowed into the opamp but seeing that it is conventional current flow does that mean electrons are flowing out of the input?Here's a trend chart of the caps voltage over a couple of minutes. When I originally discovered this I let it go for a couple of hours and it remained fairly linear upto 2+ volts. I'm doing another run now to see what happens over 24hrs

author: ve7xen
author_rank: Super Contributor
timestamp: September 05, 2013, 04:19:46 am
content: Probably thermoelectric effects, EMF or something similar. I observe the same effect connecting new 22nF polyester cap to my 10Gohm meter. Approximately 1mV/s charges up.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 05, 2013, 04:47:55 am
content: Well it's good to see that it's not just my meterI am, however a little perplexed as to how a thermoelectric voltage would provide what seems to be a constant current, and wouldn't the caps voltage plateau at this EMF and not continue up to several volts?

author: Unknown
author_rank: Unknown
timestamp: September 05, 2013, 04:55:10 am
content: What's the spec for bias current for that meter? I see no reason why bias current would have to be one particular polarity. Could be either as far as I'm concerned.Try putting resistors of a few hundred kohm in series with the meter. It should limit bias currents and will have a negligible effect on attenuation of the signal.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2013, 05:55:11 am
content: Quote from: AlfBaz on September 05, 2013, 03:44:57 amWhat you measure is nothing else but the ordinary IBias, the input bias current, or input leakage current of the input FET amplifier.If you study the specification ofanyFET OpAmp, you'll see, that this current can be of both polarities, and is exactly of that order of magnitude (a few pA).That fact about OpAmps is trivial, or did I miss a different aspect of your question?The specification of the 3457A is incomplete, IBias is not specified, also not the corresponding input resistance.If you have measured 2.6pA, then you may calculate formally an input resistance of 3V/2.6pA ~ 1 TOhm = 1000 GOhm.But as IBias is nearly constant over the input voltage (as you can see from your diagram), the formal input resistance depends on the input voltage.At 30mV, you would calculate 10 GOhm.The 3458A specifies an input leakage bias of < 20pA, > 10GOhm, so that's in the same order.You can also measure this current the same way, if you omit the external capacitor.There's already a 220pF capacitor (C101) in the DCV input path, which will be charged from 0 to 3V within ~ 5 min, assuming around 2pA input current.FrankPS: I've just found Dave's #479 video blog, where he explains that I bias stuff.Later on, what he really measures is the offset voltage instead of the bias current, obviously.Measuring the charging current on a capacitor would've been the correct way to separate both effects correctly.

author: EEVblog
author_rank: Unknown
timestamp: September 05, 2013, 06:18:59 am
content: IIRC one of the big claims of the 34461A is that it has a new input architecture that lowers the input bias current significantly over it's competitors.

author: AlfBaz
author_rank: Super Contributor
timestamp: September 05, 2013, 06:32:35 am
content: Thanks for the detail Dr FrankI have been skimming over the "Theory of operation" in the service manual, that and the schematics show a "Pre-charge" circuit, to pre-charge the input amplifier. So it seems that HP already thought about this. If that's the case I was curious as to why this is happening, or is it that without the precharge cct, bias currents would be much higher.As for posting to start with I haven't dealt with precision gear previously and have thought of voltmeters as loads not potential current sources and as such seems to be something to keep in mind when measuring small voltages from odd sources.Is this where electrometers come in to play or as Dave just stated look to upgrade?

author: EEVblog
author_rank: Unknown
timestamp: September 05, 2013, 06:41:46 am
content: Quote from: AlfBaz on September 05, 2013, 06:32:35 amYes, but they aren't zero either.Take the Keithley 6514 for example:http://www.keithley.com/data?asset=387It has 3fA input bias current.That's in current mode though, they don't actually specify it for the voltage range. The same perhaps?BTW, the 34461A is 25pA

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2013, 07:11:53 am
content: Quote from: AlfBaz on September 05, 2013, 06:32:35 amThat precharge circuitry compensates for errors due to stray capacitance vs. Auto Zero switching.. quite a complicated descriprition..It's not obvious, how the Input Bias Current has to be included ..Anyhow, the 3457A is a very nice instrument, and the schematics are just beautiful to study..Electrometers are designed for lowest leakage currents, and Auto Zero circuitry mostly is omitted for that reason.Leakage currents originate from the FETs, but are mainly due to leakage in the housing (case) of the FET, on the PCB, cabling, switches and input jacks.Therefore, Keithley is THE specialist for building electrometers in the fA range for decades, and how they really do that magic, is not so well known.Obviously, they use specialized FETs and housing, perhaps bare dies, and they implement PTFE for isolation everywhere at the junction points, cables, and jacks,.Also they implement IBias compensation, I assume.Schematics for their electrometers are not available so easily..Frank

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2013, 07:17:03 am
content: Quote from: EEVblog on September 05, 2013, 06:41:46 amI think, that's nothing spectacular, if the 3457A typically has got 2pA, 3458A is specified < 20pA (I will measure that later), 34401 < 30pA.The Keithley electrometers are really superior in that aspect.FrankPS:Just've measured the leakage current of my 3458A.It has two 82pF caps at the input FET, so I take that as 200pF, and simply measure the delta time needed for a certain delta voltage, in the 10V range, after initially shorting the input.I've first switched off autozero function, because that injects charge into the input node.AZERO OFF:1.16V => 1.20V => 47sec => Ib = 200pF * 0.04V / 47sec = 0.17pA0.3V => 0.4V => 9.5sec => Ib = 2.1pA0.1V => 0.2V => 8s => Ib = 2.5pAThe input currents are very low, and depend on the input voltage.AZERO ON:-0.05V => -0.06V => 19.66s => Ib = -0.1pAAZERO injects charge, therefore the input current has opposite polarity and a different order of magnitude in this case.On the 3457A, AZERO can also be switched off, so you could also measure the 'real' input current without disturbance from charge injection.

author: PA4TIM
author_rank: Super Contributor
timestamp: September 05, 2013, 08:08:51 am
content: I think it is not allways only input bias. I (and some friends too) once done an experiment. we used several battery powered meters, (including a 10GOhm agilent and some Flukes) , Some analog chopper/electrometers and 6,5 and 7,5 digit bench multimeters. The testsetup was an AC transformer behind a shielded 1:1 isolation transformer, bridge rectifier and a electrolytic cap. DC volt was low, can not remember but somewhere around 5VDC. Then I let that connected for 24 hours. The battery powered meters all just measured 5VDC but most of the 230V powered meters did not stop charging. In one test the voltage over the capacitor went over 100VDC and was still rising.The explanation we came up with was that of capacitive coupling of the transformer in the multimeter was causing this effect. One meter that did not show this effect mentioned in its user manual that it had a special screened transformer to prevent this effect.Just try it, it's reall funny.

author: Dave
author_rank: Super Contributor
timestamp: September 05, 2013, 09:47:05 am
content: Quote from: Dr. Frank on September 05, 2013, 07:17:03 amYou have to consider that the 3457A was a beast that could be used for calibration. The 33461A is just another everyday meter.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2013, 12:36:00 pm
content: Quote from: Dave on September 05, 2013, 09:47:05 amWell, not exactly. Description from the HP catalog of 1997:HP3458A: .. Calibration and metrology labs .. rely on its accuracy..HP3457A: Ideal for automated test systems..HP34401A: Benchtop unit delivers 11 measurement functions and 6 1/2 digits for a price of a 5 1/2 digit.Although the 3457A is a little bit more stable and faster in some ranges than the 34401A (the everyday DMM of those days..), it is not good enough for metrological use.Anyhow, this classification does not tell anything about the input bias current.. the low cost 34401A is also specified with a very low value of Ib < 30pA.Frank

author: robrenz
author_rank: Super Contributor
timestamp: September 05, 2013, 01:03:24 pm
content: Quote from: EEVblog on September 05, 2013, 06:18:59 amMarketing BS. its the same as the 34401A and Fluke 88XXA metersEDIT:From the 34461A manual.Leakage Current ErrorsThe DMM's input capacitance will "charge up" due to input bias currents when the terminals are open–circuited (if theinput resistance is >10 G?). The DMM's measuring circuitry exhibits approximately30pAof input bias current forambient temperatures from 0 to 30 °C. Bias current doubles for every 8 °C change in ambient temperature above 30°C. This current generates small voltage offsets dependent upon the source resistance of the DUT. This effect becomesevident for a source resistance of greater than 100 k?, or when the DMM's operating temperature is significantlygreater than 30 °C.Agilent

author: AlfBaz
author_rank: Super Contributor
timestamp: September 05, 2013, 04:07:29 pm
content: Quote from: PA4TIM on September 05, 2013, 08:08:51 amAs I mentioned earlier I was going to log data for 24 hours, was curious to see how high the voltage got. After a couple of hours it dropped back to zero and started charging again. Sure enough it was just over the 3V mark when it dropped, so the range change kicked in the 10MOhm impedance@ Dr FrankI didn't even consider auto zero, I believe it's on be default. I'll adjust my labview app so that it doesn't reset the meter prior to logging and try it with auto zero off

author: PA4TIM
author_rank: Super Contributor
timestamp: September 05, 2013, 04:47:03 pm
content: The meter must be mains powered, the leakage of the cap must be ultra low otherway it will not rise. And not every meter does it. If it stops at a few volt, it is probably just i-bias but if the transformer inside the meter is not shielded the voltage goes up to probably 370V if the cap is ideal.I also used a electro current meter in series with the meter to see if the current came from the diodebridge or from the meter. It came from the meter. A GR 10uF standard cap will give an other result as a high esr, high leakage sjingwhoawokky cap ;-) )I forgot this for DAVE: test these effects with a current meter in series (that can measure pA/nA)

author: Dave
author_rank: Super Contributor
timestamp: September 05, 2013, 06:45:20 pm
content: Quote from: Dr. Frank on September 05, 2013, 12:36:00 pmI stand corrected.I do own one instrument that was calibrated with a 3457A, though. An E3612A power supply. Hardly precision work at 3.5 digits.

author: Dr. Frank
author_rank: Super Contributor
timestamp: September 05, 2013, 09:15:44 pm
content: Quote from: Dave on September 05, 2013, 06:45:20 pmYep, you do!And perhaps you know, what's my passion.My nickname given by the colleagues was 'ppm-vulture' when I was 19, working in that cal - lab.Today, it's even worse.Frank