[SOLVED] Voltmeter Takes Forever To Charge Up From a High-Impedance Source - Page 1
Forum section: electronics / general

author: niconiconi
author_rank: Frequent Contributor
timestamp: June 28, 2024, 11:10:33 am
content: Update: Solved! Seehttps://www.eevblog.com/forum/metrology/why-do-my-voltmeter-take-forever-to-charge-up-from-a-high-impedance-source/msg5557145/#msg5557145I was trying to use a regular benchtop voltmeter to measure high-impedance voltage sources. I know, I should've used an electrometer or electrometer-grade opamp circuit instead to do it "properly" - but it was just for the purpose of learning something about low-current measurements, so I decided to start simple.The equivalent circuit of a DMM's input stage is supposed to be a resistor (typ. 10 MΩ / 10 GΩ), a capacitor (typ. 100 pF), a bias current source (typ. 100 pA, either direction), and an ideal voltmeter, all in parallel. See Figure 1.The source resistance and meter's input resistance are supposed to form a voltage divider with a Thévenin resistance of Rsource // Rmeter. If this equivalent circuit is a good representation of the DMM, the step response of the voltmeter's reading should always settle in a short time regardless of the source impedance - as the R in its RC time constant can't be lower than Rmeter itself. If R = 10 GΩ, C = 100 pF, 5RC = 5 s.I did an experiment: 10 V source followed by 100 MΩ resistor, measured by a Fluke 8840A 5.5-digit benchtop DMM in 20 V range (10 GΩ resistance), and the result is Figure 2. The meter took ~5 minutes to charge up, and the final DC voltage reading was 9.9160 V, so R = 11.80 GΩ as advertised by the meter.But by assuming R = 100 MΩ, the curve-fitted equivalent C is as high as 587 nF (For reference, by assuming R = 10 GΩ, C = 5.8 nF). This fitted value is huge, and I don't believe it's the meter's actual parasitic capacitance - it's probably a non-linear effect of the input stage - bias current related?Repeating the same experiment with 10 V source followed by 1 GΩ resistor, now the meter took ~1 hour to charge up, and the final DC voltage reading was 9.0929 V, so R = 10.02 GΩ, but C is 507 nF when assuming R = 1 GΩ (if R = 10GΩ, C = 50 nF).I tried to replicate the same result in a circuit simulator by adjusting the current source from pA to nA, but it cannot replicate the slow charging time. I also tried to measure the meter's input capacitance using an LCR meter (at 20 V range), and I find the readings vary wildly and are completely unstable, depending on the test frequency, and even depends on the meter's measurement speed settings, sometimes ~300 pF, sometimes ~400 nF, sometime it's even inductive.From these results, I believe the equivalent circuit is not really an accurate representation of the DMM when measuring high-impedance source. The input charging time strongly depends on the source resistance, and increases by 10x when the source resistance is increased by 10x too, unlike what the equivalent circuit suggests. Thus, the input stage is both non-linear and time-variant, with a bias current that is constantly changing (especially during sampling). Also, perhaps at near-DC, the input stage even tries to null the input current by feedback to maintain high impedance?Did I reach the correct conclusions? Do you have a better equivalent circuit for these kinds of measurement, or any other references?

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2024, 02:40:07 pm
content: A couple of things occur to me:1. I do not know what kind of resistors you used, but high value resistors may display dielectric absorption, leading to extended settling time.2. Printed circuit board materials, especially FR4 types, may display "hook" at high impedances. This is a change in capacitance versus frequency which has the effect of increasing settling time. This plagued high impedance oscilloscope inputs when the transition was made from point-to-point wiring to printed circuit board construction. Before Tektronix figured it out, they built their high impedance attenuators on plastic circuit boards. Modern instruments which have high impedance circuits sometimes suffer from this even now.3. Measuring the input capacitance or resistance of a multimeter with a given instrument may be difficult or impossible because of charge injection from the multimeter. Sometimes older instruments have an advantage for either not generating, or being immune to, charge injection.4. Leakage across dirty surfaces can cause long settling time.

author: bdunham7
author_rank: Super Contributor
timestamp: June 28, 2024, 03:00:34 pm
content: Try reversing the polarity of the meter (so you're reading -10V) and compare the results. Getting approximately 10G input resistance is likely a coincidence, the spec is >10G and the actual resistance component is typically much higher, virtually infinite based on my experience with 8842A models.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2024, 03:55:00 pm
content: The Fluke 8840/42 and a few other older meters have an analog filter at the input. For the 8840 the schenmatics shows 2 x 330 nF effectively in parallel at the filter. The filter can be turned off, but this comes with extra noise due to the sampling type (fluke specific recirculating remainder) ADC. So the filter may well be on by default.

author: mawyatt
author_rank: Super Contributor
timestamp: June 28, 2024, 05:03:10 pm
content: Quote from: David Hess on June 28, 2024, 02:40:07 pmRecall some also used alumina substrates, and Tek had those cool alumina terminal strip standoffs with the plated "grooves" for supporting leaded components lying in the grooves and soldered in place and not the ones that used phenolic and epoxy laminates.Best,

author: RoGeorge
author_rank: Super Contributor
timestamp: June 28, 2024, 05:55:32 pm
content: Maybe the input capacitor(s) have some leakage, such that a considerable percent of the 1nA coming from the 10G resistor is going to ground instead of charging the input capacitance.Or maybe a big part from the 1nA incoming current (from the 10G at 10V) is sunk as bias input current by the input amplifier of the instrument, instead of charging the input capacitor(s), so the capacitor will charge slower than expected.These are just hypotheses, I don't know how the input of the instrument looks like, and don't have hands-on experience with very low currents either.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2024, 07:04:26 pm
content: The filter input capacitors (660 nF nominal) are a reasonable good match to the "measured" 507 nF. There is the possibilty that the JFET switch to isolate the filter may be leaky - this may get unnoticed for quite some time as the filter on case is the normal use. This type of defect (leaky fet) would give nasty slow settling with the FET off resistance and some 660 nF even with a lower impedance source, just less effect (e.g.( R_source + 100 K) / R_JFET). So a leaky FET is not the problem here, but could be a not so obvious defect to check for.The precision meters have the high input impedance to measure accurate to the ppm level with source resistance in the KOhm range, not to measure with some 10 M or 100 M source resistance. Besides the settling problem there can also be current noise - the input bias is usually not noise free, but can fluctuate (at least some thermal effect).The current noise should not be too bad, as the 5.5 and 6.5 digit meters usually have series resistors in the 100K-200 K range as part of the input protection.The FR4 PCB part is usually only a few 10 pF of parasitic capacitance. The effect may be just detectable for a rel. fast (e.g. 1PLC) AZ switching cycle. From another thread that looked at this:https://www.eevblog.com/forum/metrology/analog-frontends-for-dmms-approaching-8-5-digits-discussions/msg5103669/#msg5103669the estimate is that 2 pF of FR4 capacitance result in 1 Tohm of effective resistance to ground for the 1 PLC AZ cycle. So a rather small effect for a 5.5 digit meter.

author: niconiconi
author_rank: Frequent Contributor
timestamp: June 28, 2024, 08:23:36 pm
content: Quote from: Kleinstein on June 28, 2024, 03:55:00 pmYou nailed it. We have a winner.The Fluke 8840A only gives you 5.5 digits when it's in "slow measurement rate" mode, with a huge input filter that rolls off at 7 Hz. I was already suspecting the filter when my LCR meter readings change depending on the measurement rate, but I didn't bother to double check it before going to sleep.Here are the retests with "medium" measurement rate. The step responses now settle within 1 minute or so for both 100 MΩ and 1 GΩ source impedances.For those of you who're playing at home, the Fluke 8840A manual says:QuoteHere's the block diagram:Here's the filter schematic. Active filter with 220 nF and 330 nF capacitors, looks exactly like what I was seeing.Full schematics for reference:Case closed. Conclusion: RTFM (Read The Filter Manual).

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2024, 08:33:43 pm
content: Quote from: mawyatt on June 28, 2024, 05:03:10 pmWhen they used ceramic terminal strips with fired silver in their tube era oscilloscopes, they used what looks like paper phenolic for the high impedance attenuator.Later when they had the hook problem with FR4 substrates, Tektronix used what I suspect is polysulfone, and some other white plastic that I never identified.Quote from: RoGeorge on June 28, 2024, 05:55:32 pmThe input current should be a lot less than 1 nanoamp. I would expect considerably less than 20 picoamps.

author: Sensorcat
author_rank: Regular Contributor
timestamp: June 28, 2024, 08:53:35 pm
content: It's exemplary that the OP of a threat not only marks his question as solved, but also summarizes his findings to close the thread. There should be more of this! Maximizes the usefulness of the forum and should be encouraged/advertised somehow.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2024, 08:59:19 pm
content: Quote from: niconiconi on June 28, 2024, 08:23:36 pmI would not have guessed that the switched filter is before the input buffer.I wonder if they graded the AD547 for high common mode rejection. That was the premium JFET input precision operational amplifier at that time, but it still was not very good. We have much better JFET parts now which should be drop in replacements. I am using the OP140 to replace the originally recommended AD547 in my Tektronix DM501A.

author: Kleinstein
author_rank: Super Contributor
timestamp: June 28, 2024, 09:18:18 pm
content: In the 8840 the AD547 uses a bootstrapped supply. This way the common mode rejection is way less important. It looks like the lower voltages may even bypass the OP-amp and use the JFET amplifier directly, with the AD547 only as a precharge / auxiliary signal for the FET switches.For the 20 V range and only 5.5 digits the amplifier is not that critical.The input filter is directly at the input. The source impedance adds to the 100 K in the input path and thus can lower the cross over frequency.One advantage from a filter directly at the input is that short pulses (and also much of the hum) are not clipped so easy - they are still averaged out. Also other meters (e.g. HP3456 or Datron 1281) with an input filter often have it similar at the input. For protection they have the large resistor (some 100K) anyway. So why not reuse it also for the filter. A later filter would need an extra buffer and would add resistor noise.

author: David Hess
author_rank: Super Contributor
timestamp: June 28, 2024, 11:28:50 pm
content: Quote from: Kleinstein on June 28, 2024, 09:18:18 pmI missed that in the schematic, but there it is.Quote86 dB is required for 4.5 digits, and the AD547 has a minimum common mode rejection of 76 dB. Of course this depends on how many counts are error are acceptable, but bootstrapping removes this as a consideration.