EEVblog #584 - What Effect Does Your Multimeter Input Impedance Have?

[vk6zgo]

Another situation where the DMM's 10M input impedance gave a misleading result was in the following case:

A 24 v battery supply was isolated from an external circuit by a reverse biased power diode in the positive lead.
A colleague measured between  the outer end of the diode & the negative terminal,confidently expecting a reading of zero volts.
He was astounded to see a reading of 11 volts!

A good reminder that power diodes aren't ideal,& that a reverse resistance of  around 11.8M is quite possible.

That was a very different scenario,but again confirms that the best Test Instrument is the one between your ears---closely followed by a Scientific Calculator.

[Rerouter]

Hi Dave, Given your precision current source and precision voltage standard and infinite impedance Agilent Multimeter ( I have the same model, BTW), I was wondering if you can make an accurate measurement of some 13 gig-ohm ( thats right 13 x10^9 ohms) resistors I have. See attached image. Perhaps this is an odd request, but you seem rather clever and I imagine you or someone here might  have some ideas other than applying 1000 volts to it and trying to measure 77 nano-amps. I can send you some if you promise to do a short video on it. Years ago Jim Williams at LTC measured them with some contraption he had in his lab and confirmed the value but he never disclosed how he did it ( I can't call him today....for obvious reasons) or with what.   Thanks and wet soggy cheers from Oregon --Bob

Have a look into industrial megger testers used by electricians, for 6.3KV and higher capable units gigaohm is the standard unit, with the mid end meggers for 3 phase 1000V can generally measure close to the gigaohm value your chasing,

[Rufus]

What is this "wouldn't put a low value pot in there because the wiper resistance is going to have an effect on the stability" at about 2:09?

Pots usually have poor end to end tolerance and maybe not very good end to end temperature stability so if used as part of a divider swamping them with a better fixed value resistor may be a good idea. I don't see what wiper resistance has to do with it - what if anything am I missing?

[SeanB]

13G and with a half watt power rating. Even in SF6 at 400Bar those resistors will flash over before they get to a half watt power dissipation. No Idea how those were measured, but I remember there were a few appnotes from him about measuring ultra high impedance opamp input currents, might be a good starting point.

[Pillager]

This video takes me back 20 years, when I was learning electronics.

We were shown how you can have a voltage or current error, depending on the setup, when measuring both values at the same time. Of course, we were using analog meters then, so it was even more important, especially when switching ranges...

[crusader66]

I recently discovered that my old Fluke 8810A has input resistance >= 1Gohm on it's 200mV, 2V and 20V ranges.  Probably would have been a good meter to use here.

[David Hess]

Hi Dave, Given your precision current source and precision voltage standard and infinite impedance Agilent Multimeter ( I have the same model, BTW), I was wondering if you can make an accurate measurement of some 13 gig-ohm ( thats right 13 x10^9 ohms) resistors I have. See attached image. Perhaps this is an odd request, but you seem rather clever and I imagine you or someone here might  have some ideas other than applying 1000 volts to it and trying to measure 77 nano-amps. I can send you some if you promise to do a short video on it. Years ago Jim Williams at LTC measured them with some contraption he had in his lab and confirmed the value but he never disclosed how he did it ( I can't call him today....for obvious reasons) or with what.   Thanks and wet soggy cheers from Oregon --Bob

Nothing fancy is needed.  Use the shunt resistance of your best 10 megohm input voltmeter as a voltage divider with the 13 gigohm resistor and a 10 volt source and you should get a reading of 7.69 millivolts.  Any good voltmeter will have an input bias current low enough to keep the error far below the 10% tolerance of the resistor.

[EEVblog]

Hi Dave, Given your precision current source and precision voltage standard and infinite impedance Agilent Multimeter ( I have the same model, BTW), I was wondering if you can make an accurate measurement of some 13 gig-ohm ( thats right 13 x10^9 ohms) resistors I have. See attached image. Perhaps this is an odd request, but you seem rather clever and I imagine you or someone here might  have some ideas other than applying 1000 volts to it and trying to measure 77 nano-amps. I can send you some if you promise to do a short video on it. Years ago Jim Williams at LTC measured them with some contraption he had in his lab and confirmed the value but he never disclosed how he did it ( I can't call him today....for obvious reasons) or with what.   Thanks and wet soggy cheers from Oregon --Bob

Nothing fancy is needed.  Use the shunt resistance of your best 10 megohm input voltmeter as a voltage divider with the 13 gigohm resistor and a 10 volt source and you should get a reading of 7.69 millivolts.  Any good voltmeter will have an input bias current low enough to keep the error far below the 10% tolerance of the resistor.

That's one way. The other way is to use a known measured lower value parallel resistor, to bring the value down into the region of the multimeter.
I haven't done the math on these values to work out what tolerance you'd be able to measure to, but probably good enough for a 13G resistor.
Or with a 10V source and my 480 picoameter I could measure that to 0.5% or better easily.
Or I could use my Keithley 515A megohm bridge

[plesa]

Hi Dave, Given your precision current source and precision voltage standard and infinite impedance Agilent Multimeter ( I have the same model, BTW), I was wondering if you can make an accurate measurement of some 13 gig-ohm ( thats right 13 x10^9 ohms) resistors I have. See attached image. Perhaps this is an odd request, but you seem rather clever and I imagine you or someone here might  have some ideas other than applying 1000 volts to it and trying to measure 77 nano-amps. I can send you some if you promise to do a short video on it. Years ago Jim Williams at LTC measured them with some contraption he had in his lab and confirmed the value but he never disclosed how he did it ( I can't call him today....for obvious reasons) or with what.   Thanks and wet soggy cheers from Oregon --Bob

Nothing fancy is needed.  Use the shunt resistance of your best 10 megohm input voltmeter as a voltage divider with the 13 gigohm resistor and a 10 volt source and you should get a reading of 7.69 millivolts.  Any good voltmeter will have an input bias current low enough to keep the error far below the 10% tolerance of the resistor.

That's one way. The other way is to use a known measured lower value parallel resistor, to bring the value down into the region of the multimeter.
I haven't done the math on these values to work out what tolerance you'd be able to measure to, but probably good enough for a 13G resistor.
Or with a 10V source and my 480 picoameter I could measure that to 0.5% or better easily.
Or I could use my Keithley 515A megohm bridge

I thing that picoammeter or electrometer is only possible solution. I'm using Keithley 6485 it has integrated math function, which automatically recalculate input  current to specified units (Volts, Ohms...etc). Quite handy for high impedance measurement or voltage measurment with extra high input impedance (protection circuit  above 500V is reguired).
I do not think that such a resistor will survive more than 500V. And with high voltage the voltage coefficient will degrade the measurement precision.

[David Hess]

I thing that picoammeter or electrometer is only possible solution. I'm using Keuthley 6485 it has intergrated math function, which automatically recalculate input  current to specified units (Volts, Ohms...etc). Quite handy for high impedance measurement or voltage measurment with extra high input impedance (protection circuit  above 500V is reguired).
I do not think that such a resistor will survive more than 500V. And with high voltage the voltage coefficient will degrade the measurement precision.

Challenge accepted.

Here is my Tektronix DM502 which is old enough to drink (This particular meter was produced in 1975 making it 39 years old.) measuring a 999 megohm resistor with 10 volts of excitation.  You can see the top of the resistor, a Caddock MG815-3, directly attached to the negative terminal.  I am not exactly sure about the tolerance of the Caddock resistor other than it being 1% or better.  Unfortunately I do not have any larger value resistors handy at the moment but this will serve.

99.5 millivolts across the 10 megohm internal resistance of the voltmeter indicates 9.95 nanoamps of current.  9.99 volts - 99.5 millivolts = 9.89 volts across the unknown resistance.  9.89 volts / 9.95 nanoamps = 994 megohms which is well within the various tolerances including that of the 1% or better Caddock resistor.  I could make the same measurement with any of my other digital voltmeters that have a 10 megohm input resistance and get the same although more accurate results.

As you point out, a meter with integrated math would make this more convenient.

Inside every good digital voltmeter is an electrometer struggling to get out.

[plesa]

It will work for 1G resistors pretty well and results will be OK, but on >10G you must apply lot off averaging or increase the bias voltage. And it make the picoammeters more usefull in this measurement region.

[David Hess]

It will work for 1G resistors pretty well and results will be OK, but on >10G you must apply lot off averaging or increase the bias voltage. And it make the picoammeters more usefull in this measurement region.

The only change with the 10G (or 13G resistor) is the loss of one significant digit because of the smaller signal which still leaves about 1 part in 50 including noise for this 200mV 3.5 digit voltmeter.  The resistance seen by the voltmeter is practically identical and the noise will be the same.  1G in parallel with the internal 10M shunt resistor is 9.9M.  10G in parallel with the internal 10M shunt resistor is 9.99M.  The meter bias current is already lower than 9 picoamps (*) just to meet its specifications with 11.1M of input resistance (**) and a resolution of 100 microvolts.

Now you have got me wondering how to design a chopper stabilized input stage with a bias current lower than that.

(*) One nice thing about old test equipment is that in this case, full service documentation is available.  The input bias current for the integrated buffer is specified to typically be 4pA at 25C and 40pA at 70C.  Actual use shows that it is better than that.

(**) The high impedance buffer actually sees 11.1M maximum because there are additional 1M and 100K resistors in series with its input after the 10M decade divider for overload protection.

[kedwards22]

I haven't done the calculations on this but could you use capacitive discharge to measure 11gig. Get a high quality, low self discharge cap, charge it up to a known voltage, leave it for a few hours (?) completely open circuit, then re-measure it's voltage. You'd have to do the measurement quickly so the meter doesn't discharge it too much. This would give a value for it's self discharge internal resistance, then repeat with the 11 gig resistor attached, and then a simple calculation gets you the resistor value. To get an accurate measurement you might need peak detect on the meter and a fairly constant temp room. Haven't done the numbers so I might be way off with this.

[David Hess]

I haven't done the calculations on this but could you use capacitive discharge to measure 11gig. Get a high quality, low self discharge cap, charge it up to a known voltage, leave it for a few hours (?) completely open circuit, then re-measure it's voltage. You'd have to do the measurement quickly so the meter doesn't discharge it too much. This would give a value for it's self discharge internal resistance, then repeat with the 11 gig resistor attached, and then a simple calculation gets you the resistor value. To get an accurate measurement you might need peak detect on the meter and a fairly constant temp room. Haven't done the numbers so I might be way off with this.

It would be about 5400 picofarads per minute so this is certainly feasible.  At low voltages a JFET or MOSFET could be used to buffer the signal so it may be continuously monitored.

[ConKbot]

I know this trap! I was building a lipo battery dissipative balancer circuit with a micropower reference/comparator/amp IC, and since the battery voltage divider was a high impedance node I ended up using a FET input opamp as a unity gain buffer. 

I have its power switch toggle between +/- 4.5v  and 0-9V  (powered off a 9V battery)  I found some shrouded meter sockets, and some male-male shrouded DMM test cable leads, that way I can still keep using my normal DMM probes, and put it all in a housing. The offset voltage may be an issue on a 6 digit DMM, but its fine on my 87V