Measuring 800Meg Ohms

[BradC]

So I was building a Silicon Chip EHT Stick (April 2010). It's just a 1000:1 passive divider. 80 10M resistors in series for the high side and an 820k + 30k + 50k Pot on the low side. The theory is you set it up against the 10M input impedance of your meter and adjust it for about the right output.

So I built the board up and thought "Well, my 3457a has a 3G range, let's measure it". Now to be fair, the 3G range is one of the "extended ohm" ranges and the stated accuracy is +/- 16%. Hardly worth it.
So with the board on the desk I put my trusty croc clips on and got nothing. The meter just read overload. Hrm, let's try the 4 wire mode. Nup. Nada.

So I thought since I have a PG506 with a vaguely accurate 100V output I'll set the divider up on that and use that to adjust things. That worked, For a 100.05V input I could get close enough to 0.10005V out. I measured the bottom end resistor chain and got about 868k . Put that into the spreadsheet in parallel with the 10M input and got ~798k. Calculated the top half out at roughly 798M which is pretty good for a string of 80 resistors.

While I had dinner I wondered about the interference induced in a set of 700mm test leads at the poofteenths of an amp you'd get through an 800M resistor so I hooked it up with my 100mm leads and hung it off the front of the meter. Bang, got a fairly steady reading at 100NPLC. Reading was ~7974**k. Within sneezing distance of my calculated 798M. Nice!

For future reference, when measuring more than about 20M, use the *short* leads!

[ovnr]

tl;dr: Noise makes sensitive measurements hard to get right.

[David Hess]

Sometimes older instruments have advantages.  Exposed binding posts allow the high impedance end of the high voltage resistor to be directly attached to the meter to minimize coupling of interference.  The other end of the resistor can be attached to a lead since it goes to a low impedance reference voltage.

Doing the above, my ludicrous by today's standards Tektronix DM501 and DM502 multimeters measure a Caddock MG815 999 megohm high voltage resistor that I have for testing within the tolerance of the meters using the voltage ranges as a current meter with a 10 megohm internal shunt.

[intabits]

"my 3457a ...",
"...0.10005V out.",
"in parallel with the 10M input"

Your calculations may be off somewhat, since on the 30mV to 3v DC ranges, which you would be using above, the 3457A has an input impedance of 10G (like many others, eg: 3456A).

PS: I always thought that the correct term was "poomteenth".  (but maybe that's just being homophobic...)

[BradC]

Your calculations may be off somewhat, since on the 30mV to 3v DC ranges, which you would be using above, the 3457A has an input impedance of 10G (like many others, eg: 3456A).

Not when you enable FIXEDZ and turn on the 10M resistor.

[Kleinstein]

With those very high resistors, the typical DMM type measurement with applying a constant current and measure the voltage does not work very well anymore. The better way is to apply a more or less constant voltage and than measure the current. Ideally the current is measured with a circuit like a transimpedance amplifier to keep the voltage drop low. 

Some electrometers can be used this way.
With a normal DMM one might need a reasonable stable voltage source (e.g. 30 V range) and than measure voltage and current (one at a time).

[joeqsmith]

I will test the conductance on meters that support it using up to 1Tohm.  Depending on the meter,  they can get you in the ballpark.

For real high resistances, I will use an electrometer.  Cleaning the parts becomes important.  In some cases I will use a shielded box to hold the parts.   In the picture shown, I was measuring a few capacitor's leakage current and had to add some Teflon to increase the test setup's resistance.   

[intabits]

Not when you enable FIXEDZ and turn on the 10M resistor.

OK, I didn't know there was such a feature (as far as I know, my 3456A doesn't have it)

Did you select that solely because the SC project assumes it?
Or is there some other advantage to using it for this type of measurement?
(rather than changing the resistor values in the project to suit a 10G load)

I've just made a 100:1/10:1 divider box with selectable outputs for 10M and 1M loads.
And I'm about to make a new version with an "no load" output option for the 3456A, so I'm wondering if there's a reason not to.   

[BradC]

Not when you enable FIXEDZ and turn on the 10M resistor.

OK, I didn't know there was such a feature (as far as I know, my 3456A doesn't have it)

Did you select that solely because the SC project assumes it?
Or is there some other advantage to using it for this type of measurement?
(rather than changing the resistor values in the project to suit a 10G load)

Hrm. Well, while the 3457a has a 10G load on the 3V range, none of my other multi-meters do and when I'm troubleshooting a CRT I don't want to be having to use the 3457a to measure the EHT. I'd much rather use a handheld and not be tied to the bench. So it really does need to be set up for a 10M load to be portable. I just wanted to use the 3457a to adjust the divider to get it as accurate as I have the ability to, because I could.

I think if you are making a precision divider, then setting it up for use on the meter you are likely to use it with is the way to go. So if you plan on using your divider with the 3456A then setting it up for a no-load input is probably the best option. I have a plug-in shunt I made for the 3457a to bring the 10M input impedance down to a relatively accurate 1M for use when I adjust things that need a 1M impedance (like my Tek PG506). That comes in handy. If it is something you might need, you could always build a 10M plug in shunt to use on the 3456A when you need a constant input impedance. I find the FIXEDZ mode on the 3457a really handy when it's needed.

[intabits]

That all makes good sense, thanks.
So I'll continue with my "bench" 3Kv 100:1/10:1 divider box, having selectable 1M/10M/10G outputs.
But for a 1000:1 unit, I'll do as you've done, like the SC project, and make adapters for 10G meters and 1M scopes.