Which DC volt meter / multimeter with high input impedance (>100M Ohm)? - Page 1
Forum section: electronics / testgear

author: nctnico
author_rank: Super Contributor
timestamp: December 11, 2014, 07:29:53 pm
content: Just wondering if someone knows a DC volt meter with an input impedance greater than 100M Ohm? I need to measure some sensitive circuits and all the multimeters I have are letting me down with their 10M input impedance.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 11, 2014, 07:49:58 pm
content: It appears the Keysight 34661A can do >10G/Ohms at low voltages.http://literature.cdn.keysight.com/litweb/pdf/5991-1983EN.pdf

author: robrenz
author_rank: Super Contributor
timestamp: December 11, 2014, 07:52:12 pm
content: Same for the Fluke 8846AI think many/most 6.5 digit meters have a high impedance setting.

author: MartyD
author_rank: Regular Contributor
timestamp: December 11, 2014, 07:57:44 pm
content: My Tektronix DMM4050 and also my much cheaper Siglent SDM3055 do have 10Gig-Ohms in the lower voltage ranges.

author: tautech
author_rank: Super Contributor
timestamp: December 11, 2014, 07:59:31 pm
content: HH or bench?The Siglent SDM3055 has a 10 M or 10 G Ohm input impedance settings.Thanks Marty, just to confirm your post.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 11, 2014, 08:02:18 pm
content: Does the 10G mode slow the response time?

author: MartyD
author_rank: Regular Contributor
timestamp: December 11, 2014, 08:03:59 pm
content: @tautech: Yes it does, I was positively surprised as well!

author: nctnico
author_rank: Super Contributor
timestamp: December 11, 2014, 08:05:58 pm
content: I didn't check the suggestions yet but I see 'low voltage' a couple of times. I need to be able to measure up to 100V DC. I don't need very high accuracy though. Even 1% would suit my needs perfectly. Hand held or bench doesn't matter much.

author: tautech
author_rank: Super Contributor
timestamp: December 11, 2014, 08:13:25 pm
content: Quote from: rx8pilot on December 11, 2014, 08:02:18 pmMaybe marginally, just depends when you touch the leads as the screen updates, this applies to both 10M & 10G settingsHowever 10 G is only availabe in 2V and lower ranges.Only tested a empty dry cell battery.

author: MartyD
author_rank: Regular Contributor
timestamp: December 11, 2014, 08:13:42 pm
content: @nctnico: Just checked, my Tek 4050 does 10gig up to 10 volts, the siglent up to 2 volts. So neither would be an option for you I guess...Maybe you have better luck with an older, FET-driven voltmeter from ebay or such.

author: rx8pilot
author_rank: Super Contributor
timestamp: December 11, 2014, 08:14:05 pm
content: At that voltage, you may be limited to 10M.What kind of signal can't deal with 10M at 100v? If you do not need accuracy better than 1%, 10M should be great.

author: nctnico
author_rank: Super Contributor
timestamp: December 11, 2014, 08:20:15 pm
content: I want to measure the open voltage on a load detection circuit. This circuit can trip when it sees less than 60M Ohm. Maybe I just need to put a 90M ohm resistor in series with my DMM and multiply the readings by 10.

author: edpalmer42
author_rank: Super Contributor
timestamp: December 11, 2014, 10:21:55 pm
content: Quote from: nctnico on December 11, 2014, 08:20:15 pmYes, you can use either the series resistor or, if you don't happen to have 90M worth of resistors handy, you could use a potentiometric circuit as described here:https://www.eevblog.com/forum/blog/eevblog-584-what-effect-does-your-multimeter-input-impedance-have/msg396860/#msg396860Ed

author: wiss
author_rank: Frequent Contributor
timestamp: December 11, 2014, 10:26:57 pm
content: Keithley 617: > 200 TOhm to 200 VKeithley 610: > 10^14 Ohm to 100 VNot cheap...

author: rx8pilot
author_rank: Super Contributor
timestamp: December 12, 2014, 02:45:07 am
content: That is INSANE.

author: rsjsouza
author_rank: Super Contributor
timestamp: December 12, 2014, 03:05:58 am
content: Perhaps a high voltage probe such as theFluke 80K-6?

author: skipjackrc4
author_rank: Regular Contributor
timestamp: December 12, 2014, 03:59:40 am
content: I'm not aware of any straight DMM's that will have such a high impedance at 100V. You're in electrometer territory. Luckily, the Keithley 616 can be found on eBay for quite cheap.

author: engiadina
author_rank: Contributor
timestamp: December 14, 2014, 10:06:00 pm
content: The Agilent 3458A has >10G in all Ranges.Well, not cheap either.

author: nctnico
author_rank: Super Contributor
timestamp: December 14, 2014, 10:57:20 pm
content: I decided to go with a 90M Ohm resistor (22M + 68M) in series with my DMM and go from there.

author: Marco
author_rank: Super Contributor
timestamp: December 15, 2014, 12:21:54 am
content: Quote from: engiadina on December 14, 2014, 10:06:00 pmDatasheet mentions 10Mohm for 100V and up.You could bodge a high impedance low offset buffer with something like a MCP6V31 or other CMOS auto-zero opamp in unity gain with bootstrapped supplies to extend the voltage range ... you'd need an isolated supplies which exceeds the voltage range you want to measure though.

author: MadTux
author_rank: Frequent Contributor
timestamp: December 15, 2014, 12:47:03 am
content: Anything that has written 'Electrometer' on it. Early ones use simple tubes in the preamp. Newer ones use selected 3N163 p-channel MOSFETs with an especially well isolated gate. Really good ones such as the Keithley 640 use a vibrating capacitor plate to modulate AC onto the other plate if the vibrating input plate gets charged up.

author: nctnico
author_rank: Super Contributor
timestamp: December 15, 2014, 01:40:25 am
content: Quote from: skipjackrc4 on December 12, 2014, 03:59:40 amThey are not expensive but it seems these devices need some kind of 'probe' with a triax connector.

author: David Hess
author_rank: Super Contributor
timestamp: December 20, 2014, 08:00:09 am
content: As others point out, 100 volts with a high input resistance is going to be found in electrometers and not standard multimeters.A high value resistor in series with the 10 Mohm voltmeter input will be the least expensive way to do this.