EEVblog Electronics Community Forum
Products => Test Equipment => Topic started by: nctnico on December 11, 2014, 07:29:53 pm
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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.
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It appears the Keysight 34661A can do >10G/Ohms at low voltages.
http://literature.cdn.keysight.com/litweb/pdf/5991-1983EN.pdf (http://literature.cdn.keysight.com/litweb/pdf/5991-1983EN.pdf)
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Same for the Fluke 8846A
I think many/most 6.5 digit meters have a high impedance setting.
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My Tektronix DMM4050 and also my much cheaper Siglent SDM3055 do have 10Gig-Ohms in the lower voltage ranges.
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HH or bench?
The Siglent SDM3055 has a 10 M or 10 G Ohm input impedance settings. :o
Thanks Marty, just to confirm your post.
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Does the 10G mode slow the response time?
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@tautech: Yes it does, I was positively surprised as well! :)
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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.
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Does the 10G mode slow the response time?
Maybe marginally, just depends when you touch the leads as the screen updates, this applies to both 10M & 10G settings
However 10 G is only availabe in 2V and lower ranges.
Only tested a empty dry cell battery.
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@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.
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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.
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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.
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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.
Yes, 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/#msg396860 (https://www.eevblog.com/forum/blog/eevblog-584-what-effect-does-your-multimeter-input-impedance-have/msg396860/#msg396860)
Ed
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Keithley 617: > 200 TOhm to 200 V
Keithley 610: > 10^14 Ohm to 100 V
Not cheap...
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That is INSANE.
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Perhaps a high voltage probe such as the Fluke 80K-6 (http://en-us.fluke.com/products/all-accessories/fluke-80k-6.html)?
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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.
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The Agilent 3458A has >10G in all Ranges.
Well, not cheap either.
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I decided to go with a 90M Ohm resistor (22M + 68M) in series with my DMM and go from there.
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The Agilent 3458A has >10G in all Ranges.
Datasheet 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.
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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.
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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.
They are not expensive but it seems these devices need some kind of 'probe' with a triax connector.
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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.