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Old 15th Jan 2022, 10:46 am   #1
ChristianFletcher
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Default DVM Input Impedance

The several digital volt meter that I own all have 10 meg ohms of input resistance and I am aware this is the standard. But why was 10 meg chosen given the FET inputs on a DVM it could have been 1 20 or 100 so why 10 ?

Maybe the first meter was 10 and everyone just copied that specification? Who built the first DVM ? Not counting analog meters with a valve or FET front end.

Thanks Chris
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Old 15th Jan 2022, 11:41 am   #2
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Default Re: DVM Input Impedance

There are enough around that are not always 10 megOhm to catch you out.

Take especial care when using a high voltage multiplier probe!

One factor pushing the 10 MehOhm standard is keeping the necessary resistor values reasonable for the other voltage ranges.

David
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Old 15th Jan 2022, 11:58 am   #3
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Default Re: DVM Input Impedance

Ok not seen any. Although I do have a meter that has a low Z feature for when I am masquerading as an electrician
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Old 15th Jan 2022, 2:17 pm   #4
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Default Re: DVM Input Impedance

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Originally Posted by ChristianFletcher View Post
Who built the first DVM ? Not counting analog meters with a valve or FET front end.

Thanks Chris
Non-Linear Systems did in the early 1950's apparently; http://www.hp9825.com/html/dvms.html

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Old 15th Jan 2022, 3:22 pm   #5
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Default Re: DVM Input Impedance

Now I know - Thanks David
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Old 15th Jan 2022, 6:31 pm   #6
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Default Re: DVM Input Impedance

oftenly better DMM have this 10megs input impedance except the ranges 10V and lower, there they work as electrometer , Gigaohms are possible.
They are easy to identify: in the 10V range they becom a static load voltage at the input without any cable in the plug. switch to 100V = 0,000 nothing bec. there is the 10M

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Old 16th Jan 2022, 7:15 pm   #7
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Default Re: DVM Input Impedance

ElectronicsStackExchange has a discussion on this subject. Consensus of opinion seems to be 'high enough not to bias a circuit too much, low enough not to be compromised by factors like leakage inside the meter'

The ubiquitous ultra-cheap yellow fag packet meters (with brilliant LCD display contrast but not much else) are usually 1meg and 0.5meg on AC

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Old 17th Jan 2022, 3:39 pm   #8
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Default Re: DVM Input Impedance

That’s interesting that the cheap meters may not follow the standard arrangement
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Old 17th Jan 2022, 10:52 pm   #9
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Default Re: DVM Input Impedance

Note that a lot of bench DVMs have a 1M input on AC rather than 10M. Handheld meters can sometimes be 1M or 10M on AC or something in between.
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Old 18th Jan 2022, 12:03 am   #10
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Default Re: DVM Input Impedance

Quote:
oftenly better DMM have this 10megs input impedance except the ranges 10V and lower, there they work as electrometer , Gigaohms are possible.
I've got a couple of Keithley DVMs that offer >10G on the 10V DC range and below.

I've also got an old DataTech DVM that is similar on the 5V and 0.5V ranges. If I feed it 5V DC and then disconnect the leads the meter will keep displaying 5.000V for a few seconds. It typically loses about 1mV every few seconds. It doesn't even drop 1mV from 5.000V when measuring 5V via a 10M series resistor so the input impedance is very high on these lower ranges.
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Old 18th Jan 2022, 12:34 am   #11
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Default Re: DVM Input Impedance

The Fluke 8060A DVM was 10M if you selected V and a range. But if you left all the function switches disengaged, and selected either 200mV or 2V ranges, they had a 1G to 10G input impedance.

Fluke called this the "high impedance mode".

AFAIK that was the only Fluke handheld DVM to have that feature.

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Old 18th Jan 2022, 12:40 am   #12
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Default Re: DVM Input Impedance

Actually the Fluke Differential Voltmeters (the precursor to DVM's) were essentially infinite input impedance. They were a null detecting instrument with a stable zener voltage standard, and a 5 or 6 digit Kelvin-Varley divider. When adjusted to the null positon, the KV setting was the measured voltage. But at null, no current flows and the input impedance is infinity.

Well actually it is limited by leakage paths within the instrument and external connecting leads. But it is very high indeed.

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Old 19th Jan 2022, 2:08 am   #13
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Default Re: DVM Input Impedance

Perhaps one side effect is the accuracy of the rated 10Meg - my HP3457A specifies that to be 0.5%, but typically the accuracy is not mentioned (but was was more accurate for the higher digit DVM's I have).

That can be of use to know when using a DVM in voltage mode to measure a low leakage current level accurately. I recently used that technique to measure the accuracy of 100M and 200M Welwyn resistors to better than 1%.
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Old 20th Jan 2022, 9:11 pm   #14
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Default Re: DVM Input Impedance

hello Craig,

I love that Oldies. They are time less

883A
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Old 21st Jan 2022, 12:51 am   #15
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Default Re: DVM Input Impedance

For those of us working in a nice environment, a high impedance DVM is nice. True reading. But in rail Signal working, there's always the chance that that high impedance DVM might pick up stray voltage from the above 25kv line and contaminate the reading. So although rail signal engineers use Fluke quality stuff, the input is shunted by ( circa) 150K ohm to reduce this effect.
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Old 21st Jan 2022, 10:12 am   #16
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Default Re: DVM Input Impedance

Quote:
Originally Posted by 6AL5W-Martin View Post
hello Craig,

I love that Oldies. They are time less

883A
Good to see you like the Fluke differential voltmeters too! I have three of them with different characteristics, and one sits on my bench shelf.

The late great Jim Williams sacrificed one for the 5-digit Kelvin Varley that he needed for one of his rather famous Apps Notes.

I have a 7-digit KV, the Fluke 720A as part of a precision voltage calibration rig. That is quite a tour de force. The most significant digit resistors have to be accurate to one part in 10^7 and sit in a sealed oil bath to keep their temperature stable and tracking. There is a trimming process to make sure those resistors and lower digits are correct.

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Old 22nd Jan 2022, 1:44 pm   #17
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Default Re: DVM Input Impedance

Although it is something I was already aware of it makes a interresting reminder to see how different meters have different measurement error for different source impedances. I have plotted a accurate 10V source feeding a volt meter via a series resistor. (10V at 0.0005%)
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Last edited by ChristianFletcher; 22nd Jan 2022 at 1:51 pm.
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Old 23rd Jan 2022, 2:16 pm   #18
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Default Re: DVM Input Impedance

-I could have done with an assistant holding an Avo up while sorting an (isolated) ceiling rose yesterday, with a 10meg DVM i was cursing the 15v and 90v phantom voltages across certain terminals. The Fluke ones with dual impedance sound like a good idea. (if your assistant keeps wandering off!)

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