Input Impedances for VTVMs, DMMs, etc

[Dave Edwards]

I work for Hewlett-Packard, and have my choice of any of the latest
lab quality DVM's.
On my bench, next to the 410C is a much used Heath IM18 that I built
when I was in High School.
It is very hard to make tweaks with a DVM, and even the analog bargraph
of the portable DVMs does not have the resolution to do the trick.
Regards...Dave










 11:21 AM 4/26/98 -0700, you wrote:
>Hey all,
>
>At the risk of getting off topic there seem to be so many questions about
>VTVMs, DMMs, VOMs, etc and what to use and what the specs mean that maybe
>some basic definitions are in order (If I'm wrong in doing this feel free
>to flame me):
>
>VTVM: DC volts input impedance - 10 megohm meter, 1 megohm probe for a
>total of 11 megohm. Typical DC accuracy - 3%. AC volts input impedance -
>1 megohm shunted by about 200pf at the probe tip. Typical AC accuracy -
>5% over a specific frequency range (25Hz to 1Mhz). Typical of the breed,
>Heath IM-28 (Heath tie-in!). A VTVM is a very good all around measurement
>tool for most troubleshooting, unless you are very heavily into digital
>circuits. In those cases it may not be accurate enough.
>
>VOM (Volt/ohm meter): Rather than using an 'input impedance' spec VOMs
>express it as a 'sensitivity' of 'ohm/volt'. Typical are 20K ohm/volt DC
>and 5K ohm/volt AC. Examples are Heath MM-1 and Simpson 260. In general,
>these specs translate into a much lower input impedance and less
>accuracy. IMHO, VOMs are not suitable for serious troubleshooting. They
>can load down a circuit which results in inaccurate readings. But need to
>know if 120VAC is present on that outlet? Good choice here. And another
>caveat: many VOMs use ohms batteries much greater than 1.5 volts. They
>can damage semiconductor junctions.
>
>Good Quality DMM (DVM): emphasis on good quality. Input impedance for
>both DC volts and AC volts - 10 megohm or greater. DC basic accuracy -
>.3% or better. AC accuracy - 1.0% or better, within a specific frequency
>range. Typical of the breed, Fluke 70 series, Fluke 80 series, TEK DMM900
>series. These DMMs even have an analog bargraph to spot trends. Best
>choice for your troubleshooting needs, even on vintage equipment.
>
>So....if you want/need an accurate voltmeter, what to do? If you have a
>VTVM, use it! It will serve you well, unless you're into digital. Want a
>DMM? When shopping around ask two basic questions. Basic DC accuracy -
>better be .5% or better. Input impedance - better be 10 megohm or
>greater. I'll bethca many of the $40 DMMs don't qualify. After all, when
>you are staring at a piece of non-working equipment what is usually the
>first test equipment you grab? Your voltmeter. Invest in a good one.
>
>Shameless plug - Yesterday I received a Tech America Flyer. On the front
>page is a Fluke 77 Series 11 DMM on sale for $99.95. That is an absolute
>bargain! Fluke is closing out the Series 11 and coming out with the
>Series 111. I have no connection what so ever with Tech America. Just
>passing on that info.
>
>Mike Donnelly
>MJATS at JUNO.COM
>
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>

Dave Edwards  WB2TBB
____________________________________________________________________
"It's a dog-eat-dog world . . . and I'm wearing Milkbone underwear."
--Norm Peterson
____________________________________________________________________

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