How do digital multimters work?

[Shaka]

Hi,

I was wondering how a digital multimeter works. Specifically, how does it measure voltage? What principle of physics does it use?

 

[etech]

It's been awhile, but I believe it is a voltage to frequency converter.
Take a 0 to 2 volt range. At 2 volts assume a count of 2000 per period of time. The meter will take the frequency from the VF converter and apply it to a counter. At the end of the time period that count will clocked to a register and displayed on the display. The counter will be reset to 0 and the cycle will start over again.
The voltage will be scaled to 0 to 2 volts before being applied to the voltage to frequency converter. This can be done as simply as using a voltage divider network of resistors and a switch. So if you go to a 0 to 20 volt range the voltage is divided by 10 before being switched to the VF. The decimal point is scaled accordingly on the display.
Current is measured as voltage across a precision resitor.

 

[Shaka]

How does the voltage to frequency converter work? I've looked up diagrams on the internet and they all have opamps in them. My basic circuit analysis class didn't really explain what the opamp is used for, just how to analyze circuits containing opamps. Any further information would be appreciated.

 

[etech]

Op-amps are amplifiers. I'd have to see the circuit to know what they are using if for in that application. I would imagine that it is a buffer to limit loading of the signal applied.

check the application note at the bottom, I didn't read this one but Nationals an's are usually pretty good.
Freq. to Volt. converter

 

[Shaka]

Here is a link to the pdf document with a VF converter diagram:

VF converter diagram

The National Semiconductor document was very helpful. But I would really like to know how the VF converter works.