Hi all, I am trying to find out the method of how a multimeter measures voltage. Am I right in thinking that it measures the voltage at point A and point B then calculates the drop with A - B?

Also if the measurement is negative, is the voltage scaled to a positive value say 0 to 5V for an adc to calculate this?

 

Am I right in thinking that it measures the voltage at point A and point B then calculates the drop with A - B?

Yes and no. It's correct to say the meter can only report a ∆V, the difference between the voltages on its probes. It's incorrect to say "it measures the voltage at point A and point B". It cannot separately determine the voltage at both points, only how much B is different than A. Voltage has no meaning without a reference. You could declare yourself as having a voltage of 10 million volts, and it would be true, if useless. Measurement might show you have only a small voltage compared to Earth ground, which is a common reference point for zero voltage.

 

So how does a voltmeter determine if the difference is pos or neg? We determine the difference by subtracting e.g. A = 2V and B = 5V to get -3V

 

So how does a voltmeter determine if the difference is pos or neg? We determine the difference by subtracting e.g. A = 2V and B = 5V to get -3V

The A/D converer in the meter can measure either positive or negative voltages.
It doesn't have to do any math.

Again you are using voltage values without any reference.
Where are voltages A and B referenced to?
Some arbitrary ground point?
But the meter doesn't care about ground. It's isolated from ground.
All it measures is the voltage (difference) between A and B.
Until you understand that you will keep confusing yourself.

 

Hi all, I am trying to find out the method of how a multimeter measures voltage. Am I right in thinking that it measures the voltage at point A and point B then calculates the drop with A - B?

Also if the measurement is negative, is the voltage scaled to a positive value say 0 to 5V for an adc to calculate this?

Most voltmeters cannot measure voltage. They measure current and convert that into a voltage.
A typical digital voltmeter (DVM) has an input impedance of 10MΩ.
Let us assume that the meter leads are placed across points A and B and the potential difference from A to B is 10V. The current through the voltmeter is 1μA. The instrument can measure that and display the result as units of volt. At the same time the meter can determine the direction of the current and display either a + or - sign.

 

I am trying to find out the method of how a multimeter measures voltage.

It uses an analog to digital converter (ADC).
https://en.wikipedia.org/wiki/Integrating_ADC

Am I right in thinking that it measures the voltage at point A and point B then calculates the drop with A - B?

No.

 

Most voltmeters cannot measure voltage.

What?
All digital voltmeters measure voltage.
The 10meg input resistance is from the input attenuators.
The ADC measures the voltage across the selected divider resistor, depending upon the range selected.

They measure current by measuring the voltage across a shunt resistor.

 

Most voltmeters cannot measure voltage. They measure current and convert that into a voltage.

I see where you are going, but I disagree. It would be better to say that a voltmeter conducts a current through a calibrated resistance, and measures the voltage across it. How it does that varies with the meter.

Of course anywhere in anything having to do with DC circuits there has to be a current for there to be a voltage. But in the context of this question that seems a bit pedantic. The core elements of a dual-slope converter are a voltage controlled current source and a voltage comparator.

ak

 

I feel like I need buttons for,
"Not how you talk to noobies" and,
"Off Topic"
This seems to always happen when you throw a noobie question into a shark tank full of seriously qualified engineers.

 

I feel like I need buttons for,
"Not how you talk to noobies" and,
"Off Topic"
This seems to always happen when you throw a noobie question into a shark tank full of seriously qualified engineers.

You can't win, if you try to answer a voltage measurement question with a person who is a bit pedantic you get nuked.

 

You can't win, if you try to answer a voltage measurement question with a person who is a bit pedantic you get nuked.

And we seem to have at least 6 of them here.

C'mon guys, let the T.S. define the intellectual level at which he can operate.

 

I am not going to attempt to defend my statements or try to be pedantic. CC and AK both make valid points. I am just trying to get the TS to understand that you cannot measure voltage at point A, nor can you measure the voltage at point B by themselves.

When you connect an analog voltmeter across points A and B, current flows through the meter and that is what is measured.

When you connect a digital voltmeter across points A and B, current flows through the meter and that is what is measured. Yes, Ohm's Law applies and the voltage across the resistance is directly proportional to the current.

I assume that in the voltmeter circuitry there is an amplifier that may look something like this:

As far as I can recall, you can solve for the gain of the amplifier by analyzing the flow of currents i1, i2, i3, and i4 in the circuit.

An electrometer responds to voltage, not current.

 

I was thinking CC handled this in post #5, i.e. The meter doesn't know anything about true ground or relative ground. It only knows what is connected to its two probes.

All voltage is a voltage compared to someplace else, that's why a volt meter always has two probes. It is up to the person measuring to be cognizant of the relative ground potential because the meter has no clue where "ground" is. The meter always thinks its black (negative) lead is, "ground".

 

still none the wiser. Can someone explain how the ADC processes a pos and neg input and displays it.

 

still none the wiser. Can someone explain how the ADC processes a pos and neg input and displays it.

Are you asking specifically about voltage measurement or the sign of the measurement?

 

both.

 

still none the wiser. Can someone explain how the ADC processes a pos and neg input and displays it.

A good way of understanding that would be to look at the data sheet for a typical high-resolution ADC, such as an LTC2440. Perhaps over-simplifying, but you can think of it is being that the ADC has an input range spanning both negative and positive voltages, with zero volts being exactly at the midpoint; and the binary codes generated by the ADC are positive and negative 2's-compliment numbers, to match.

 

still none the wiser. Can someone explain how the ADC processes a pos and neg input and displays it.

Did you read the wiki article referenced in post #6?

 

I feel like I need buttons for,
"Not how you talk to noobies" and,
"Off Topic"
This seems to always happen when you throw a noobie question into a shark tank full of seriously qualified engineers.

So true
I'm often guilty of going a bit sideways on a topic!

 

both.

Ok. Here goes.

An ADC takes a current or voltage and determines which bin in fits into, i.e. each bin has a low threshold and a high threshold.
Let us for the moment say there are 100 bins, i.e. the result can be a value from 0 to 99.
Now we define the minimum bin threshold and the maximum threshold, i.e. we select our current/voltage references.

Let us set the lowest bin threshold to -1V and the highest bin threshold to +1V so that if the input voltage is 0V it falls into bin 50.
Any input voltage between -1V and <0V will fall into one of bins 0 to 49.
Any input voltage above 0V to 1V will fall into one of bins 50 to 99.

Hence the system now can determine the sign of the input voltage.
The system applies a conversion factor to convert from the number 0 to 49 to a display value in volts including a -ve sign.
The system applies a conversion factor to convert from the number 50 to 99 to a display value in volts and does not display the -ve sign.