Multimeter fluctuates when measuring voltage of mV region(DC)

ronsimpson said:

Noise in the A to D converter will cause fluctuations. Some meters will average several readings before displaying a value. This helps hide the noise. In the "mV scale" there may be a amplifier added ahead of the A to D converter. This amp will add some noise. In some cases to get the ADC to work down to "mVs" the reference voltage on the A to D converter was reduced to 1/10 (only in the mV scale mode). This also adds noise.

Another way of thinking about it: If you have a record player, and the recording has a quite place so you turn up the volume, the noise also increases. The noise of the amplifier + record noise will be easy to hear when you turn up the volume. Just like the meter when you turned up the volume (mV scale).

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cowboybob said:

Is the meter analog or digital?

Digital's often will have the last digit oscillate between one or two numbers (say .004 and .005 when the actual value is 0.0045) due to rounding of the value being fed to the A to D converter and any noise that may be finding its way onto the leads of the meter or an inherent source variable (a little AC noise).

Analog's, generally, don't have the sensitivity to display low level noise of this sort. The meter movement simply cannot physically move that rapidly.

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williame.newtist said:

...'cause it fluctuates with three digits.

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cowboybob said:

How so? And what is your "testing circuit" (signal source)?

Does a display of, say, 234, have ALL three digits fluctuating? Or, if there is a decimal point, just the digits to the right of the decimal? Or what?

With the "testing circuit" removed from the meter, what does the meter display? What does it display with the meter leads shorted together?

Please explain in more detail.

Just curious now.

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WTP Pepper said:

The chances are you have associated noise with your signal which is why an oscilloscope can be of help as you can see the noise.

Assuming you have a 10mV signal with 1mV of noise. Your DVM will easily pick up the noise and try to average it, but with the random nature of noise it will not give a steady reading.

On a larger signal of say 1V, the DVM will be on a higher range so the 1mV noise will be insignificant and possibly be below the least significant bit of the of the analog to digital convertor in the DVM so won't show up. Even if it can be measured, it maybe below the resolution of the display. i.e its measuring it but cannot display it.

Measuring low level signals will always be a problem without a proper measuring environment as mains hum (50/60Hz depending where you are) can get everywhere. Especially onto multimeter leads. Strip ceiling lights, energy saving lamps and CRT TV's are terrible radiators of mush.

You could try twisting the measuring leads together to increase the common mode rejection of interference. i.e. a meter measures the difference between its inputs. By twisting together, you reduce mains born (and other external noise from motors) as this noise is induced in both leads at the same time and the meter can reject this to a certain level.

The technical term for this is Signal to Noise Ratio. The higher the better.

As previously mentioned, an analog meter won't physically respond to such changes unless the are large spikes. The kind caused by motors (fridges) coming on and off.

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