Short note: Digital multimeter (DMM)

A Digital Multimeter (DMM) is an electronic instrument that measures voltage (AC/DC), current (AC/DC) and resistance, and shows the value directly as numbers on a digital display (LCD or LED). Many DMMs also measure capacitance, frequency and temperature, and can test diodes and continuity.

Block diagram

 Input -> Range   -> Signal      -> ADC -> Decoder/ -> Display
          selector   conditioner          driver
          (attenuator/ (amp, AC-DC          ^
           shunt)       converter, I-V)    Control logic + clock

Working

• Voltage: an attenuator scales the input, and the ADC converts it.
• Current: the current flows through a precision shunt resistor, and the voltage drop across it is measured.
• Resistance: a constant current is passed through the unknown resistor, and the voltage across it is measured.
• The ADC output goes to a decoder, which drives the display.

Advantages: high accuracy (0.1 % or better), high input impedance (about 10 MΩ) so little loading error, easy reading with no parallax error, auto-ranging and auto-polarity, and small size. Disadvantages: it needs a battery, and it is poor at showing quickly changing readings.