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DMM block diagram: V, I and R measurement

Short answer

A DMM uses a function/range switch to select an attenuator for DC voltage (high input resistance ~10 MΩ), a shunt resistor for DC current (voltage drop I×Rsh), or a constant current source for resistance (voltage proportional to Rx). The conditioned signal is digitized by an ADC and displayed.

Draw the block diagram of DMM (Digital Multimeter). And explain how it measures DC voltage, DC current and resistance.

Block diagram of DMM

 Probes -> Function / -> Signal     -> ADC -> Decoder -> Display
           range switch   conditioner         driver    (LCD)
            |  |  |
          Attenuator  Shunt  Constant current source
            (V)       (I)         (R)
                 Clock, reference, control logic

The function and range switch connects the input to the right circuit, then the conditioner (amplifier) feeds the ADC. The digital output is decoded and displayed.

Measuring DC voltage

  • The probes are connected across the component.
  • A high-resistance attenuator (voltage divider, input resistance about 10 MΩ) scales the voltage down to the ADC input range (e.g. 0 to 200 mV).
  • The ADC converts it to a number, which is displayed after multiplying by the attenuation factor (range). The high input resistance stops circuit loading.

Measuring DC current

  • The meter is connected in series; the current flows through a precision low-value shunt resistor Rsh.
  • The voltage drop V = I × Rsh is amplified, converted by the ADC and displayed as current.

Measuring resistance

  • The unknown resistor Rx is connected to the probes (circuit power off).
  • The DMM passes a known constant current Iref through Rx from an internal source.
  • The voltage across Rx is V = Iref × Rx, which is proportional to Rx.
  • The ADC converts it and the display shows R = V/Iref in Ω, kΩ or MΩ.

✓ Answer checked by Claude Opus 5.5 (high effort)

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