What is the working principle of digital voltmeter?

# voltmeters usually employ an electronic circuit that acts as an http://www.answers.com/topic/integrator,

# linearly ramping output voltage when input voltage is constant (this can be easily realized with an http://www.answers.com/topic/operational-amplifier).

# The dual-slope integrator method applies a known reference voltage to the integrator for a fixed time to ramp the integrator's output voltage up, then the unknown voltage is applied to ramp it back down, and the time to ramp output voltage down to zero is recorded (realized in an http://www.answers.com/topic/analog-to-digital-converter implementation).

# The unknown voltage being measured is the product of the voltage reference and the ramp-up time divided by the ramp-down time.

# The voltage reference must remain constant during the ramp-up time, which may be difficult due to supply voltage and temperature variations. .

Digital voltmeters necessarily have input amplifiers, and, like vacuum tube voltmeters, generally have a constant input resistance of 10 megohms regardless of set measurement range