Ramp type Digital Voltmeter

Ramp type Digital Voltmeter Points : Ramp type Digital Voltmeter The operating principle of a ramp type digital voltmeter is to measure the time that a linear ramp voltage takes to change from level of input voltage to zero voltage (or vice versa). This time interval is measured with an electronic time interval counter and the count is displayed as a number of digits on electronic indicating tubes of the output readout of the voltmeter.

At the start of measurement a ramp voltage is initiated. A negative going ramp is shown in the said fig. But a positive going ramp may also be used. The ramp voltage value is continuously compared with the voltage being measured (unknown voltage). At the instant the value of ramp voltage is equal to that of unknown voltage of a coincidence circuit, called an input comparator, generates a pulse which opens a gate. The ramp voltage continues to decrease till it reaches ground level (zero volt). At this instant another comparator called ground comparator generates a pulse and closes a gate.

The time elapsed between opening and closing of the gate is “t” as indicated in the fig. During this time internal pulses from a clock pulse generator pass through the gale and are counted and displayed. The decimal number as indicated by the readout is a measure of the value of Input Voltage.

The sample rate multivibrator determines the rate at which the measurement cycles are initiated. The sample rate circuit provides an initiating pulse for the ramp generator to start its next ramp voltage. At the same time it sends a pulse to counters which sets all of them to 0. This momentarily removes the digital display of the readout.