Draw the schematic circuit of an Analog to Digital converter using Voltage-to Frequency conversion and explain its principle of operation. Draw its relevant Waveforms.

Ans:

Analog to Digital Converter Using Voltage-to-Frequency Conversion: An analog voltage can be converted into digital form by producing pulses whose frequency is proportional to the analog voltage. These pulses are counted by a counter for a fixed duration and the reading of the counter will be proportional to the frequency of the pulses and hence to the analog voltage.

A voltage-to-frequency converter is shown in Fig. (a). The analog voltage Va is applied to an integrator whose output is applied at the inverting input terminal of a comparator. The non-inverting input terminal of the comparator is connected to a reference voltage -VR.

Initially, the switch S is open and the voltage vo decreases linearly with time (vo = Vat/τ ) which is shown in Fig. 10(b). When the decreasing vo reaches -VR at t =T, the comparator output  VC   goes  HIGH.  This  is  used  to  close  the  switch  S  through  a  monostable multivibrator. When the switch S is closed, the capacitor C discharges, thereby returning the integrator output vo  to 0. Since the pulse width of the waveform V_C is very small, therefore, a monostable multivibrator is used to keep the switch S closed for a sufficient time to discharge the capacitor completely. The rate at which the capacitor discharges depends upon the resistance of the switch.

Let the pulse width of the monostable multivibrator be T_d. Therefore, the switch S remains closed for Td after which it opens and vo starts decreasing again.

If the integration time T >> Td, the frequency of the waveforms V_o and VC is given by

 f = 1/( T + T_d)  ≅  1/T = (1/ τ) (V_a/V_R)

Therefore we get an output waveform whose frequency is proportional to the analog input voltage. Using the voltage-to-frequency (V/F) converter an A/D converter is demonstrated in Fig.(c). The output of the voltage-to-frequency converter is applied at the clock (CK) input of a counter throughout an AND gate. For a fixed time interval T₁, the AND gate is enabled. The reading of the counter at t = T₁ is specified by

n =  fT₁ =  (1/ τ) (V_a/V_R) T₁  that is proportional to V_a

Fig.(a) Logic diagram of Voltage-to-Frequency Converter

Fig.(b) Waveforms of Voltage-to-Frequency Converter

Fig.(c) Schematic circuit of A/D converter using a V/F converter