Question 1
Consider the circuit shown with an ideal OPAMP. The output voltage $V_o$ is __________V (rounded off to two decimal places).

 A 0.5 B 0.75 C -0.5 D -0.75
GATE EC 2022   Digital Circuits
Question 1 Explanation:

$V_A=\frac{V_R}{2} \; and R_{th}=R$
$V_B=\frac{V_B}{2}$

$V_C=\frac{(V_R+V_R/4)}{2}=\frac{5}{8}V_R$
$R_{th}=2R||2R=R$
Similarly,
$V_D=\frac{V_C}{2}=\frac{5}{16}V_R$
So, $\frac{S}{16}V_R=\frac{5}{16} \times 1.6V$
$V_o=\frac{-3R}{(2R+R)} \times \frac{5}{16} \times 1.6=-\frac{3}{2} \times \frac{5}{16} \times 1.6$
$V_o=-0.5V$
 Question 2
An 8-bit unipolar (all analog output values are positive) digital-to-analog converter (DAC) has a full-scale voltage range from $0\; V$ to $7.68\:V$. If the digital input code is $10010110$ (the leftmost bit is $\text{MSB}$), then the analog output voltage of the DAC (rounded off to one decimal place) is ___________ V.
 A 2.4 B 4.5 C 8.4 D 5.4
GATE EC 2021   Digital Circuits
Question 2 Explanation:
\begin{aligned} \text { Given: } \qquad \qquad V_{p s}&=7.68 \mathrm{~V} \\ n &=8 \mathrm{bit} \\ \text { Resolution }(k) &=\frac{V_{F S}}{2^{n}-1}=\frac{7.68}{2^{8}-1}=0.03\\ \text { Now, } \quad V_{\mathrm{DAC}}&=k \times\{\text { Decimal equivalent }\}\\ &=0.03 \times\{150\} \\ &=4.5 \mathrm{~V} \end{aligned}

 Question 3
A 10-bit D/A converter is calibrated over the full range from 0 to 10 V. If the input to the D/A converter is 13A (in hex), the output (rounded off to three decimal places) is _________ V.
 A 3.069 B 1.214 C 2.124 D 0.257
GATE EC 2020   Digital Circuits
Question 3 Explanation:
Given, n=10
$V_{FS}=10\, V$
Input Voltage=$(13A)_{16}=(314)_{10}$
Output Voltage=Resolution X Decimal Equivalent of input
$V_{o}=\frac{10}{2^{10}-1}\times 314=3.069\, V$
 Question 4
In an N bit flash ADC, the analog voltage is fed simultaneously to $2^{N}-1$ comparators. The output of the comparators is then encoded to a binary format using digital circuits. Assume that the analog voltage source $V_{in}$ (whose output is being converted to digital format) has a source resistance of 75 $\Omega$ as shown in the circuit diagram below and the input capacitance of each comparator is 8 pF. The input must settle to an accuracy of 1/2 LSB even for a full scale input change for proper conversion. Assume that the time taken by the thermometer to binary encoder is negligible.

If the flash ADC has 8 bit resolution, which one of the following alternatives is closest to the maximum sampling rate ?
 A 1 megasamples per second B 6 megasamples per second C 64 megasamples per second D 256 megasamples per second
GATE EC 2016-SET-2   Digital Circuits
 Question 5
Consider a four bit D to A converter. The analog value corresponding to digital signals of values 0000 and 0001 are 0 V and 0.0625 V respectively. The analog value (in Volts) corresponding to the digital signal 1111 is ________.
 A 0.7 B 0.93 C 1.13 D 1.07
GATE EC 2015-SET-1   Digital Circuits
Question 5 Explanation:
Step size $=0.0625 \mathrm{V}$
Decimal equivalent $=15$
Analog output $=15 \times 0.0625=0.9375 Volts$

There are 5 questions to complete.