# Linear Integrated Circuit Questions and Answers – Basic DAC Techniques – 1

This set of Linear Integrated Circuit Multiple Choice Questions & Answers (MCQs) focuses on “Basic DAC Techniques – 1”.

1. Express the output voltage of digital to analog converter?
a) Vo =KVFS(d12-1+d22-2+….dn2-n)
b) Vo =VFS/k(d12-1+d22-2+….dn2-n)
c) Vo =VFS(d12-1+d22-2+….dn2-n)
d) Vo =K(d12-1+d22-2+….dn2-n)

Explanation: The input is an n-bit binary word D and is combined with the reference voltage VR to give on analog output signal. Mathematically it is described as
Vo =KVFS(d12-1+d22-2+….dn2-n) where, K -scaling factor, VFS-full scale output voltage.

2. Why the switches used in weighted resistor DAC are of single pole double throw (SPDT) type?
a) To connect the resistance to reference voltage
b) To connect the resistance to ground
c) To connect the resistance to either reference voltage or ground
d) To connect the resistance to output

Explanation: SPDT are electronic switches controlled by a binary word. If the binary input to a switch is 1, it connects the resistance to the reference voltage and if the input is 0, the switch connects the resistor to ground.

3. Determine the output current for an n-bit weighted resistor DAC?

a) (VR/R )× (do/2 +d1/22 + ……dn/2n)
b) (VR/R )× (d1/21 +d2/22 + ……dn/2n)
c) (VR/R )× (d02/2 +d12/22 + ……dn2/2n)
d) None of the mentioned

Explanation: The output current, Io= I1+I2+….In
Io= (VR/2R )×(d1) +(VR/22R)× (d2) ….+(VR/2nR )×(dn)
Io =(VR/R)× (d1/21 +d2/22 + ……dn/2n).

4. In a D-A converter with binary weighted resistor, a desired step size can be obtained by
a) Selecting proper value of VFS
b) Selecting proper value of R
c) Selecting proper value of RF
d) All of the mentioned

Explanation: The size of the steps depends on the value of RF, provided that the maximum output voltage does not exceed the saturation level of an op-amp.
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5. Determine the Full scale output in a 8-bit DAC for 0-15v range?
a) Full scale output=15.1v
b) Full scale output=15.2v
c) Full scale output=14.5v
d) Full scale output=14.94v

Explanation: Full scale output = (Full scale voltage -LSB)
= [15v-(15v/28)] = (15v-0.0586) = 14.94v.

6. Pick out the incorrect statement “In a 3 bit weighted resistor DAC”
a) Although the op-amp is connected in inverting mode, it can also be connected in non-inverting mode
b) The op-amp simply work as a current to voltage converter
c) The polarity of the reference voltage is chosen in accordance with the input voltage
d) None of the mentioned

Explanation: The polarity of the reference voltage is accordance with the type of the switch used. For example, in TTL switches, the reference voltage should be +5v and the output will be negative.

7. What is the disadvantage of binary weighted type DAC?
a) Require wide range of resistors
b) High operating frequency
c) High power consumption
d) Slow switching

Explanation: For better resolution of output, the input binary word length has to be increased. As the number of bit increases, the range of resistance value increases.

8. The smallest resistor in a 12 bit weighted resistor DAC is 2.5kΩ, what will be the largest resistor value?
a) 40.96MΩ
b) 10.24MΩ
c) 61.44 MΩ
d) 18.43MΩ

Explanation: The largest resistor value for 12-bit DAC= 2n×R = 212×2.5kΩ = 4096×2.5kΩ =10.24MΩ.

9. CMOS inverter is used as SPDT switch in resistor DAC and is connected to the op-amp line. Find the output of CMOS, if the input applied is 1
a) Resistance is connected to ground
b) Resistance is connected to input line
c) Resistance is connected to bit line
d) None of the mentioned

Explanation:

When Qbar =1, it makes transistor Q1-ON and Q2-OFF. The output of the CMOS inverter is 0v, connecting the resistance R1 to ground.

10. How to overcome the limitation of binary weighted resistor type DAC?
a) Using R-2R ladder type DAC
b) Multiplying DACs
c) Using monolithic DAC
d) Using hybrid DAC

Explanation: Usage wide range of resistors is the limitation of binary weighted resistor type DAC, this can be avoided by using R-2R ladder type DAC Where only two value of resistor are required.

11. Find output voltage equation for 3 bit DAC converter with R and 2R resistor?
a) Vo= -RF [(b2/8R) +(b1/4R) +(b0/2R)].
b) Vo= -RF [(b2/R) +(b1/2R) +(b0/4R)].
c) Vo= -RF [(b2/2R)+(b1/4R) +(b0/8R)].
d) Vo= -RF [(b0/4R)+(b1/2R) +(b2/R)].

Explanation: The output voltage corresponding to all possible combination of binary input in a 3-bit R-2R DAC is given as
Vo=-RF [(b2/2R) +(b1/4R) +(b0/8R)].

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