In Digital Electronics (DE), Analog-to-Digital (A/D) and Digital-to-Analog (D/A) converters are very important topics. ECET often includes questions about their working principles, resolution, and conversion formulas.
📘 Concept Notes
🔹 Analog and Digital Conversion
- Analog signal: Continuous-time signal (e.g., sound, temperature).
- Digital signal: Discrete-time signal represented in binary (0s and 1s).
- Conversion:
- A/D Converter: Converts analog → digital.
- D/A Converter: Converts digital → analog.
⚙️ Digital-to-Analog Converter (DAC)
A D/A Converter takes a digital binary input and gives an analog output voltage or current.
🔸 1. Weighted Resistor DAC
- Each bit is assigned a resistor proportional to its binary weight.
- Formula for output voltage:
Where:
= Reference voltage
= Binary bits (1 or 0)
= Number of bits
Example:
If 
and input = 1010 (4-bit), then:
🔸 2. R–2R Ladder DAC
- Uses only two resistor values: R and 2R.
- Easier to design and more accurate than weighted resistor type.
- Works on the principle of binary-weighted current division.
⚙️ Analog-to-Digital Converter (ADC)
An A/D Converter takes an analog voltage and converts it into a digital binary number.
🔸 1. Successive Approximation ADC
- Uses a comparator and successive approximation register (SAR).
- Conversion time:
Example:
For 8-bit ADC with clock = 1 MHz:
🔸 2. Flash (Parallel) ADC
- Fastest type of ADC.
- Uses multiple comparators (2ⁿ–1).
- Comparators required:
Example:
For 3-bit ADC → 
comparators.
🔸 3. Dual Slope ADC
- Integrates the input voltage for a fixed period and then de-integrates.
- Highly accurate and noise-resistant (used in digital voltmeters).
📏 Important Formulas
- Resolution (Analog Value per Bit):
Quantization Error:
Output Voltage of n-bit DAC:
Number of Levels:
💡 Example Problem
Q: Find the output voltage for an 8-bit DAC if 
and digital input = 10000000.
Solution:
, 
🔟 10 Expected MCQs – ECET 2026
Q1. A/D converter converts:
A) Analog to Analog
B) Digital to Analog
C) Analog to Digital
D) Digital to Digital
Q2. D/A converter converts:
A) Analog to Digital
B) Digital to Analog
C) Analog to Analog
D) None
Q3. In an n-bit ADC, number of quantization levels =
A)
B) 
C) 
D) 
Q4. Resolution of a 10-bit ADC with :
A) 5/1024
B) 5/1023
C) 10/1023
D) 2.5/512
Q5. The fastest ADC type is:
A) Dual Slope
B) Flash
C) SAR
D) Counter type
Q6. For 4-bit ADC, number of comparators in flash ADC =
A) 8
B) 15
C) 7
D) 16
Q7. In DAC, reference voltage used is:
A) Output voltage
B) Control voltage
C) Scaling voltage
D) Reference voltage source
Q8. Quantization error is:
A) Zero
B) Half of one LSB
C) Equal to one LSB
D) Two LSBs
Q9. If 
and 3-bit DAC input = 100, then 
= ?
A) 1V
B) 2V
C) 4V
D) 8V
Q10. The advantage of R–2R ladder DAC is:
A) More resistors required
B) Only two resistor values needed
C) Uses no resistors
D) Complex design
✅ Answer Key
| Q.No | Answer |
|---|---|
| Q1 | C |
| Q2 | B |
| Q3 | B |
| Q4 | B |
| Q5 | B |
| Q6 | C |
| Q7 | D |
| Q8 | B |
| Q9 | C |
| Q10 | B |
🧠 Explanations
- Q1 → C: A/D converter converts analog input to digital output.
- Q2 → B: DAC converts digital binary data into analog output.
- Q3 → B: Quantization levels = .
- Q4 → B: Resolution =
. - Q5 → B: Flash ADC is the fastest.
- Q6 → C:
comparators needed, not 7. - Q7 → D: Reference voltage defines the full-scale output.
- Q8 → B: Error = ±½ LSB.
- Q9 → C:
; 
. - Q10 → B: R–2R ladder DAC uses only two resistor values → simplifies design.
🎯 Why Practice Matters
A/D and D/A converters are conceptual + numerical topics.
By practicing resolution, levels, and conversion time problems, students can easily gain 2–3 marks in ECET.
