DAC — Digital to Analog Converter Types and Working

What is a DAC?

A DAC (Digital-to-Analog Converter) converts a binary digital code into a proportional analog voltage or current.

Output voltage formula:

Weighted Resistor DAC

Uses different resistor values (R, 2R, 4R, 8R...) proportional to bit weights.

Problem: For 8-bit, ratio is 1:128. Large spread in resistor values makes precision difficult. Advantage: Simple concept; good for low bit counts.

Numerical Examples

Q1: A 4-bit DAC has Vref = 5V. Find output for digital input 1010.

Q2: An 8-bit DAC has Vref = 10V. What is the step size (LSB voltage)?

Q3: A 12-bit DAC has full-scale output of 4.095V. What digital code gives 2.048V?

ADC vs DAC Comparison

Interview Questions

Q1: Why is R-2R ladder preferred over weighted resistor DAC? R-2R only uses two resistor values (R and 2R), making it easy to achieve precise matching in IC fabrication. Weighted resistors require values of R, 2R, 4R, ..., 2^(n-1)R — for 16-bit, ratio is 1:32768 which is impossible to match accurately. R-2R scales to any bit count with consistent accuracy.

Q2: What is settling time in a DAC? Settling time is the time after a digital code change for the output to reach and remain within a specified error band (usually ±1/2 LSB) of the final value. It limits the maximum update rate. For audio DACs, settling time < 1/sampling_rate is required.

Q3: A 10-bit DAC is used to control a motor speed from 0 to 1000 RPM. What is the speed resolution? Resolution = 1000 RPM / (2^10 - 1) = 1000 / 1023 ≈ 0.977 RPM ≈ 1 RPM. The minimum speed change controllable is about 1 RPM.