Digital-to-Analog Conversion | Sahithyan's Notes

A DAC converts a digital value to a proportional analog output voltage.

V_{out} = \frac{D}{2^n - 1} \times V_{ref}

Here:

$V_{out}$ : output voltage
$D$ : digital input value
$n$ : resolution in bits
$V_{ref}$ : reference voltage

Applications: motor speed control, audio signals, variable voltage references.

Types

Binary-Weighted Resistor DAC

Each bit $k$ drives a resistor of value $\frac{R}{2^k}$ . Currents from all bits sum at a virtual ground. Output voltage is proportional to total current.

Simple circuit
Requires resistors spanning a wide range ( $R$ to $R/256$ for 8 bits)
Precision difficult at high resolutions

R-2R Ladder DAC

Uses only 2 resistor values ( $R$ and $2R$ ) in a ladder network. Each rung adds the contribution of 1 bit. Both ends terminated with $2R$ so every node sees the same impedance.

Only 2 resistor values regardless of bit depth
Scales to high resolutions with good fabrication accuracy
Most common DAC in MCUs and audio hardware

PWM DAC

A PWM signal passed through a low-pass RC filter. The filter averages the square wave. Output DC voltage equals $V_{supply} \times \text{duty cycle}$ .

No dedicated DAC hardware required
Ripple depends on filter cutoff vs PWM frequency
Suitable for motor control and LED dimming. Not suitable for audio or precision output.