Digital-to-analogue conversion: the summing-amplifier (binary-weighted) DAC, the R-2R ladder, resolution and the analogue output, and reconstruction filtering.

Weights (up to 2 marks): in a binary-weighted DAC each bit contributes a voltage proportional to its place value. With the LSB contributing $1.0\ \text{V}$, the bits (from LSB) contribute $1, 2, 4, 8\ \text{V}$.

Code $1011$ (up to 2 marks): the set bits are bit 0 ($1\ \text{V}$), bit 1 ($2\ \text{V}$) and bit 3 ($8\ \text{V}$); bit 2 ($4\ \text{V}$) is $0$. Output $= 8 + 2 + 1 = 11\ \text{V}$ (this matches the denary value $11$ of $1011$).

Full-scale $1111$ (up to 2 marks): all four bits set: $8 + 4 + 2 + 1 = 15\ \text{V}$ (denary $15$).

Markers reward the binary-weighted contributions, the output $11\ \text{V}$ for $1011$, and the full-scale $15\ \text{V}$ for $1111$.

Binary-weighted disadvantage (up to 2 marks): it needs a wide range of precisely matched resistor values (each double the last), which is hard to manufacture accurately for many bits; small resistor errors in the most significant bits cause large output errors.

R-2R ladder (up to 2 marks): the R-2R ladder uses only two resistor values ($R$ and $2R$) in a repeating network, which are far easier to match accurately, so it scales to many bits with good accuracy.

Reconstruction filter (up to 1 mark): the DAC output is a staircase that jumps at each new sample; a low-pass (reconstruction) filter smooths the steps into a continuous analogue signal and removes the high-frequency switching components.

Markers reward the wide spread of matched resistors as the binary-weighted weakness, the two-value R-2R network as the fix, and the smoothing/reconstruction low-pass filter at the output.