Audio Engineering

484 Chapter 15

2R resistors is halved at each stage the current that is switched is also halved. The
currents are summed at the input to the output buffer amplifi er. The limit to the ultimate
resolution and accuracy is determined partly by the accuracy of adjustment and matching
of the characteristics of the resistors used and also by the care with which the converter is
designed into the surrounding circuitry. Implementation of a 16-bit converter requires that
all of the resistors are trimmed to within 0.0007% (half of an LSB) of the ideal value and,
further, that they all maintain this ratio over the operational temperature of the converter.
The buffer amplifi er must be able to settle quickly and it must not contribute to the output
noise signifi cantly.





MSB

Switch control – digital value to be converted to analogue

LSB

Vin R

2 R 2 R 2 R 2 R 2 R 2 R

RRRR

Vref Vref/2 Vref/4 Vref/8 Vref/16

R

Vout

Figure 15.25 : The basic form of the R–2R digital to analogue (DAG) converter is shown here

implemented by ideal current switches. The reference voltage can be an audio bandwidth

signal and the DAC can be used as a true four quadrant multiplying converter to implement

digitally controlled analogue level and equalization changes. Other forms of switching

currents are also used and these may not offer a true multiplication facility.