0195136047.pdf

6.2 DIGITAL SYSTEM COMPONENTS 307

V 0 analog output voltage

Vref reference analog input voltage

b 0 most significant bit of binary input code

bn− 1 least significant bit of binary input code

In order to provide current-to-voltage conversion and/or buffering, an op amp is used at the output.
However, in some high-speed applications where a limited output voltage range is acceptable, a
resistor, instead of an op amp, is used for the current-to-voltage conversion, thereby eliminating
the delay associated with the op amp.

WEIGHTED-RESISTORD/A CONVERTER

Figure 6.2.9 shows a 4-bit weighted-resistor D/A converter which includes a reference voltage
source, a set of four electronically controlled switches, a set of four binary-weighted precision
resistors, and an op amp. Each binary bit of digital input code controls its own switch. The switch
closes with a bit value of 1, and the switch stays open with binary 0. The resistor connected to
the most significant bit (MSB),b 0 , has a value ofR;b 1 is connected to 2R,b 2 to 4R, andb 3 to 8R.
Thus, each low-order bit is connected to a resistor that is higher by a factor of 2. For a 4-bit D/A
converter, the binary input range is from 0000 to1111.
An important design parameter of a D/A converter is theresolution, which is the smallest
output voltage change,V, which for ann-bit D/A converter is given by

V=

Vref

2 n−^1

(6.2.2)

The range of resistor values becomes impractical for binary words longer than 4 bits. Also,
the dynamic range of the op amp limits the selection of resistance values. To overcome these
limitations, theR–2Rladder D/A converter is developed.

R–2R LADDERD/A CONVERTER

Figure 6.2.10 shows a 4-bitR–2Rladder D/A converter, which contains a reference voltage source,
a set of four switches, two resistors per bit, and an op amp. The analog output voltage can be
shown to be

1010110 0

R

I Vo

in

Vref

I 0

b 0 b 1 b 2 b 3

I 1 I 2 I 3

2 R 4 R 8 R

R

+

−

Figure 6.2.94-bit weighted-resistor D/A converter.