Audio Engineering

490 Chapter 15

Convert

VRT

VRT

R 1

R

O 1

O 8

R

R 2

R

R

R 2

Vinput

Outputs

Latch

255 to 8 encoder

Differential

comparators (255)

1

2

126

127

254

255

Figure 15.29 : A chain of resistors provides a series of reference voltages for a set of

comparators whose other input is the input signal. An 8-bit encoder will need 255

comparators. Their output will drive an encoder that maps the output state of the

255 comparators onto an 8-bit word. The NMINV control line is used to convert the

output word from an offset binary count to a two’s complement form. A 16-bit converter

would require an impracticably large number of comparators (65536) in addition to

posing serious diffi culties to setting the 65536 resistor values to the required tolerance

value. The technique does have one virtue in speed and in not needing a sample and

hold amplifi er to precede it.