®

PCM78

8

THD+N vs CONVERSION TIME (–60dB)

5.2

4.4

26

10

Convert Time (µs)

48

4.8

4.6

5.0

THD+N vs CONVERSION TIME (–20dB)

0.051

0.049

0.045

26

10

Convert Time (µs)

0.047

48

0.050

0.048

0.046

THD+N vs CONVERSION TIME (0dB)

0.008

0.007

0.005

26

10

Convert Time (µs)

0.006

48

TYPICAL

THEORY OF OPERATION

The PCM78P is a successive approximation A/D converter;

this type of converter is well suited to high speed and

resolution. The accuracy of a successive approximation

converter is described by the transfer function shown in

Figure 1. All successive-approximation A/ D converters

have an inherent quantization error of

±1/2LSB. The

remaining errors in the A/ D converter are combinations of

analog errors due to the linear circuitry, matching and

tracking properties of the ladder and scaling networks,

power supply rejection, and reference errors. In summary,

these errors consist of initial errors including Gain, Offset,

Linearity, Differential Linearity, and Power Supply Sensi-

tivity. Gain drift over temperature rotates the line (Figure 1)

about zero, and Offset drift shifts the line left or right over

the operating temperature range. Total Harmonic Distortion

+ Noise (THD+N) is a measure of the magnitude and

distribution of the Linearity Error, Differential Linearity

Error, and Noise, as well as quantization errors. The

THD+N specification is most useful in audio or dynamic

signal processing applications. To be useful, THD+N

should be specified for both high level and low level input

signals. This error is unadjustable and is the most meaning-

ful indicator of A/ D converter accuracy for dynamic

applications.

DYNAMIC RANGE

Dynamic range is a measure of the ratio of the smallest

signals the converter can resolve to the full scale range and

is usually expressed in decibels. The theoretical dynamic

range of a converter is approximately 6 x n, where n is the

number of bits of resolution. A 16-bit converter would thus

have a theoretical dynamic range of 96dB. The actual useful

dynamic range is limited by noise and linearity errors and

is therefore somewhat less than the theoretical limit.

FIGURE 1. Input vs Output for Ideal Bipolar A/D Converter.

0111…1111

0111…1110

0000…0010

0000…0001

0000…0000

1111…1111

1111…1110

1000…0001

1000…0000

*See Input/Output Relationship Table for code definitions.

–FSR

2

Analog Input

All Bits On

Gain

Error

+1/2LSB

All Bits Off

Offset

Error

–1/2LSB

+FSR

2

–1LSB

On

IN

E

Off

IN

E

(

()

)