PCM78 Burr-Brown Corporation, PCM78 Datasheet - Page 8

• Manufacturer Part Number: PCM78
• Description: 16-Bit Audio ANALOG-TO-DIGITAL CONVERTER
• Manufacturer: Burr-Brown Corporation

TYPICAL
PERFORMANCE CURVES
At T
0.008
0.007
0.006
0.005
0.051
0.050
0.049
0.048
0.047
0.046
0.045
A
= +25 C, V
5.2
5.0
4.8
4.6
4.4
2
2
2
CC
®
= 15V, unless otherwise noted.
PCM78
THD+N vs CONVERSION TIME (–20dB)
THD+N vs CONVERSION TIME (–60dB)
THD+N vs CONVERSION TIME (0dB)
4
4
4
Convert Time (µs)
Convert Time (µs)
Convert Time (µs)
6
6
6
8
8
8
(CONT)
10
10
10
8
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
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
)
Offset
Error
E
All Bits Off
–1/2LSB
IN
Off
Analog Input
Gain
Error
+1/2LSB
All Bits On
E
(
IN
+FSR
On
2
1/2LSB. The
–1LSB
)