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CODING SCHEMES USED WITH
DATA CONVERTERS
Jason Albanus
With the recent proliferation of analog-to-digital converters
(ADCs) and digital-to-analog converters (DACs), and the
variety of digital coding schemes which they use, has come
a need to understand these different coding schemes which
converters use to talk to the 鈥渄igital world鈥? The purpose of
this article is to describe the individual coding schemes used
with ADCs and DACs manufactured by Burr-Brown, and
explain their relationships.
Following this text is a list of abbreviations and definitions
intended to clarify any questions regarding the nomenclature
which has been used.
Throughout this guide, examples and tables given are for a
4-bit data converter. In unipolar and bipolar examples alike,
the Full Scale Range (FSR) is 10V creating a V
LSB
of
0.625V. For unipolar examples, minus full scale (鈥揊S ) is 0V
and plus full scale (+FS) is 10V; for bipolar examples, 鈥揊S
is 鈥?V and +FS is +5V.
USB 鈥?UNIPOLAR STRAIGHT BINARY
The Unipolar Straight Binary coding is perhaps the simplest
coding scheme to understand. As the name implies, it is a
coding scheme which is used only for unipolar voltages.
When using USB coding, the digital count begins at all zeros
(0000) at a V
CODE
of 0V (V
t+
= 0V + 1/2V
LSB
and there is
no V
t鈥?/div>
). As the digital code increments, the analog voltage
increases one V
LSB
at a time, and the digital count ends
(1111) at the positive full scale value. Table I shows how the
USB codes correspond to analog voltages for a 4-bit digital
system.
MNEMONIC
Zero
+1 V
LSB
DIGITAL CODE
0000
0001
0010
0011
0100
0101
0110
0111
1000
1001
1010
1011
1100
1101
1110
1111
0.3125
0.9375
1.5625
2.1875
2.8125
3.4375
4.0625
4.6875
5.3125
5.9375
6.5625
7.1875
7.8125
8.4375
9.0625
V
t鈥?/div>
V
CODE
0.000
0.625
1.250
1.875
2.500
3.125
3.750
4.375
5.000
5.625
6.250
6.875
7.500
8.125
8.750
9.375
V
t+
0.3125
0.9375
1.5625
2.1875
2.8125
3.4375
4.0625
4.6875
5.3125
5.9375
6.5625
7.1875
7.8125
8.4375
9.0625
Unipolar Straight Binary is the coding scheme used by the
ADC7802 and ADS7803.
CSB 鈥?COMPLEMENTARY STRAIGHT BINARY
The Complementary Straight Binary coding scheme is the
exact digital opposite (one鈥檚 complement) of Unipolar
Straight Binary. CSB coding, like its counterpart USB, is
also restricted to unipolar systems.
When using CSB coding with a digital system, the digital
count begins at all zeros (0000) at the positive full scale
value. As the digital code increments, the analog voltage
decreases one V
LSB
at at time, until 0V is reached at a digital
code of 1111. The relationship between CSB coding and its
corresponding analog voltages can be seen in Table II.
MNEMONIC
Zero
+1V
LSB
DIGITAL CODE
1111
1110
1101
1100
1011
1010
1001
1000
0111
0110
0101
0100
0011
0010
0001
0000
V
t鈥?/div>
V
CODE
0.000
V
t+
0.3125
0.9375
1.5625
2.1875
2.8125
3.4375
4.0625
4.6875
5.3125
5.9375
6.5625
7.1875
7.8125
8.4375
9.0625
0.3125
0.9375
1.5625
2.1875
2.8125
3.4375
4.0625
4.6875
5.3125
5.9375
6.5625
7.1875
7.8125
8.4375
9.0625
0.625
1.250
1.875
2.500
3.125
3.750
4.375
5.000
5.625
6.250
6.875
7.500
8.125
8.750
9.375
1/4 FSR
1/2 FSR
3/4 FSR
+FS
TABLE II. CSB Coding Scheme.
BOB 鈥?BIPOLAR OFFSET BINARY
Bipolar Offset Binary coding, as the name implies, is for use
in bipolar systems (where the analog voltage can be positive
and negative). This coding scheme is very similar to USB
coding since, as the analog voltage increases, the digital
count also increases.
BOB coding begins with digital zero (0000) at the negative
full scale. By incrementing the digital count, the correspond-
ing analog value will approach the positive full scale in
1V
LSB
steps, passing through bipolar zero on the way. This
鈥渮ero crossing鈥?occurs at a digital code of 1000 (see Table
1/4 FSR
1/2 FSR
3/4 FSR
+FS
TABLE I. USB Coding Scheme.
漏
1991 Burr-Brown Corporation
AN-175
Printed in U.S.A. March, 1991
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