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Zeta User Manual Rev 1.0
Page 40
14.2.4
A/D Operation
In essence, an A/D converter converts an analog voltage to a number that indicates the ratio between that voltage
and the full range of voltages that the A/D can measure. A 16-bit A/D converter means that all input voltages are
represented by a 16-bit binary number ranging from 0 to 65535 (2
16
-1), or binary 0000 0000 0000 0000 to 1111
1111 1111 1111. There are two groups of input ranges, unipolar and bipolar. Unipolar ranges consist of only
positive voltages (0-5V, 0-10V), and bipolar ranges include both negative and positive voltages (+/-5V, +/-10V).
For a unipolar input range, the A/D values are straight binary and will range from 0 (binary 0000 0000 0000 0000)
for 0V to 65535 (binary 1111 1111 1111 1111) for the top end of the range. For a bipolar input range, the A/D
values are 2s complement and will range from -32768 ( binary 1000 0000 0000 0000) to +32767 (binary 0111 1111
1111 1111).
The tables below summarize the relationship between input voltage and A/D values for the general case. Note
that the nominal upper limit of the input range (5.0000V or 10.0000V) is not achievable, since this voltage would
require a 17 bit number (2
16
or 1 0000 0000 0000 0000). V
FS
means the full-scale input voltage, either 5V or 10V.
Bipolar Input Ranges
A/D Code
A/D code binary
Formula
+/-5V input range +/-10V input range
-32768
1000 0000 0000 0000
-V
FS
-5.0000V
-10.0000V
-32767
1000 0000 0000 0001
-V
FS
+ 1 LSB
-4.9998V
-9.9997V
-1
1111 1111 1111 1111
-1 LSB
-0.153mV
-0.305mV
0
0000 0000 0000 0000
0V
0.0000V
0.0000V
1
0000 0000 0000 0001
+1 LSB
0.153mV
.305mV
32767
0111 1111 1111 1111
V
FS
- 1 LSB
4.9998V
9.9997V
Unipolar Input Ranges
A/D Code
A/D code binary
Formula
0-5V input range
0-10V input range
0
0000 0000 0000 0000
0V
0.0000V
0.0000V
1
0000 0000 0000 0001
1 LSB (V
FS
/ 65536)
0.076mV
0.153mV
32767
0111 1111 1111 1111
V
FS
/ 2 - 1 LSB
2.4999V
4.9998V
32768
1000 0000 0000 0000
V
FS
/ 2
2.5000V
5.0000V
32769
1000 0000 0000 0001
V
FS
/ 2 + 1 LSB
2.5001V
5.0002V
65535
1111 1111 1111 1111
V
FS
- 1 LSB
4.9999V
9.9998V
14.2.5
A/D Resolution
The smallest change in input voltage that can be detected is 1/(2
16
), or 1/65536, of the full-scale input range.
This smallest change results in an increase or decrease of 1 in the A/D code, and is referred to as 1 LSB (1 Least
Significant Bit). The resolution is always 16 bits, but the value of 1 LSB will vary with the input range.
Polarity
Input Range
Resolution (1LSB)
Bipolar
±10V
305µV
Bipolar
±5V
153µV
Unipolar
0 - 10V
153µV
Unipolar
0 - 5V
76µV
