Analog-to-Digital Input Ranges and Resolution
Overview
Poseidon uses a 16-bit A/D converter. This means that the analog input voltage can be measured to the precision of
a 16-bit binary number. The maximum value of a 16-bit binary number is 216 - 1, or 65535, so the full range of
numerical values that you can get from a Poseidon analog input channel is 0 - 65535.
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 one in the A/D code, and is referred to as one Least Significant
Bit (1 LSB).
Unipolar and Bipolar Inputs
Poseidon can measure both unipolar (positive only) and bipolar (positive and negative) analog voltages. The full-
scale input voltage range depends on the Gain, Range, and Polarity bit settings in the Analog Configuration Register
at Base+11. In front of the A/D converter is a programmable gain amplifier that multiplies the input signal before it
reaches the A/D. This gain circuit has the effect of scaling the input voltage range to match the A/D converter for
better resolution. In general, you should select the highest gain you can that will allow the A/D converter to read the
full range of voltages over which your input signals will vary. If the gain is too high, the A/D converter will clip at
either the high end or low end, and you will not be able to read the full range of voltages on your input signals.
Input Ranges and Resolutions
The table below lists the full-scale input range for each valid analog input configuration. The parameters Polarity,
Range and Gain are combined to create the value “Code”, which is the value that you must write to the analog
configuration register at Base+11 to get the input range shown. A total of nine different input ranges are possible.
Note that the range programming codes 4, 5, 6 and 7 are invalid and that range codes 9 – 11 are equivalent to range
codes 0 – 2.
Polarity
Range
Gain
Code
Input Range Resolution (1 LSB)
Bipolar
5V
1
0
±
5V
153
µ
V
Bipolar
5V
2
1
±
2.5V
76
µ
V
Bipolar
5V
4
2
±
1.25V
38
µ
V
Bipolar
5V
8
3
±
0.625V
19µ
V
Unipolar
5V
1
4
Invalid Setting
Unipolar
5V
2
5
Invalid Setting
Unipolar
5V
4
6
Invalid Setting
Unipolar
5V
8
7
Invalid Setting
Bipolar
10V
1
8
±
10V
305
µ
V
Bipolar
10V
2
9
±
5V
153
µ
V
Bipolar
10V
4
10
±
2.5V
76
µ
V
Bipolar
10V
8
11
±
1.25V
38
µ
V
Unipolar
10V
1
12
0-10V
153
µ
V
Unipolar
10V
2
13
0-5V
76
µ
V
Unipolar
10V
4
14
0-2.5V
38
µ
V
Unipolar
10V
8
15
0-1.25V
19µ
V
Diamond Systems Corporation
Poseidon User Manual
Page 78