Hercules-EBX CPU User Manual V1.02
Page 79
12. ANALOG-TO-DIGITAL INPUT RANGES AND RESOLUTION
12.1.1 OVERVIEW
Hercules-EBX uses a 16-bit A/D converter. The full range of numerical values for a 16-bit number
is 0 - 65535. However the A/D converter uses twos complement notation, so the A/D value is
interpreted as a signed integer ranging from –32768 to +32767.
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 so
this change is referred to as 1 LSB, or 1 Least Significant Bit.
The analog inputs on Hercules-EBX have three configuration options:
♦
Single-ended or differential mode
♦
Unipolar or bipolar mode
♦
Input range (gain)
The single-ended / differential configuration, unipolar / bipolar configuration, and the input range
selection are all handled in software.
12.1.2 INPUT RANGE SELECTION
Hercules-EBX can be configured to measure both unipolar (positive only) and bipolar (positive
and negative) analog voltages. This configuration is done via the Analog Configuration Register
(Page 0:Base+1) and applies to all inputs. In addition you can select a gain setting for the inputs,
which causes them to be amplified before they reach the A/D converter. The gain setting is
controlled in software, so it can be changed on a channel-by-channel basis. In general you should
select the highest gain (smallest input range) that will allow the A/D converter to read the full
range of voltages over which your input signals will vary. If you pick too high a gain, then 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.
12.1.3 INPUT RANGE TABLE
The table below indicates the analog input range for each possible configuration. The polarity is
set in the Analog Configuration Register (Page 0:Base+1), and the gain is set with the G1 and G0
bits in the Input Range Control Register (Page 0:Base+4). The Gain value in the table is provided
for clarity. Note that the single-ended vs. differential setting has no impact on the input range or
the resolution.
Polarity G1 G0 Input
Range Resolution (1 LSB)
Bipolar 0
0
±
10V 305
µ
V
Bipolar 0
1
±
5V 153
µ
V
Bipolar 1
0
±
2.5V 76
µ
V
Bipolar 1
1
±
1.25V 38
µ
V
Unipolar
0
0
0 – 10V
153
µ
V
Unipolar
0
1
0 – 5V
76
µ
V
Unipolar
1
0
0 – 2.5V
38
µ
V
Unipolar
1
1
0 – 1.25V
19
µ
V
Table 33: Data Acquisisition : Analog Input Range
