User manual

ATHENA CPU User Manual V1.40 Page 51
11. ANALOG-TO-DIGITAL INPUT RANGES AND RESOLUTION
11.1.1 OVERVIEW
Athena 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 Athena have three configuration options:
♦ Single-ended or differential mode
♦ Unipolar or bipolar mode
♦ Input range (gain)
The single-ended / differential and unipolar / bipolar configuration is done with a jumper on
jumper block J13 (see page 23), and the input range selection is done in software.
11.2 Input Range Selection
Athena can be configured to measure both unipolar (positive only) and bipolar (positive and
negative) analog voltages. This configuration is done with a jumper 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. However, 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.
11.3 Input Range Table
The table below indicates the analog input range for each possible configuration. The polarity is
set with a jumper on jumper block J13, and the gain is set with the G1 and G0 bits in the register
at Base + 3. 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 305µV
Bipolar 0 1
±5V 153µV
Bipolar 1 0
±2.5V 76µV
Bipolar 1 1
±1.25V 38µV
Unipolar 0 0 --- Invalid ---
Unipolar 0 1 0 – 8.3V
153µV
Unipolar 1 0 0 – 5V
76µV
Unipolar 1 1 0 – 2.5V
38µV
Table 20: Data Acquisition : Analog Input Range