User manual

ATHENA CPU User Manual V1.40 Page 57
14. ANALOG OUTPUT RANGES AND RESOLUTION
14.1 Description
Athena uses a 4-channel 12-bit D/A converter (DAC) to provide 4 analog outputs. A 12-bit DAC
can generate output voltages with the precision of a 12-bit binary number. The maximum value of
a 12-bit binary number is 2
12
- 1, or 4095, so the full range of numerical values that the DACs
support is 0 - 4095. The value 0 always corresponds to the lowest voltage in the output range,
and the value 4095 always corresponds to the highest voltage minus 1 LSB (the theoretical top
end of the range corresponds to an output code of 4096 which is impossible to achieve).
⇒
⇒⇒
⇒ Note: In this manual, the terms analog output, D/A, and DAC are all used interchangeably to
mean the same thing.
14.2 Resolution
The resolution is the smallest possible change in output voltage. For a 12-bit DAC the resolution
is 1/(2
12
), or 1/4096, of the full-scale output range. This smallest change results from an increase
or decrease of 1 in the D/A code, and so this change is referred to as 1 LSB, or 1 least significant
bit. The value of this LSB is calculated as follows:
1 LSB = Output voltage range / 4096
Example: Output range = 0-10V;
Output voltage range = 10V – 0V = 10V
1 LSB = 10V / 4096 = 2.44mV
Example: Output range = ±10V;
Output voltage range = 10V – (-10V) = 20V
1 LSB = 20V / 4096 = 4.88mV
14.3 Output Range Selection
Jumper block J13 is used to select the DAC output range. See page 23 for configuration data.
The DACs can be configured for 0-10V or ±10V.
Two parameters are configured: unipolar/bipolar mode and power-up/reset clear mode. In most
case, for unipolar mode set the board to reset to zero scale, and for bipolar mode configure the
board for reset to mid-scale. In each case the DACs will reset to 0V.