Poseidon uses a 4-channel 12-bit D/A converter (DAC) to provide four 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 212 - 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 conversion of digital data originating from the Poseidon computer hardware to an analog signal terminating at an external source.
The resolution is the smallest possible change in output voltage. For a 12-bit DAC the resolution is 1/(212), 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, so this change is referred to as 1 least significant bit (1 LSB ). The value of this LSB is calculated as follows.
1 LSB = Output voltage range / 4096
The maximum voltage swing is defined as the difference between the highest nominal output voltage and the lowest output voltage. For an output range of 0-10V or +/-5V, the maximum voltage swing is 10V.
Example:
For, Output range = ±5V;
Maximum voltage swing = 10V
Therefore,
1 LSB = 10V / 4096 = 2.44mV
The D/A converter chip on Poseidon requires two references, one for the low end and one for the high end of the range. The high end can be set to 5V, 10V, or Programmable, and the low end can be either 0V (for unipolar output ranges) or minus the high-end voltage. All channels are set to the same output range.
On power up, the D/A automatically resets to mid-scale, which is 0V in bipolar mode and 1/2 full-scale voltage in unipolar mode.