Vaisala FD12P - Manual (Page 91)

Chapter 4 _________________________________________________________________ Operation
VAISALA ________________________________________________________________________ 91
Type
AN AMBL
↵ ↵↵ ↵
WAITING FOR MULTIPLEXER
ANALOG INPUT, AMBL
118 0.1
119 0.3
Analog Output Commands
Analog Output Calibration
The DAC output voltage is converted to current, 0 to 22 mA unscaled.
This current is then software-calibrated to give 4 mA at the minimum
visibility and 20 mA at the maximum visibility. The minimum and
maximum visibility values are set in the configuration session. A
hardware error is indicated by 0 mA.
The ACAL command sets two-bit values, 4000 and 800, to the
digital-to-analog converter. The corresponding currents measured by a
multimeter must be given as answers to the questions asked in the
commands. The analog output scaling factors, which define the
bits/mA relation, are then calculated by the software. The scaling
factors are Scale 0 and Scale 1. The FD12 calculates them as follows:
Scale 0 = 4 ×((4000-800)/(high current - low current))
The bit value that gives 4 mA. Scale 1 depends on the mode.
In linear mode:
Scale 1 = bits16mA / (maximum vis - minimum vis)
In logarithmic mode:
Scale 1 = (ln(max vis) - ln(min vis)) / bits16mA
bits16mA = (3200 / (high current - low current)) Ч 16