LI-COR LI-7500, Instruction Manual

Page: 59 / 155

Section 3
3-30 Setup and Operation
DAC Configuration
Each DAC is configured by specifying the source channel (e.g. CO
2
mmol/m
3
) that drives the
analog signal, the source channel value which corresponds to zero volts, and the source
channel value which corresponds to full scale voltage (5V).
For example, to configure DAC #1 to output a voltage signal proportional to CO
2
mmol/m
3
,
20 mmol/m
3
full scale, select CO
2
mmol/m
3
under DAC1 'Source', and set 0V = 0, and 5V =
20.
0V
→
0 (X
o
, zero volts corresponds to 0 mmol/m
3
)
5V
→
20 (X
F
, full scale corresponds to 20 mmol/m
3
)
When a voltage range R is selected, the DAC output voltage V resulting from a CO
2
molar
value X is given by
V R
X X
X X
o
F o
=
−
−
where R = 5V.
Delay Time
The output signal from the LI-7500 optical bench is sampled by a high-speed A/D converter
and input into a Digital Signal Processor (DSP). This signal is processed digitally and gas
densities are computed from it. There is a fixed delay in this process, and an additional user-