Section 6
Interfacing
6-9
NOTE:
The 20 mA signal will not go negative. If you wish to use the
20mA option, configure the DAC so that the signal will always be
positive. For example, to allow a range of -100 to +100
µ
mol/mol, set
X
0
= -100, rather than 0. Then, -40
µ
mol/mol will generate an 8.8 mA
signal on DAC 1 20 mA.
Resolution
The DAC's are 12-bit bipolar devices, so resolution is given by (total
range)/2
12
. The voltage ranges can go positive or negative, so resolution is
±
R/2
12
= R/2
11
. This corresponds to 2.44 mV on the 5V range, and 48.8
µ
V
on the 100 mV range. The 4 to 20mA range does not go negative, so its
resolution is (20-4)/2
12
= 3.91
µ
A. The smallest difference that can be
resolved for any variable is,
∆
X = 2
-11
(X
F
- X
o
) = 4.88 x 10
-4
(X
F
- X
o
)
[voltage]
and
∆
X = 2
-12
(X
F
- X
o
) = 2.44 x 10
-4
(X
F
- X
o
)
[current]
As an example, if X
F
= 1000 and X
o
= 0, the resolution will be about 0.5
µ
mol/mol on either voltage channel, and 0.25
µ
mol/mol on the current
channel.
Timing
The DAC outputs change whenever the target channel changes. All of the
CO
2
values (FCTs 22-27) are computed at 5 Hz. The H
2
O values (FCTs 32-
38), however, come at an
irregular
interval; they change at 5 Hz, but 2 of the
5 measurements each second are “missed” (while the temperature and
auxiliary input channel are measured), so actually only 3 changes occur in 1
second. This irregularity makes outputting H
2
O signals on the DAC
inappropriate for high speed, regular sampling applications such as eddy
correlation. Users needing high speed regular sampling should use the non-
linear outputs for H
2
O, and possibly CO
2
, if >5 Hz is required.
