Theory of Operation
Chapter 4
SCXI-1124 User Manual
4-4
© National Instruments Corporation
range. Using these two values, you can do a linear interpolation to determine the correct code to
write to the DAC to get a particular voltage by using the following formula:
B
U
= B
L
+ (V
U
- V
L
)*(B
H
- B
L
)/(V
H
- V
L
)
where
B
L
= bit pattern to be written to get the low value of the range (stored in EEPROM)
B
H
= bit pattern to be written to get the high value of the range (stored in EEPROM)
V
L
= low value of the range
V
H
= high value of the range
V
U
= voltage you want
B
U
= necessary bit pattern to write
If you are using National Instruments software, the reading of constants from the EEPROM and
calculation of bit patterns is done for you automatically. If you are writing your own software,
see the SCXI-1124 Register-Level Programmer Manual for information on how to read the
EEPROM and decode the offset information.
The output voltage is referenced to the isolated ground reference and is short-circuit protected.
Therefore, you will not damage the module if you short the voltage output of a channel to its own
ground. However, you must not short the voltage output of a channel to earth ground. If the
isolated ground of the channel is at a large common-mode voltage with respect to earth, shorting
the voltage output to earth ground damages the module.
The voltage output can drive up to
±
5 mA. If you are driving a long cable, you must account for
any voltage drop in the cable. For example, 100 ft of 22 AWG stranded copper wire has about
1.5
Ω
of resistance. Thus, driving a load 50 ft away at 5 mA will produce an error at the load of
7.5 mV. Solutions to this problem include:
•
Locate the load closer to the SCXI-1124 module.
•
Use heavy gauge wire to minimize resistive loss.
•
Add a high-impedance input buffer near the load.
•
Calibrate your system to account for the resistive losses of the cable.
Noise Reduction
If you need less noise on the voltage output, you can put a filter on the output, provided you are
driving a high-impedance input. Most of the noise at the SCXI-1124 outputs is due to the
onboard switching supply that provides the power for the isolated circuitry. This supply operates
at 100 kHz switching frequency, so an effective filter should filter out 100 kHz and higher
frequencies. A simple RC filter does this job very well. The cutoff frequency of the RC filter
shown in Figure 4-2 is:
f = 1/(2
π
RC)
For example, a 1 k
Ω
resistor and a 0.1
µ
F capacitor produces a cutoff frequency of 1.6 kHz.
