Calibration
Chapter 5
SCXI-1124 User Manual
5-2
© National Instruments Corporation
Each channel and each range has different values for B
L
and B
H
. There are six channels and
seven ranges, which makes 6 x 7 = 42 pairs of calibration constants to characterize the module.
These constants are determined at the factory and stored in the onboard EEPROM. Due to drift
of the channels over time and temperature, you may want to calibrate the module yourself. In
addition to holding the factory calibration constants, the EEPROM has space for a set of user
constants and a set of load constants. The load constants are the numbers that National
Instruments software uses to calculate bit patterns. When the module is shipped, the load
constants are the same as the factory constants. Information on the structure of these tables is in
your SCXI-1124 Register-Level Programmer Manual or software reference manual. This chapter
describes how to determine the calibration constants.
Calibration Methods
Two-Point Calibration Method
With this method, you program a channel with two different DAC codes, measure the circuit
output for each code, and calculate the calibration constants.
To calibrate the SCXI-1124 module, perform the following steps:
1. For the range and channel that you are calibrating write a code, C
1
, to the DAC, where
0
≤
C
1
≤
4,095.
2. Measure the output voltage or current, O
1
.
3. For the same range and channel that you are calibrating write a code, C
2
, to the DAC, where
0
≤
C
2
≤
4,095.
4. Measure the output voltage or current, O
2
.
5. Calculate B
L
from the following formula:
B
L
= C
1
+ (O
L
- O
1
)*(C
2
- C
1
)/(O
2
- O
1
)
where O
L
is the low end of the output range (-1 V, -5 V, or -10 V for bipolar; 0 V for
unipolar; 0 mA for current output), and B
L
is the necessary bit pattern to write (rounded to
the nearest integer) to get O
L
.
6. Calculate B
H
from the following formula:
B
H
= C
1
+ (O
H
- O
1
)*(C
2
- C
1
)/(O
2
- O
1
)
where O
H
is the high end of the output range (1 V, 5 V, 10 V, or 20 mA), and B
H
is the
necessary bit pattern you write (rounded to the nearest integer) to get O
H
.
