Chapter 4
Calibration Procedures
© National Instruments Corporation
4-3
AT-MIO-16 User Manual
•
R7—Offset trim, analog output channel 0
•
R3—Offset trim, analog output channel 1
Analog Input Calibration
To null error sources that compromise the quality of measurements, you must calibrate the
analog input circuitry by adjusting the following potential sources of error:
•
Offset error at the input of the instrumentation amplifier
•
Offset error at the input of the ADC
•
Gain error of analog input circuitry
Offsets at the input to the instrumentation amplifier contribute gain-dependent offset error to the
analog input circuitry. This offset is multiplied by the gain of the instrumentation amplifier. To
calibrate this offset, you must ground the analog input, read it at two different gain settings, and
adjust a trimpot until the readings match at the two different gain settings.
Offset error at the input of the ADC is the total of the voltage offsets contributed by the circuitry
from the output of the instrumentation amplifier to the ADC input, including the offsets of the
ADC itself. Offset errors appear as a voltage added to the input voltage being measured. To
calibrate this offset, you must apply V
-
fs
+
1
/
2
LSB to the analog input circuitry and adjust a
trimpot until the ADC returns readings that flicker between its most negative count and the most
negative count plus one. The voltages corresponding to V
-
fs
and 1 LSB are given in Table 4-1.
All the stages up to and including the input of the ADC contribute to the gain error of the analog
input circuitry. With the instrumentation amplifier set to a gain of 1, the gain of analog input
circuitry is ideally 1. The gain error is the deviation of the gain from 1 and appears as a
multiplication of the input voltage being measured. To calibrate this offset, you must apply
V
+fs
-
3
/
2
LSB to the analog input circuitry and adjust a potentiometer until the ADC returns
readings that flicker between its most positive count and the most positive count minus 1. The
voltages corresponding to V
+fs
and 1 LSB are given in Table 4-1.
The voltages corresponding to V
-
fs
,
which is the most negative voltage that the ADC can read,
V
+fs
- 1, which is the most positive voltage the ADC can read, and 1 LSB, which is the voltage
corresponding to one count of the ADC, depend on the input range selected. The value of these
voltages for each input range is given in Table 4-1.
Table 4-1. Voltage Values for Calculating Offset Error
Input Range
V
-fs
V
+fs
- 1
1 LSB
1
/
2
LSB
-10 to +10 V
-10 V
+9.99512 V
4.88 mV
2.44 mV
-5 to +5 V
-5 V
+4.99756 V
2.44 mV
1.22 mV
0 to 10 V
0 V
+9.99756 V
2.44 mV
1.22 mV
