National Instruments Low-Cost Multifunction I/O Board for ISA Lab-PC+ - Manual (Page 76)

Chapter 5 Calibration
© National Instruments Corporation 5-3 Lab-PC+ User Manual
Analog Input Calibration
To null out error sources that compromise the quality of measurements, you must calibrate the
analog input circuitry by adjusting the following potential sources of error:
• Offset errors
• Gain error of the analog input circuitry
You must perform the calibration if you change the input configuration from bipolar (the factory
setting) to unipolar.
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 instrumentation amplifier. Other
sources of offset error include the track-and-hold amplifier and the ADC. On the Lab-PC+, two
trimpots are used to null out all of these offset sources. The first trimpot is used to null out the
input offset (up to and including the instrumentation amplifier). To null out this offset, ground
the input channel and adjust R7 until the readings at gains of 1 and 50 are the same. Then, to
null out the output offset, adjust R6 until the readings are
±
0.5 LSB. Because one of these error
sources is gain-dependent, you should check and recalibrate the offset, if necessary, whenever
the gain is changed significantly. Alternatively, you can calibrate the input offset at gain = 1 and
note the offset errors for all other gains. You can then apply a software correction to the readings
at gains higher than one by subtracting the offset errors. With this method, you can use the board
at all available gain levels without recalibrating the input.
The maximum offset at the gain amplifier is specified at 0.5 mV. The maximum possible
contribution of the gain amplifier to the total offset is therefore 0.5 mV multiplied by the gain.
To find the error in LSBs, divide this voltage by the voltage of 1 LSB. Hence, with a large gain
change, such as from 1 to 100, the number of LSBs offset from this source changes from about
0.2 to almost 20. Clearly, an adjustment that is acceptable for a 0.2 LSB error is probably not
suitable when the error is multiplied by 100. For small changes in the gain, the error that
accompanies changes in gain is much less. If the gain is changed from 1 to 2 or 5, the offset
probably does not need to be recalibrated. Likewise, changes between gains of 20, 50, or 100
probably do not require recalibration.
All the stages up to and including the input to the ADC contribute to the gain error of the analog
input circuitry. With the amplifier set to a gain of 1, the gain of the 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
-1.5 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 5-1.