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Calibration
2
Calibration
Before delivery from the factory, the VMIVME-3126A High Resolution, Isolated
Analog-to-Digital Converter board is fully calibrated for each range and conforms to
all specifications.
The calculated gain and offset coefficients are only as good as the source used for
calibration. The calibration source should have very little noise. Although the
calibration routine takes an average of the input samples, a low noise source is
essential on the lower ranges to get maximum results. If the accuracy of the source is
in question, a precision multimeter can be connected to the board’s input to measure
the exact input voltage. This reading would then be used in computing the value to
enter into the TCVH and TCVL registers.
A recommended source is the Hewlett-Packard HP3245A Universal Source. In
addition to the accurate, low noise output, this source also has a General-Purpose
Instrumentation Bus (GPIB) interface. This enables calibration to be handled entirely
by a PC, commanding the source which determines the voltage to output. A
recommended multimeter is the Hewlett-Packard HP3458A. This meter also has a
GPIB interface to aid in automating the calibration process.
The voltages used for calibration do not need to be symmetrical around ground for
bipolar or mid-scale for unipolar scales. Due to the presence of noise in any system,
the voltages used should not be very close to positive or negative full-scale. If voltages
near the extremes were used, noise could cause the value to clip to full-scale and
degrade the accuracy. With this in mind, Table 2-3 below shows a guideline to follow
for the calibration voltages.
Table 2-3 Calibration Guidelines
Input Scale
3-Point, %Full-Scale
5-Point, %Full-Scale
7-Point, %Full-Scale
Unipolar
75%, 50%, 25%
80%, 60%, 50%, 40%,
20%
80%, 70%, 60%, 50%,
40%, 20%, 10%
Bipolar
±75%, 0%
±75%, ±25%, 0%
±75%, ±50%, ±25%, 0%
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