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Calibration Procedures
Chapter 6
AT-MIO-64F-5 User Manual
6-8
© National Instruments Corporation
Offset error in the analog output circuitry is the total of the voltage offsets contributed by the
components in the output circuitry. This error, which is independent of the DAC output voltage,
is the amount of voltage generated by the DAC when it is set to produce 0 V. To correct this
offset error, the calibration routine should write a value of 0 to each DAC and adjust CALDAC4
and CALDAC6 until it measures the same voltage between each analog output and AO GND as
it measures with both inputs grounded.
Gain error in the analog output circuitry is the sum of the gain errors contributed by the
components in the output circuitry. This error is a voltage difference between the desired voltage
and the actual output voltage generated that is proportional to the DAC output voltage. To
correct the gain error, the output calibration routine should set each analog output to 5 V and
measure the difference between each output and the internal voltage reference. It should then set
each analog output to 0 and measure the difference between each output and AOGND. Finally, it
should measure the difference between AOGND and AIGND. All these measurements need to
be combined with the value of the onboard reference, as recovered from the EEPROM, to
accurately determine the actual magnitude of a nominal 5 V step on the analog outputs.
CALDAC5 and CALDAC7 should then be adjusted so that this value is exactly 5 V. This
procedure is insensitive to offset, gain, and linearity errors in the analog input circuitry. Gain
adjustment may, however, have a small effect on offset error, so for best results analog output
gain should be calibrated before analog output offset.
The nominal unadjusted gain error of each analog output channel is +0.25%. CALDAC5 and
CALDAC7 adjust this gain error by lowering the value of the reference voltage by an adjustable
amount between 0 and 50 mV, or 0 to 0.5% of 10 V. Thus, the nominal gain adjustment range
for a 10 V referenceÐinternal or externalÐis ±0.25%. However, the calibration mechanism is not
designed to accommodate other reference voltages. Thus, when using an external reference, it is
advisable to set the gain CALDAC to 0 and to account for the +0.25% gain error of the analog
output channel, either in software or with external hardware.
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