User`s manual

4 SELF-CALIBRATION in the PCI-DAS1602/16
The PCI-DAS1602/16 provides self-calibration of the analog source and measure systems thereby
eliminating the need for external equipment and user adjustments. All adjustments are made via 8-bit
calibration DACs or digital potentiometers referenced to an on-board factory calibrated standard. The
PCI-DAS1602/16 is shipped fully-calibrated from the factory with cal coefficients stored in nvRAM. At
run time, these calibration factors are loaded into system memory and are automatically retrieved each
time a different DAC/ADC range is specified. The user has the option to recalibrate with respect to the
factory-measured voltage standards at any time by simply selecting the "Calibrate" option in InstaCal.
Full calibration typically requires less than two minutes and requires no user intervention.
4.1 ANALOG INPUT CALIBRATION
A variety of methods are used to calibrate the different elements on the board. The analog front-end has
several "knobs" to turn. Offset calibration is performed in both the instrumentation amplifier gain stage
and the ADC itself. Front-end gain adjustment is performed only via the ADC reference. This strategy
was chosen since the gain tolerance of the in-amp circuit is quite good and there is adequate gain tuning
range using only the ADC.
The analog output circuits are calibrated for gain and offset as well. Offset adjustments for the analog
output are made in the output buffer section. The tuning range of this adjustment allows for max DAC
and output buffer offsets. Gain calibration of the analog outputs are performed via DAC reference
adjustments.
Figure 4.1 below is a block diagram of the analog front-end calibration system:
Analog-In
ADC
Cal
Ref
Trim Dac
Offset
RefOffset Adj
Offset Adj
Uni/Bip
Digital Gain Pot
Digital Offset Pot
Figure 4-1. Analog Input Front-End Calibration System
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