NXP Semiconductors MPC5644A, Reference Manual, Page: 1129 / 1735

Enhanced Queued Analog-to-Digital Converter (EQADC)
MPC5644A Microcontroller Reference Manual, Rev. 6
Freescale Semiconductor 1129
25.6.6.7 ADC Calibration Feature
25.6.6.7.1 Overview
NOTE
Details of the recommended calibration procedure can be found in
Applications Note AN2989 “Design, Accuracy and Calibration of Analog
to Digital Converts on the MPC5500 family” available from
www.freescale.com. This contains recommended settings to avoid
unexpected behavior when performing calibration.
There are three sets of calibration coefficients for each ADC. Each set is composed by a gain factor and
an offset factor: GCCn/OCCn, ALTGCCn1/ALTGCCn1, and ALTGCCn2/ALTGCCn2, where n is the
ADC number 0 or 1. The pair GCCn/OCCn is selected when it is used the normal configuration or the
alternate configurations 3 to 8. The pair ALTGCCn1/ALTGCCn1 is used only when the alternate
configuration 1 is selected. And the pair ALTGCCn2/ALTGCCn2 is for the alternate configuration 2. The
description below is for a generic pair of gain/offset GCC/OCC.
The EQADC provides a calibration scheme to remove the effects of gain and offset errors from the results
generated by the on-chip ADCs. Only results generated by the on-chip ADCs are calibrated. The results
generated by ADCs on the external device are directly sent to RFIFOs unchanged. The main component
of calibration hardware is a Multiply-and-Accumulate (MAC) unit, one per on-chip ADC, that is used to
calculate the following transfer function which relates a calibrated result to a raw, uncalibrated one.
CAL_RES = GCC * R OCC+2;
where:
• CAL_RES is the calibrated result corresponding the input voltage V
i
.
• GCC is the gain calibration constant.
• RAW_RES is the raw, uncalibrated result with resolution adjustment corresponding to an specific
input voltage V
i
.
• OCC is the offset calibration constant.
• The addition of two reduces the maximum quantization error of the ADC. See Section 25.7.6.3,
Quantization Error Reduction During Calibration .
Calibration constants GCC and OCC are determined by taking two samples of known reference voltages
and using these samples to calculate the values for the constants. For details and an example about how to
calculate the calibration constants and use them in result calibration refer to
Section 25.7.6, ADC Result
Calibration . Once calculated, GCC is stored in the Section 25.5.3.4, ADC0/1 Gain Calibration Constant
Registers (ADC0_GCCR and ADC1_GCCR) , and OCC in Section 25.5.3.5, ADC0/1 Offset Calibration
Constant Registers (ADC0_OCCR and ADC1_OCCR) , from where their values are fed to the MAC unit.
The alternate gain values are stored in Section 25.5.3.7, ADC0/1 Alternate Gain Registers
(ADC0_AGR1-2 and ADC1_AGR1-2) , and the alternate offset values in Section 25.5.3.8, ADC0/1
Alternate Offset Register (ADC0_AOR1-2 and ADC1_AOR1-2) . Since the analog characteristics of each
on-chip ADCs differs, each ADC has an independent pair of calibration constants.