MAC7200 datasheet(857/976 Pages) FREESCALE

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Initialization/Application Information

MAC7200 Microcontroller Family Reference Manual, Rev. 2

Freescale Semiconductor

779

Figure 33-26. Offset Error Compensated via OCC

33.12.2.3 Steps Required For Calibration

To reduce the total unadjusted error of the conversion results the ATD provides a multiply-and-add

mechanism (MADD) which will adjust the raw conversion result. This adjustment requires two factors:

Gain Calibration Constant (GCC) and Offset Calibration Constant (OCC). The determination of these two

factors is called “calibration.” To determine the two factors the following must be done:

1. Execute initialization steps 1 to 4. (see Section 33.12.1, “Initialization”)

2. Sample special channel 75% x (VRH -VRL) multiple times

3. Sample special channel 25% x (VRH -VRL) multiple times

4. Determine gain calibration constant GCC from Gain

5. Determine offset calibration constant OCC from Offset

6. Write GCC and OCC to ATDCAL

This sequence is described in details in the following sections.

The calibration must always be repeated when influence factors for the ATD accurracy change. As the

ATD supply voltages are not expected to drift the most important influence factor is the temperature.

Although the ATD is very temperature resistant in high accurracy applications a drift might get visible.

The temperature usually changes rapidly when the device is turned on and the ambient temperature is very

low. During this time the gain and the offset are expected to change. But once the device heated up the gain

and offset should remain constant.

VRH-VRL

VANx

Counts

0x3FF

VRH-VRL

Ideal transfer

curve

0x000

TUE [counts]

VANx

0xF

Adjusted

via OCC

Offset > 0

Offset < 0

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