MAC7200 datasheet(859/976 Pages) FREESCALE

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Initialization/Application Information

MAC7200 Microcontroller Family Reference Manual, Rev. 2

Freescale Semiconductor

781

33.12.2.3.3

Determine Gain Constant GCC / Determine Offset Constant OCC

The ideal transfer curve put into a coordinate system is linear, starting in the origin with a slope of 1. The

real device behavior can slightly differ. As the ATD is very linear the only two transfer curve factors that

can vary are the slope (gain) and the origin (offset). Both values must be determined and adjusted.

To determine them two measurements are required. The first measurement (RAW75) is done at 75% of the

maximum voltage (See 33.12.2.3.1 Sample Special Channel 75% x (VRH - VRL)). The second one

(RAW25) is done at 25% of the maximum voltage (See 33.12.2.3.2 Sample Special Channel

25% x (VRH - VRL)). Both values are compared with the expected values (EXP75 and EXP25):

Eqn. 33-6

Eqn. 33-7

The expected values are:

Eqn. 33-8

Eqn. 33-9

Now the gain can be calculated:

Eqn. 33-10

The offset can be calculated either like this:

Eqn. 33-11

or like this:

Eqn. 33-12

The ‘0.5’ in Equation 33-11 and Equation 33-12 moves the ATD transfer curve down by half an LSB and

is therefore reducing the Total Unadjusted Error (TUE) by half an LSB.

The Gain Calibration Constant can be calculated like this:

Eqn. 33-13

For details about GCC, see Section 33.10.2.5, “ATD Calibration Register (ATDCAL).

For offset values >= 0 the Offset Calibration Constant can be calculated like this:

Eqn. 33-14

For offset values < 0 the Offset Calibration Constant can be calculated like this:

Eqn. 33-15

EXP

Gain

RAW

Offset

0.5

EXP

Gain

RAW

Offset

0.5

EXP

0.75

4096

3072

EXP

0.25

4096

1024

Gain

EXP

–

RAW

–

---------------------------------------------

3072

1024

–

RAW

–

---------------------------------------------

2048

RAW

–

---------------------------------------------

Offset

EXP

Gain

RAW

0.5

–

3072

Gain

RAW

–

0.5

–

Offset

EXP

Gain

RAW

0.5

–

1024

Gain

RAW

0.5

–

GCC

Gain

16384

OCC

Offset

OCC

Offset

16384

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