MAC7200 datasheet(854/976 Pages) FREESCALE

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Initialization/Application Information

MAC7200 Microcontroller Family Reference Manual, Rev. 2

776

Freescale Semiconductor

33.12.2.1 Gain error

33.12.2.1.1

Gain Error Correction via GCC (Gain Too High)

If the ATD has a gain which is too high, the maximum result value of 0xFFF will already be reached at Vx

(see the blue curve in Figure 33-23). This means that for all voltages higher than Vx the transfer curve will

be clipped and therefore the result will be 0xFFF.

The total unadjusted error (TUE), which is the difference between the ideal and the actual result value, is

shown in the bottom part of Figure 33-23: With the gain too high the TUE reaches its maximum at Vx.

From there it decreases again:

Figure 33-23. Gain Error (Gain Too High) Compensated via GCC and OCC

If the GCC value is chosen correctly the adjusted transfer curve (green curve in Figure 33-23a) will fit the

ideal transfer curve and the TUE will become zero for all results until Vx is reached. When reaching Vx

the TUE will increase to the maximum value (TUEmax) which is the same as in the unadjusted case.

Therefore TUEmax values in the unadjusted and adjusted cases are always the same.

33.12.2.1.2

Gain Error Correction via OCC (Gain Too High)

Another way to reduce the TUE is the offset adjustment. In the given example it is possible to half the

TUEmax by changing the offset: When setting the offset to TUEmax / 2 the transfer curve (magenta curve

in Figure 33-23b) will be moved down. The TUE in this case is shown at the bottom of Figure 33-23b. It

VRH-VRL

VANx

Counts

0xFFF

VRH-VRL

Ideal transfer

curve

Gain too high

Corrected

0x000

TUE [counts]

VANx

Counts

VRH-VRL

Gain too high

0x000

Corrected via OCC

Ideal transfer

curve

TUE [counts]

VANx

via GCC

VRH-VRL

a) Gain adjustment: GCC < 1

b) Offset adjustment: OCC < 0

TUEmax

Gain too high

Corrected via

OCC and GCC

0xFFF

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