AT90PWM2 datasheet(242/361 Pages) ATMEL

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4317J–AVR–08/10

AT90PWM2/3/2B/3B

21.6.3

Offset Compensation Schemes

The gain stage has a built-in offset cancellation circuitry that nulls the offset of differential mea-

surements as much as possible. The remaining offset in the analog path can be measured

directly by shortening both differential inputs using the AMPxIS bit with both inputs unconnected.

(See “Amplifier 0 Control and Status register – AMP0CSR” on page 255. and See “Amplifier

1Control and Status register – AMP1CSR” on page 256.). This offset residue can be then sub-

tracted in software from the measurement results. Using this kind of software based offset

correction, offset on any channel can be reduced below one LSB.

21.6.4

ADC Accuracy Definitions

An n-bit single-ended ADC converts a voltage linearly between GND and V

REF in 2

n steps

(LSBs). The lowest code is read as 0, and the highest code is read as 2

n-1.

Several parameters describe the deviation from the ideal behavior:

•

Offset: The deviation of the first transition (0x000 to 0x001) compared to the ideal transition

(at 0.5 LSB). Ideal value: 0 LSB.

Figure 21-10. Offset Error

•

Gain Error: After adjusting for offset, the Gain Error is found as the deviation of the last

transition (0x3FE to 0x3FF) compared to the ideal transition (at 1.5 LSB below maximum).

Ideal value: 0 LSB

Output Code

VREF Input Voltage

Ideal ADC

Actual ADC

Offset

Error

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