Z8F082ASH020SC datasheet(132/244 Pages) ZILOG

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PS024705-0405

P R E L I M I N A R Y

Analog-to-Digital Converter

Z8 Encore! XP® F08xA Series

Product Specification

114

Calibration and Compensation

The Z8 Encore! XP® F08xA Series ADC can be factory calibrated for offset error and gain

error, with the compensation data stored in Flash memory. Alternatively, user code can

perform its own calibration, storing the values into Flash themselves. Thirdly, the user

code can perform a manual offset calibration during DIFFERENTIAL mode operation.

Factory Calibration

Devices that have been factory calibrated contain 30 bytes of calibration data in the Flash

option bit space. This data consists of 3 bytes for each input mode. See “ZiLOG Calibra-

tion Bits” on page 145. There is 1 byte for offset, 2 bytes for gain correction.

User Calibration

If the user has precision references available, its own external calibration can be per-

formed using any of the three available input modes. Because the calibration data consid-

ers buffer offset and non-linearity, it is recommended that this calibration be performed

separately for each of the ADC input modes planned for use.

Manual Offset Calibration

When uncalibrated, the ADC has significant offset (up to 35 mV with unity gain and up to

250 mV in 20x gain mode). Subsequently, manual offset calibration capability is built into

the block. When the ADC Control Register 0 sets the input mode (ANAIN[2:0]) to MAN-

UAL OFFSET CALIBRATION mode, the differential inputs to the ADC are shorted

together by an internal switch. Reading the ADC value at this point produces 0 in an ideal

system. The value actually read is the ADC offset. This value can be stored in flash and

accessed by user code to compensate for the input offset error.

There is no provision for manual gain calibration.

Software Compensation Procedure

The value read from the ADC high and low byte registers are uncompensated. The user

mode software must apply gain and offset correction to this uncompensated value for

maximum accuracy. The following formula yields the compensated value:

ADCcomp = (ADCuncomp - OFFCAL) + ((ADC uncomp - OFFCAL)*GAINCAL)/2

where GAINCAL is the gain calibration byte, OFFCAL is the offset calibration byte and

ADCuncomp is the uncompensated value read from the ADC. The OFFCAL value is in

two’s complement format, as are the compensated and uncompensated ADC values.

The offset compensation is performed first, followed by the gain compensation. One bit of

resolution is lost because of rounding on both the offset and gain computations. As a result

the ADC registers read back 13 bits: 1 sign bit, two calibration bits lost to rounding and 10

data bits.

Note:

Z8F082ASH020SC Datasheet(PDF) 132 Page - Zilog, Inc.

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