AD7854ARS datasheet(21/28 Pages) AD

21 / 28 page

AD7854/AD7854L

REV. B

–21–

Self-Calibration Timing

Figure 29 shows the timing for a software full self-calibration.

Here the BUSY line stays high for the full length of the self-

calibration. A self-calibration is initiated by writing to the con-

trol register and setting the STCAL bit to 1. The BUSY line

goes high at the end of the write to the control register, and

BUSY goes low when the full self-calibration is complete after a

time tCAL as show in Figure 29.

t23

DATA LATCHED INTO

CONTROL REGISTER

Hi-Z

DATA

VALID

tCAL

DATA

BUSY

Figure 29. Timing Diagram for Full Self-Calibration

For the self-(gain + offset), self-offset and self-gain calibrations,

the BUSY line is triggered high at the end of the write to the

control register and stays high for the full duration of the self-

calibration. The length of time for which BUSY is high depends

on the type of self-calibration that is initiated. Typical values are

given in Table VIII. The timing diagram for the other self-

calibration options is similar to that outlined in Figure 29.

System Calibration Description

System calibration allows the user to remove system errors

external to the AD7854/AD7854L, as well as remove the errors

of the AD7854/AD7854L itself. The maximum calibration

range for the system offset errors is

±5% of VREF, and for the

system gain errors it is

±2.5% of VREF. If the system offset or

system gain errors are outside these ranges, the system calibration

algorithm reduces the errors as much as the trim range allows.

Figures 30 through 32 illustrate why a specific type of system

calibration might be used. Figure 30 shows a system offset cali-

bration (assuming a positive offset) where the analog input

range has been shifted upwards by the system offset after the

system offset calibration is completed. A negative offset may

also be removed by a system offset calibration.

MAX SYSTEM OFFSET

IS ±5% OF VREF

ANALOG

INPUT

RANGE

SYSTEM OFFSET

CALIBRATION

SYS OFFSET

AGND

VREF + SYS OFFSET

VREF – 1LSB

MAX SYSTEM FULL SCALE

IS ±2.5% FROM VREF

ANALOG

INPUT

RANGE

MAX SYSTEM OFFSET

IS ±5% OF VREF

VREF – 1LSB

SYS OFFSET

AGND

Figure 30. System Offset Calibration

Figure 31 shows a system gain calibration (assuming a system

full scale greater than the reference voltage) where the analog

input range has been increased after the system gain calibration

is completed. A system full-scale voltage less than the reference

voltage may also be accounted for a by a system gain calibration.

ANALOG

INPUT

RANGE

SYSTEM OFFSET

CALIBRATION

AGND

SYS FULL S.

VREF – 1LSB

MAX SYSTEM FULL SCALE

IS ±2.5% FROM VREF

ANALOG

INPUT

RANGE

VREF – 1LSB

SYS FULL S.

AGND

MAX SYSTEM FULL SCALE

IS ±2.5% FROM VREF

Figure 31. System Gain Calibration

Finally in Figure 32 both the system offset error and gain error

are removed by the system offset followed by a system gain cali-

bration. First the analog input range is shifted upwards by the

positive system offset and then the analog input range is

adjusted at the top end to account for the system full scale.

MAX SYSTEM FULL SCALE

IS ±2.5% FROM VREF

MAX SYSTEM OFFSET

IS ±5% OF VREF

ANALOG

INPUT

RANGE

SYSTEM OFFSET

CALIBRATION

FOLLOWED BY

SYSTEM GAIN

CALIBRATION SYS OFFSET

AGND

VREF + SYS OFFSET

VREF – 1LSB

ANALOG

INPUT

RANGE

MAX SYSTEM OFFSET

IS ±5% OF VREF

VREF – 1LSB

SYS OFFSET

AGND

SYS F.S.

MAX SYSTEM FULL SCALE

IS ±2.5% FROM VREF

Figure 32. System (Gain + Offset) Calibration

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