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–13–

REV. A

AD7851

FINISHED

LAST

WRITE

OPERATION

ABORT

YES

CAL REGISTER POINTER IS

AUTOMATICALLY INCREMENTED

READ CAL REGISTER

CAL REGISTER POINTER IS

AUTOMATICALLY RESET

WRITE TO CONTROL REGISTER SETTING STCAL = 0, RDSLT1 = 1,

RDSLT0 = 0, AND CALSLT1, CALSLT0 = 00, 01, 10, 11

START

Figure 9. Flowchart for Reading from the Calibration

Registers

Adjusting the Offset Calibration Register

The offset calibration register contains 16 bits, two leading zeros

and 14 data bits. By changing the contents of the offset register,

different amounts of offset on the analog input signal can be

compensated for. Increasing the number in the offset calibration

nal, and decreasing the number in the offset calibration register

compensates for positive offset on the analog input signal. The

default value of the offset calibration register is 0010 0000 0000

0000 approximately. This is not an exact value, but the value in

the offset register should be close to this value. Each of the 14

data bits in the offset register is binary weighted; the MSB has a

weighting of 5% of the reference voltage, the MSB-1 has a

weighting of 2.5%, the MSB-2 has a weighting of 1.25%, and so

on down to the LSB which has a weighting of 0.0006%.

This gives a resolution of

±0.0006% of V

REF approximately.

More accurately the resolution is

±(0.05 × V

REF)/2

13 volts =

±0.015 mV, with a 2.5 V reference. The maximum offset that

can be compensated for is

±5% of the reference voltage, which

equates to

±125 mV with a 2.5 V reference and ±250 mV with a

5 V reference.

Q. If a +20 mV offset is present in the analog input signal and the

reference voltage is 2.5 V, what code needs to be written to the

offset register to compensate for the offset?

A. 2.5 V reference implies that the resolution in the offset reg-

ister is 5%

× 2.5 V/213 = 0.015 mV. +20 mV/0.015 mV =

1310.72; rounding to the nearest number gives 1311. In

binary terms this is 0101 0001 1111, therefore decrease the

offset register by 0101 0001 1111.

This method of compensating for offset in the analog input sig-

nal allows for fine tuning the offset compensation. If the offset

on the analog input signal is known, there will be no need to

apply the offset voltage to the analog input pins and do a system

calibration. The offset compensation can take place in software.

Adjusting the Gain Calibration Register

The gain calibration register contains 16 bits, two leading 0s

and 14 data bits. The data bits are binary weighted as in the off-

set calibration register. The gain register value is effectively mul-

tiplied by the analog input to scale the conversion result over the

full range. Increasing the gain register compensates for a

smaller analog input range and decreasing the gain register com-

pensates for a larger input range. The maximum analog input

range that the gain register can compensate for is 1.025 times

the reference voltage, and the minimum input range is 0.975

times the reference voltage.

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