AD7262BSTZ datasheet(28/32 Pages) AD

28 / 32 page

AD7262

Rev. 0 | Page 28 of 32

ADJUSTING THE OFFSET CALIBRATION REGISTERS

The internal offset calibration register can be adjusted manually

to compensate for any signal path offset from the sensors to the

ADC. Here, no internal calibration is required, and the CAL pin

can remain at a low logic state. By changing the contents of the

offset register, different amounts of offset on the analog input

signal can be compensated for. To determine the digital code to

be written to the offset register

Configure the sensor to its offset state.

Perform a number of conversions using the AD7262/

AD7262-5.

Take the mean digital output code from both DOUTA

and DOUTB. This is a 12-bit result and the offset register

is 12 bits; thus, the result can be stored directly in the

offset register.

Write the digital code to the offset registers to calibrate the

AD7262/AD7262-5.

If a +10 mV offset is present in the analog input signal and the

gain of the PGA is 2, the code that needs to be written to the

offset register to compensate for the offset is

0000

0001

0000

(

If a − 10 mV offset is present in the analog input signal and the

gain of the PGA is 2, the code that needs to be written to the

offset register to compensate for the offset is

)

μV/

305

(

−

= −16.39 = 1000 0001 0000

SYSTEM GAIN CALIBRATION

The AD7262/AD7262-5 also allow the user to write to an

external gain register, thus enabling the removal of any overall

system gain error. Both ADC A and ADC B have independent

external gain registers, allowing the user to calibrate

independently the gain on both ADC A and ADC B signal

paths. The gain calibration feature can be used to implement

accurate gain matching between ADC A and ADC B.

The system calibration function is used by setting the sensors to

which the AD7262/AD7262-5 are connected to a 0 gain state.

The AD7262/AD7262-5 convert this analog input to a digital

output code, which corresponds to the system gain and is avail-

able on the DOUTx pins. This digital output code can then be stored

in the appropriate external register. For details on how to write to

a register, see the Writing to a Register section and Table 11.

The gain calibration register contains seven bits of data. By

changing the contents of the gain register, different amounts of

gain on the analog input signal can be compensated for. The

MSB is a sign bit, while the remaining six bits store the multiplica-

tion factor, which is used to adjust the analog input range. The

gain register value is effectively multiplied by the analog input

to scale the conversion result over the full range. Increasing the

gain register multiplication factor compensates for a larger

analog input range, and decreasing the gain register multiplier

compensates for a smaller analog input range. Each bit in the

gain calibration register has a resolution of 2.4 × 10−4 V (1/4096).

A maximum of 1.538% of the analog range can be calibrated for.

The multiplier factor stored in the gain register can be decoded

as outlined in Table 13.

The gain registers can be cleared by writing all 0s to each register,

as described in the Writing to a Register section. For accurate

gain calibration, both the positive and negative full-scale digital

output codes should be measured prior to determining the

multiplication factor that is written to the gain register.

Table 13. Decoding of Multiplication Factors for Gain Calibration

Analog Input

Digital Gain

Error

Gain Register

Code

Multiplier

Equation

Multiplier

Value

Comments

LSB

(Sign bit + 6 bits)

(1 ± x/4096)

VIN max

0 LSB

0 000000

1 − 0/4096

Sign bit = 0, which implies negative sign

in multiplier equation

VIN max – 244 μV

−2 LSB

0 000001

1 − 1/4096

0.999755859

Sign bit = 0, which implies negative sign

in multiplier equation

VIN max − (63 × 244 μV)

−126 LSB

0 111111

1 − 63/4096

0.98461914

Sign bit = 0, which implies negative sign

in multiplier equation

VIN max

0 LSB

1 000000

1 + 0/4096

Sign bit = 1, which implies plus sign in

multiplier equation

VIN max + 244 μV

+2 LSB

1 000001

1 + 1/4096

1.000244141

Sign bit = 1, which implies plus sign in

multiplier equation

VIN max + (63 × 244 μV)

+126 LSB

1 111111

1 + 63/4096

1.015380859

Sign bit = 1, which implies plus sign in

multiplier equation

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