AD7264BSTZ datasheet(26/32 Pages) AD

26 / 32 page

AD7264

Rev. A | Page 26 of 32

ADJUSTING THE OFFSET CALIBRATION REGISTER

The internal offset calibration register can be adjusted manually

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

ADC. 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. Use the following steps 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 AD7264.

Take the mean digital output code from both DOUTA and

DOUTB. This is a 14-bit result but the offset register is only

12 bits; thus, the 14-bit result needs to be converted to a

12-bit result that can be stored in the offset register. This is

achieved by keeping the sign bit and removing the second

and third MSBs.

The resultant digital code can then be written to the offset

registers to calibrate the AD7264.

Example:

Mean digital code from DOUTA = 8100 (01 1111 1010 0100)

Code written to offset register = 0111 1010 0100

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

(

= 65.57 = 0000 0100 0001

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

(

−

= −65.57 = 1000 0100 0001

SYSTEM GAIN CALIBRATION

The AD7264 also allows the user to write to an external 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 AD7264 is connected to a 0 gain state. The AD7264

converts this analog input to a digital output code, which

corresponds to the system gain and is available on the DOUT 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 (V)

Digital Gain

Error (LSB)

Gain Register

Code

(Sign Bit + 6 Bits)

Multiplier

Equation

(1 ± x/4096)

Multiplier

Value

Comments

VIN max

0 LSB

0 000000

1 − 0/4096

Sign bit = 0; negative sign in multiplier

equation

VIN max − 244 μV

−2 LSB

0 000001

1 − 1/4096

0.999755859

Sign bit = 0; negative sign in multiplier

equation

VIN max − (63 × 244 μV)

−126 LSB

0 111111

1 − 63/4096

0.98461914

Sign bit = 0; negative sign in multiplier

equation

VIN max

0 LSB

1 000000

1 + 0/4096

Sign bit = 1; plus sign in multiplier

equation

VIN max + 244 μV

+2 LSB

1 000001

1 + 1/4096

1.000244141

Sign bit = 1; plus sign in multiplier

equation

VIN max + (63 × 244 μV)

+126 LSB

1 111111

1 + 63/4096

1.015380859

Sign bit = 1; plus sign in multiplier

equation

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