AD7705/AD7706

–11–

REV. A

Table VI. Register Selection

RS2

RS1

RS0

Register

Register Size

0

0

0

Communications Register

8 Bits

0

0

1

Setup Register

8 Bits

0

1

0

Clock Register

8 Bits

0

1

1

Data Register

16 Bits

1

0

0

Test Register

8 Bits

1

0

1

No Operation

1

1

0

Offset Register

24 Bits

1

1

1

Gain Register

24 Bits

R/

W

Read/

Write Select. This bit selects whether the next operation is a read or write operation to the selected register.

A “0” indicates a write cycle for the next operation to the appropriate register, while a “1” indicates a read opera-

tion from the appropriate register.

STBY

Standby. Writing a “1” to this bit puts the part into its standby or power-down mode. In this mode, the part con-

sumes only 10

µA of power supply current. The part retains its calibration coefficients and control word informa-

tion when in STANDBY. Writing a “0” to this bit places the part in its normal operating mode.

CH1–CH0

Channel Select. These two bits select a channel for conversion or for access to the calibration coefficients as out-

lined in Table VII. Three pairs of calibration registers on the part are used to store the calibration coefficients

following a calibration on a channel. They are shown in Tables VII for the AD7705 and Table VIII for the AD7706

to indicate which channel combinations have independent calibration coefficients. With CH1 at Logic 1 and CH0

at a Logic 0, the part looks at the AIN1(–) input internally shorted to itself on the AD7705 or at COMMON

internally shorted to itself on the AD7706. This can be used as a test method to evaluate the noise performance of

the part with no external noise sources. In this mode, the AIN1(–)/COMMON input should be connected to

an external voltage within the allowable common-mode range for the part.

Table VII. Channel Selection for AD7705

CH1

CH0

AIN(+)

AIN(–)

Calibration Register Pair

0

0

AIN1(+)

AIN1(–)

Register Pair 0

0

1

AIN2(+)

AIN2(–)

Register Pair 1

1

0

AIN1(–)

AIN1(–)

Register Pair 0

1

1

AIN1(–)

AIN2(–)

Register Pair 2

Table VIII. Channel Selection for AD7706

CH1

CH0

AIN

Reference

Calibration Register Pair

0

0

AIN1

COMMON

Register Pair 0

0

1

AIN2

COMMON

Register Pair 1

1

0

COMMON

COMMON

Register Pair 0

1

1

AIN3

COMMON

Register Pair 2