AD7707

Rev. B | Page 18 of 52

ON-CHIP REGISTERS

The AD7707 contains eight on-chip registers that can be

accessed via the serial port of the part. The first of these is a

communications register that controls the channel selection,

decides whether the next operation is a read or write operation

and selects which register the next read or write operation

accesses. All communications to the part must start with a write

operation to the communications register. After power-on or

RESET, the device expects a write to its communications register.

The data written to this register determines whether the next

operation to the part is a read or a write operation and

determines to which register this read or write operation occurs.

Therefore, write access to any of the other registers on the part

starts with a write operation to the communications register

followed by a write to the selected register. A read operation

from any other register on the part (including the communications

register itself and the data register) starts with a write operation

to the communications register followed by a read operation

from the selected register. The communications register also

controls the standby mode and channel selection and the DRDY

status is available by reading from the communications register.

The second register is a setup register that determines calibration

mode, gain setting, bipolar/unipolar operation, and buffered

mode. The third register is the clock register and contains the

filter selection bits and clock control bits. The fourth register is

the data register from which the output data from the part is

accessed. The final registers are the calibration registers, which

store channel calibration data. The registers are described in

more detail in the following sections.

COMMUNICATIONS REGISTER (RS2, RS1, RS0 = 0,

0, 0)

The communications register is an 8-bit register from which

data can either be read or to which data can be written. All

communications to the part must start with a write operation to

the communications register. The data written to the commu-

nications register determines whether the next operation is a

read or write operation and to which register this operation

takes place. When the subsequent read or write operation to the

selected register is complete, the interface returns to where it

expects a write operation to the communications register. This

is the default state of the interface, and on power-up or after a

RESET, the AD7707 is in this default state waiting for a write

operation to the communications register. In situations where

the interface sequence is lost, if a write operation of sufficient

duration (containing at least 32 serial clock cycles) takes place

with DIN high, the AD7707 returns to this default state.

outlines the bit designations for the communications register.

Table 14

Table 14. Communications Register

0/DRDY (0)

RS2 (0)

RS1 (0)

RS0 (0)

R/W (0)

STBY (0)

CH1 (0)

CH0 (0)

Table 15. Communications Register Bit Descriptions

Bit

Description

0/DRDY

For a write operation, a 0 must be written to this bit so that the write operation to the communications register actually takes place. If a

1 is written to this bit, the part does not clock on to subsequent bits in the register. The serial interface stays at this bit location

until a 0 is written to this bit. Once a 0 is written to this bit, the next seven bits are loaded to the communications register. For a read

operation, this bit provides the status of the DRDY flag from the part. The status of this bit is the same as the DRDY output pin.

RS2 to

RS0

Register selection bits. These three bits select to which one of eight on-chip registers the next read or write operation takes

place, as shown in Table 16, along with the register size. When the read or write operation to the selected register is complete,

the part waits for a write operation to the communications register. It does not remain in a state where it continues to access the

register.

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 operation 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 consumes only

8 μA of power supply current. The part retains its calibration coefficients and control word information when in standby. Writing

a 0 to this bit places the part in its normal operating mode. The serial interface on the AD7707 remains operational when the

part is in standby mode.

CH1,

CH0

Channel select. These two bits select a channel for conversion or for access to the calibration coefficients as outlined in Table 17.

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 Table 17 for the AD7707 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 LOCOM input internally shorted to itself. 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 LOCOM input should be

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