AD7788 데이터 시트보기 (PDF) - Analog Devices

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AD7788

Low Power, 16-/24-Bit Sigma-Delta ADC

Analog Devices 

AD7788/AD7789

ON-CHIP REGISTERS

The ADC is controlled and configured via a number of on-chip registers, which are described on the following pages. In the following

descriptions, set implies a Logic 1 state and cleared implies a Logic 0 state, unless otherwise stated.

COMMUNICATIONS REGISTER (RS1, RS0 = 0, 0)

The communications register is an 8-bit write-only register. All communications to the part must start with a write operation to the com-

munications register. The data written to the communications register determines whether the next operation is a read or write operation,

and to which register this operation takes place. For read or write operations, once 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 ADC is in this default state waiting for a write operation to the communications regis-

ter. In situations where the interface sequence is lost, a write operation of at least 32 serial clock cycles with DIN high returns the ADC to

this default state by resetting the entire part. Table 7 outlines the bit designations for the communications register. CR0 through CR7 indi-

cate the bit location, CR denoting the bits are in the communications register. CR7 denotes the first bit of the data stream. The number in

brackets indicates the power-on/reset default status of that bit.

CR7

CR6

WEN(0)

0(0)

CR5

RS1(0)

CR4

RS0(0)

CR3

R/W(0)

CR2

CREAD(0)

CR1

CH1(0)

CR0

CH0(0)

Table 7. Communications Register Bit Designations

Bit Location Bit Name

Description

CR7

WEN

Write Enable Bit. A 0 must be written to this bit so that the write to the communications register actually

occurs. If a 1 is the first bit written, the part will not clock on to subsequent bits in the register. It will stay

at this bit location until a 0 is written to this bit. Once a 0 is written to the WEN bit, the next seven bits

will be loaded to the communications register.

CR6

0

This bit must be programmed with a Logic 0 for correct operation.

CR5–CR4

RS1–RS0

Register Address Bits. These address bits are used to select which of the ADC’s registers are being

selected during this serial interface communication. See Table 8.

CR3

R/W

A 0 in this bit location indicates that the next operation will be a write to a specified register. A 1 in this

position indicates that the next operation will be a read from the designated register.

CR2

CREAD

Continuous Read of the Data Register. When this bit is set to 1 (and the data register is selected), the

serial interface is configured so that the data register can be continuously read, i.e., the contents of the

data register are placed on the DOUT pin automatically when the SCLK pulses are applied. The commu-

nications register does not have to be written to for data reads. To enable continuous read mode, the

instruction 001111XX must be written to the communications register. To exit the continuous read

mode, the instruction 001110XX must be written to the communications register while the RDY pin is

low. While in continuous read mode, the ADC monitors activity on the DIN line so that it can receive the

instruction to exit continuous read mode. Additionally, a reset will occur if 32 consecutive 1s are seen on

DIN. Therefore, DIN should be held low in continuous read mode until an instruction is to be written to

the device.

CR1–CR0

CH1–CH0

These bits are used to select the analog input channel. The differential channel can be selected

(AIN(+)/AIN(–)) or an internal short (AIN(–)/AIN(–)) can be selected. Alternatively, the power supply can

be selected, i.e., the ADC can measure the voltage on the power supply, which is useful for monitoring

power supply variation. The power supply voltage is divided by 5 and then applied to the modulator for

conversion. The ADC uses a 1.17 V ± 5% on-chip reference as the reference source for the analog to

digital conversion. Any change in channel resets the filter and a new conversion is started.

Table 8. Register Selection

RS1 RS0 Register

0

0

Communications Register

during a Write Operation

0

0

Status Register during a

Read Operation

0

1

Mode Register

1

0

Reserved

1

1

Data Register

Register Size

8-Bit

8-Bit

8-Bit

8-Bit

16-Bit (AD7788)

24-Bit (AD7789)

Table 9. Channel Selection

CH1 CH0 Channel

0 0 AIN(+) – AIN(–)

0 1 Reserved

1 0 AIN(–) – AIN(–)

1

1

VDD Monitor

Rev. 0 | Page 11 of 20

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