AD7687

Data Sheet

Rev. E | Page 20 of 26

CS MODE, 3-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7687 is connected

to an SPI-compatible digital host having an interrupt input.

Figure 36 shows the connection diagram and Figure 37 gives

the corresponding timing.

With SDI tied to VIO, a rising edge on CNV initiates a conversion,

selects the CS mode, and forces SDO to high impedance. SDO

is maintained in high impedance until the completion of the

conversion irrespective of the state of CNV. Prior to the minimum

conversion time, CNV can be used to select other SPI devices,

such as analog multiplexers, but CNV must be returned low

before the minimum conversion time and held low until the

maximum conversion time to guarantee the generation of the

is complete, SDO goes from high to low impedance. With a

pull-up on the SDO line, this transition can be used as an

interrupt signal to initiate the data reading controlled by the

digital host. When using this option, select the value of the pull-

up resistor such that it maintains an appropriate rise time on the

SDO line for the application. This is a function of the resistance

of the pull-up and the capacitance of the SDO line. The AD7687

then enters the acquisition phase and powers down. The data

bits are then clocked out, MSB first, by subsequent SCK falling

edges. The data is valid on both SCK edges. Although the rising

edge can capture the data, a digital host using the SCK falling

edge allows a faster reading rate (provided it has an acceptable

hold time). After the optional 17th SCK falling edge, or when

CNV goes high, whichever is earlier, SDO returns to high

impedance.

If multiple AD7687 devices are selected at the same time, the

SDO output pin handles this contention without damage or

induced latch-up. Keep this contention as short as possible to

limit extra power dissipation.

DATA IN

IRQ

CLK

CONVERT

VIO

DIGITAL HOST

47k

Ω

CNV

SCK

SDO

SDI

VIO

AD7687

Figure 36. CS Mode, 3-Wire with BUSY Indicator

Connection Diagram (SDI High)

SDO

D15

D14

D1

D0

SCK

1

2

3

15

16

17

CNV

CONVERSION

ACQUISITION

ACQUISITION

SDI = 1

Figure 37. CS Mode, 3-Wire with BUSY Indicator Serial Interface Timing (SDI High)