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AD7984

Data Sheet

CS MODE, 4-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7984 is connected

to an SPI-compatible digital host with an interrupt input and

when it is desired to keep CNV, which is used to sample the

analog input, independent of the signal used to select the data

reading. This independence is particularly important in

applications where low jitter on CNV is desired.

The connection diagram is shown in Figure 34, and the

corresponding timing is given in Figure 35.

With SDI high, a rising edge on CNV initiates a conversion,

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

mode, CNV must be held high during the conversion phase and

the subsequent data readback. (If SDI and CNV are low, SDO is

driven low.) Prior to the minimum conversion time, SDI can be

used to select other SPI devices, such as analog multiplexers,

but SDI must be returned low before the minimum conversion

time elapses and then held low for the maximum possible

conversion time to guarantee the generation of the busy signal

indicator. When the conversion is complete, SDO goes from

high impedance to low impedance. With a pull-up on the SDO

line, this transition can be used as an interrupt signal to initiate

the data readback controlled by the digital host. The AD7984

then enters the acquisition phase and goes into standby mode.

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 be used to 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 19th SCK

falling edge or SDI going high (whichever occurs first), SDO

returns to high impedance.

Figure 34. CS Mode, 4-Wire with Busy Indicator Connection Diagram

Figure 35. CS Mode, 4-Wire with Busy Indicator Serial Interface Timing

AD7984

SDI

SDO

CNV

SCK

CONVERT

DATA IN

CLK

DIGITAL HOST

IRQ

VIO

47kΩ

CS1

SDO

D17

D16

tDIS

SCK

tSCK

tSCKL

tSCKH

tHSDO

tDSDO

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

SDI

CNV

tSSDICNV

tHSDICNV

Rev. B | Page 20 of 24

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