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AD7983

Data Sheet

Rev. B | Page 20 of 24

CS MODE, 4-WIRE WITH BUSY INDICATOR

This mode is usually used when a single AD7983 is connected

to an SPI-compatible digital host that has 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 requirement is particularly important in

applications where low jitter on CNV is desired.

The connection diagram is shown in Figure 32, and the

corresponding timing is given in Figure 33.

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 conversion time to

guarantee the generation of the busy signal indicator. When the

conversion is complete, SDO goes from high impedance to low.

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 AD7983 then enters the acquisition phase

and goes into standby mode. The data bits are 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 17th SCK falling edge or SDI going high, whichever is

earlier, the SDO returns to high impedance.

AD7983 SDO

SDI

DATA IN

IRQ

DIGITAL HOST

CONVERT

CS1

CLK

VIO

47kΩ

CNV

SCK

Figure 32. CS Mode, 4-Wire with Busy Indicator Connection Diagram

tCONV

tCYC

ACQUISITION

tSSDICNV

ACQUISITION

tACQ

tSCK

tSCKH

tSCKL

CONVERSION

SDI

tHSDICNV

SCK

CNV

SDO

tEN

D15

D14

tHSDO

tDSDO

tDIS

Figure 33. CS Mode, 4-Wire with Busy Indicator Serial Interface Timing

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