AD7980BCPZ-R2 datasheet(23/28 Pages) AD

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Data Sheet

AD7980

Rev. F | Page 21 of 26

4-WIRE CS MODE WITH BUSY INDICATOR

This mode is usually used when a single AD7980 is connected

to an SPI-compatible digital host that has an interrupt input,

and 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 37, and the

corresponding timing is given in Figure 38.

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 AD7980 then enters the acquisition phase

and powers down. 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.

Figure 37. 4-Wire CS Mode with Busy Indicator Connection Diagram

Figure 38. 4-Wire CS Mode with Busy Indicator Serial Interface Timing

AD7980 SDO

SDI

DATA IN

IRQ

DIGITAL HOST

CONVERT

CS1

CLK

VIO

47kΩ

CNV

SCK

tCONV

tCYC

AQUISITION

tSSDICNV

AQUISITION

tACQ

tSCK

tSCKH

tSCKL

CONVERSION

SDI

tHSDICNV

SCK

CNV

SDO

tEN

D15

D14

tHSDO

tDSDO

tDIS

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