AD7982 datasheet(20/23 Pages) AD

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AD7982

Preliminary Technical Data

Rev PrC | Page 20 of 23

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy chain multiple AD7982s on

a 3-wire serial interface while providing a BUSY indicator. This

feature is useful for reducing component count and wiring

connections, e.g., in isolated multiconverter applications or for

systems with a limited interfacing capacity. Data readback is

analogous to clocking a shift register.

A connection diagram example using three AD7982s is shown

in Figure 26 and the corresponding timing is given in Figure 27.

When SDI and CNV are low, SDO is driven low. With SCK

high, a rising edge on CNV initiates a conversion, selects the

chain mode, and enables the BUSY indicator feature. In this

mode, CNV is held high during the conversion phase and the

subsequent data readback. When all ADCs in the chain have

completed their conversions, the nearend ADC ( ADC C in

Figure 26) SDO is driven high. This transition on SDO can be

used as a BUSY indicator to trigger the data readback controlled

by the digital host. The AD7982 then enters the acquisition

phase and powers down. The data bits stored in the internal

shift register are then clocked out, MSB first, by subsequent

SCK falling edges. For each ADC, SDI feeds the input of the

internal shift register and is clocked by the SCK falling edge.

Each ADC in the chain outputs its data MSB first, and 18 × N +

1 clocks are required to readback the N ADCs. Although the

rising edge can be used to capture the data, a digital host using

the SCK falling edge allows a faster reading rate and,

consequently more AD7982s in the chain, provided the digital

host has an acceptable hold time.

CNV

SCK

SDO

SDI

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

CNV

SCK

SDO

SDI

CNV

SCK

SDO

SDI

AD7982

Figure 26. Chain Mode with BUSY Indicator Connection Diagram

SDOA = SDIB

DA17 DA16 DA15

SCK

123

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDIA

DA1

417

tSCK

tSCKH

tSCKL

DA0

SDOB = SDIC

DB17 DB16 DB15

DA1

DB1DB0DA17 DA16

tSSDISCK

tHSDISC

tHSDO

tDSDO

SDOC

DC17 DC16 DC15

DA1DA0

DC1DC0DA16

DB1DB0DA17

DB17 DB16

tDSDOSDI

tSSCKCNV

tHSCKCNV

DA0

tDSDOSDI

Figure 27. Chain Mode with BUSY Indicator Serial Interface Timing

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