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Data Sheet

AD7982

Rev. D | Page 23 of 25

CHAIN MODE WITH BUSY INDICATOR

Chain mode with busy indicator can also daisy-chain multiple

AD7982 devices on a 3-wire serial interface while providing a

busy indicator. This chain mode with busy indicator feature

reduces component count and wiring connections, for example,

in isolated multiconverter applications or for systems with a

limited interfacing capacity. Data readback is analogous to

clocking a shift register.

Figure 39 shows a connection diagram example using three

AD7982 devices, and Figure 40 shows the corresponding timing.

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 SDO pin of the ADC

closest to the digital host (see the AD7982 ADC labeled C in

Figure 39) is driven high. The transition of driving the SDO pin

of the ADC to high 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 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 read back the N ADCs.

Although the rising edge can capture the data, a digital host using

the SCK falling edge allows a faster reading rate and

consequently more AD7982 devices in the chain, provided the

digital host has an acceptable hold time.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7982

SDI

SDO

CNV

SCK

AD7982

SDI

SDO

CNV

SCK

IRQ

AD7982

SDI

SDO

CNV

SCK

Figure 39. 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

tHSDISCK

tHSDO

tDSDO

SDOC

DC17 DC16 DC15

DA1DA0

DC1DC0DA16

DB1DB0DA17

DB17 DB16

tDSDOSDI

tSSCKCNV

tHSCKCNV

DA0

tDSDOSDI

Figure 40. Chain Mode with Busy Indicator Serial Interface Timing

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