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AD7690

Data Sheet

Rev. C | Page 22 of 24

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy-chain multiple AD7690s

on a 3-wire serial interface while providing a busy indicator.

This feature is useful for reducing 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.

A connection diagram example using three AD7690s is shown

in Figure 44, and the corresponding timing is given in Figure 45.

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 AD7690 ADC labeled C in Figure 44) 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 AD7690 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 be used to

capture the data, a digital host using the SCK falling edge allows

a faster reading rate and consequently more AD7690s in the

chain, provided the digital host has an acceptable hold time.

Figure 44. Chain Mode with Busy Indicator Connection Diagram

Figure 45. Chain Mode with Busy Indicator Serial Interface Timing

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

CNV

SCK

SDO

SDI

CNV

SCK

SDO

SDI

CNV

SCK

SDO

SDI AD7690

AD7690

SDOA = SDIB

DA17 DA16 DA15

SCK

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

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