AD7687BCPZ-R2 datasheet(25/27 Pages) AD

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AD7687

Data Sheet

Rev. E | Page 24 of 26

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy-chain multiple AD7687

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

indicator. This feature is useful for reducing component count

and wiring connections, for isolated multiconverter applications

or for systems with a limited interfacing capacity (for example).

Data readback is analogous to clocking a shift register.

Figure 44 shows a connection diagram example using three

AD7687 devices, and Figure 45 gives the corresponding timing.

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

and 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 AD7687 C in Figure 44) is driven

high. This transition on SDO can act as a BUSY indicator to

trigger the data readback controlled by the digital host. The

AD7687 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;

these data bits are also shifted in by the SCK falling edge. Each

of the N ADCs in the chain outputs its data MSB first. The data

is valid on both SCK edges. Although the rising edge can capture

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

reading rate and, consequently, more AD7687 devices in the

chain (provided the digital host has an acceptable hold time).

After the optional (16 × N) + 1th SCK falling edge or CNV

rising edge, whichever is earlier, SDO is driven low again. The

maximum conversion rate may be reduced due to the total

readback time. For example, using a digital host with a 3 ns set-

up time and 3 V interface, up to eight AD7687 devices daisy-

chained on a 3-wire port can be run at a maximum effective

conversion rate of 220 kSPS.

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7687

CNV

SCK

SDO

SDI

AD7687

CNV

SCK

SDO

SDI

AD7687

Figure 44. Chain Mode with BUSY Indicator Connection Diagram

SDOA = SDIB

DA15 DA14 DA13

SCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDIA

DA1

tSCK

tSCKH

tSCKL

DA0

SDOB = SDIC

DB15 DB14 DB13

DA1

DB1

DB0 DA15 DA14

tSSDISCK

tHSDISC

tHSDO

tDSDO

SDOC

DC15 DC14 DC13

DA1

DA0

DC1

DC0

DA14

DB1

DB0 DA15

DB15 DB14

tDSDOSDI

tSSCKCNV

tHSCKCNV

DA0

tDSDOSDI

Figure 45. Chain Mode with BUSY Indicator Serial Interface Timing

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