AD7940 datasheet(22/28 Pages) AD

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AD7686

Rev. B | Page 22 of 28

CHAIN MODE WITH BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7686s

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 AD7686s is shown in

Figure 43, and the corresponding timing is given in Figure 44.

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 43) 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

AD7686 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 16 × 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 AD7686s in the chain, provided

the digital host has an acceptable hold time. For instance,

with a 3 ns digital host setup time and 3 V interface, up to four

AD7686s running at a conversion rate of 360 kSPS can be daisy

chained to a single 3-wire port.

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7686

CNV

SCK

SDO

SDI

AD7686

CNV

SCK

SDO

SDI

AD7686

Figure 43. Chain Mode with Busy Indicator Connection Diagram

SDOA = SDIB

DA15 DA14 DA13

SCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDIA

DA1

415

tSCK

tSCKH

tSCKL

DA0

SDOB = SDIC

DB15 DB14 DB13

DA1

DB1DB0DA15 DA14

tSSDISCK

tHSDISCK

tHSDO

tDSDO

SDOC

DC15 DC14 DC13

DA1DA0

DC1DC0DA14

DB1DB0DA15

DB15 DB14

tDSDOSDI

tSSCKCNV

tHSCKCNV

DA0

tDSDOSDI

Figure 44. Chain Mode with Busy Indicator Serial Interface Timing

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