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AD7686

Rev. B | Page 21 of 28

CHAIN MODE, NO BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7686s on a

3-wire serial interface. 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 two AD7686s is shown in

Figure 41, and the corresponding timing is given in Figure 42.

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

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

mode, and disables the busy indicator. In this mode, CNV is

held high during the conversion phase and the subsequent data

readback.

When the conversion is complete, the MSB is output onto SDO,

and the AD7686 enters the acquisition phase and powers down.

The remaining data bits stored in the internal shift register are

then clocked 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 clocks are required to read back the N

ADCs. The data is valid on both SCK edges. 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. The maximum conversion rate can be

reduced due to the total readback 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

on a 3-wire port.

CLK

CONVERT

DATA IN

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7686

CNV

SCK

SDO

SDI

AD7686

Figure 41. Chain Mode, No Busy Indicator Connection Diagram

SDOA = SDIB

DA15

DA14

DA13

SCK

303132

tSSDISCK

tHSDISCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV

DA1

tSCK

tSCKL

tSCKH

DA0

SDIA = 0

SDOB

DB15

DB14

DB13

DA1

DB1DB0DA15

DA14

tHSDO

tDSDO

tSSCKCNV

tHSCKCNV

DA0

Figure 42. Chain Mode, No Busy Indicator Serial Interface Timing

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