AD8021 datasheet(22/28 Pages) AD

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AD7685

Rev. C | Page 22 of 28

CHAIN MODE, NO BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7685s 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 AD7685s is shown in

Figure 42, and the corresponding timing is given in Figure 43.

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

a rising edge on CNV initiates a conversion and selects the

chain mode. 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

AD7685 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 readback 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 will allow a faster reading rate and, consequently,

more AD7685s in the chain, provided the digital host has an

acceptable hold time. The maximum conversion rate may be

reduced due to the total readback time. For instance, with a 5 ns

digital host setup time and 3 V interface, up to eight AD7685s

running at a conversion rate of 220 kSPS can be daisy-chained

on a 3-wire port.

CLK

CONVERT

DATA IN

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7685

CNV

SCK

SDO

SDI

AD7685

Figure 42. Chain Mode 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 43. Chain Mode Serial Interface Timing

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