AD8021 datasheet(23/28 Pages) AD

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AD7685

Rev. C | Page 23 of 28

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy chain multiple AD7685s 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 AD7685s 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 near-end ADC (ADC C in

Figure 44) 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 AD7685 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 also

using the SCK falling edge allows a faster reading rate and,

consequently, more AD7685s in the chain, provided the digital

host has an acceptable hold 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

to a single 3-wire port.

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7685

CNV

SCK

SDO

SDI

AD7685

CNV

SCK

SDO

SDI

AD7685

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

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 45. Chain Mode with BUSY Indicator Serial Interface Timing

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