AD7985 datasheet(24/28 Pages) AD

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AD7985

Rev. 0 | Page 24 of 28

CHAIN MODE WITH BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7985 devices

on a 3-wire serial interface while providing a busy indicator. It

is available only in normal conversion mode (TURBO is low).

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 AD7985 devices is shown in Figure 36, and

the corresponding timing is given in Figure 37.

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

a rising edge on CNV initiates a conversion, selects 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 AD7985

ADC labeled C in Figure 36) is driven high. This transition on

SDO can be used as a busy indicator to trigger the data read-

back controlled by the digital host. The AD7985 then enters the

acquisition phase and powers down. The data bits stored in the

internal shift register are 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 read back 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

AD7985 devices in the chain, provided that the digital host has

an acceptable hold time.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7985

SDI

SDO

CNV

SCK

AD7985

SDI

SDO

CNV

SCK

IRQ

AD7985

SDI

SDO

CNV

SCK

TURBO

Figure 36. 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

tHSCKCNV

DA0

tDSDOSDI

TURBO = 0

tSSCKCNV

Figure 37. Chain Mode with Busy Indicator Serial Interface Timing

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