AD8021 datasheet(24/28 Pages) AD

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AD7986

Rev. B | Page 24 of 28

CHAIN MODE WITH BUSY INDICATOR

This mode can also be used to daisy-chain multiple AD7986

devices 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 AD7986 devices is

shown in Figure 35, and the corresponding timing is given in

Figure 36.

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 SDO pin of the ADC closest to

the digital host (see the AD7986 ADC labeled C in Figure 35) 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

AD7986 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 18 × 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 AD7986 devices in the chain, provided that

the digital host has an acceptable hold time.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7986

SDI

SDO

CNV

SCK

AD7986

SDI

SDO

CNV

SCK

IRQ

AD7986

SDI

SDO

CNV

SCK

TURBO

Figure 35. Chain Mode with Busy Indicator Connection Diagram

SDOA = SDIB

DA17 DA16 DA15

SCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDIA

DA1

tSCK

tSCKH

tSCKL

DA0

SDOB = SDIC

DB17 DB16 DB15

DA1

DB1

DB0 DA17 DA16

tSSDISCK

tHSDISCK

tHSDO

tDSDO

SDOC

DC17 DC16 DC15

DA1

DA0

DC1

DC0

DA16

DB1

DB0 DA17

DB17 DB16

tDSDOSDI

tHSCKCNV

DA0

tDSDOSDI

TURBO = 0

tQUIET

Figure 36. Chain Mode with Busy Indicator Serial Interface Timing

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