AD8021 datasheet(23/28 Pages) AD

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AD7986

Rev. B | Page 23 of 28

CHAIN MODE WITHOUT BUSY INDICATOR

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

on a 3-wire serial interface. It is only available in normal conver-

sion mode (TURBO = 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 two AD7986 devices is

shown in Figure 33, and the corresponding timing is given in

Figure 34.

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 AD7986 enters the acquisition phase and

powers down. The remaining data bits stored in the internal

shift register are 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 18 × 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 AD7986 devices in the chain,

provided that the digital host has an acceptable hold time. The

maximum conversion rate may be reduced due to the total

readback time.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7986

SDI

SDO

CNV

SCK

AD7986

SDI

SDO

CNV

SCK

TURBO

Figure 33. Chain Mode Without 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

tSSCKCNV

Figure 34. Chain Mode Without Busy Indicator Serial Interface Timing

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