AD7984 datasheet(21/24 Pages) AD

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AD7984

Rev. 0 | Page 21 of 24

CHAIN MODE WITHOUT BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7984s 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 AD7984s 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 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 AD7984 enters the acquisition phase and

goes into standby mode. 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

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 will allow a faster

reading rate and consequently more AD7984s 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.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7984

SDI

SDO

CNV

SCK

AD7984

SDI

SDO

CNV

SCK

Figure 36. Chain Mode Without Busy Indicator Connection Diagram

SDOA = SDIB

DA17

DA16

DA15

SCK

343536

tSSDISCK

tHSDISCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV

DA1

tSCK

tSCKL

tSCKH

DA0

SDIA = 0

SDOB

DB17

DB16

DB15

DA1

DB1DB0DA17

DA16

tHSDO

tDSDO

tSSCKCNV

tHSCKCNV

DA0

Figure 37. Chain Mode Without Busy Indicator Serial Interface Timing

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