AD7982 datasheet(19/23 Pages) AD

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Preliminary Technical Data

AD7982

Rev PrC | Page 19 of 23

CHAIN MODE, NO BUSY INDICATOR

This mode can be used to daisy chain multiple AD7982s on a 3-

wire serial interface. This feature is useful for reducing

component count and wiring connections, e.g., 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 AD7982s is shown in

Figure 24 and the corresponding timing is given in Figure 25.

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 AD7982 enters the acquisition phase and

powers down. The remaining data bits stored in the internal

shift register are then 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

readback 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 AD7982s 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.

CNV

SCK

SDO

SDI

CLK

CONVERT

DATA IN

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7982

Figure 24. Chain Mode, No BUSY Indicator Connection Diagram

SDOA = SDIB

DA17

DA16

DA15

SCK

tSSDISCK

tHSDISC

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 25. Chain Mode, No BUSY Indicator Serial Interface Timing

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