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AD7982

Data Sheet

Rev. D | Page 22 of 25

CHAIN MODE WITHOUT BUSY INDICATOR

Chain mode without busy indicator can be used to daisy-chain

multiple AD7982 devices on a 3-wire serial interface. The chain

mode without busy indicator feature reduces 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.

Figure 37 shows a connection diagram example using two AD7982

devices, and Figure 38 shows the corresponding timing.

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 completes, 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

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 capture the data, a digital host using the SCK falling edge

allows a faster reading rate and consequently more AD7982

devices in the chain, provided the digital host has an acceptable

hold time. The maximum conversion rate can be reduced due to

the total readback time.

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7982

SDI

SDO

CNV

SCK

AD7982

SDI

SDO

CNV

SCK

Figure 37. Chain Mode Without Busy Indicator Connection Diagram

SDOA = SDIB

DA17

DA16

DA15

SCK

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 38. Chain Mode Without Busy Indicator Serial Interface Timing

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