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AD7691

Rev. 0 | Page 22 of 28

Chain Mode Without Busy Indicator

This mode can be used to daisy-chain multiple AD7691s 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 AD7691s is shown in

Figure 42, and the corresponding timing is given in Figure 43.

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 AD7691 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 can allow a faster reading

rate and consequently more AD7691s 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.

CLK

CONVERT

DATA IN

DIGITAL HOST

CNV

SCK

SDO

SDI

CNV

SCK

SDO

SDI

AD7691

Figure 42. Chain Mode Without 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 43. Chain Mode Without Busy Indicator Serial Interface Timing

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