AD7680 datasheet(21/28 Pages) AD

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AD7688

Rev. A | Page 21 of 28

CHAIN MODE, NO BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7688s 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 AD7688s 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 AD7688 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 16 × 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 allows a faster reading

rate and, consequently more AD7688s 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.

For instance, with a 3 ns digital host set-up time and 3 V

interface, up to four AD7688s running at a conversion rate of

360 kSPS can be daisy-chained on a 3-wire port.

CLK

CONVERT

DATA IN

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7688

CNV

SCK

SDO

SDI

AD7688

Figure 42. Chain Mode, No BUSY Indicator Connection Diagram

SDOA = SDIB

DA15

DA14

DA13

SCK

tSSDISCK

tHSDISC

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV

DA1

tSCK

tSCKL

tSCKH

DA0

SDIA = 0

SDOB

DB15

DB14

DB13

DA1

DB1DB0DA15

DA14

tHSDO

tDSDO

tSSCKCNV

tHSCKCNV

DA0

Figure 43. Chain Mode, No BUSY Indicator Serial Interface Timing

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