AD7985BCPZ datasheet(24/29 Pages) AD

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Data Sheet

AD7985

Rev. C | Page 23 of 28

CHAIN MODE WITHOUT BUSY INDICATOR

This mode can daisy-chain multiple AD7985 devices on a 3-wire

serial interface. It is available only in normal conversion mode

(TURBO is low). This feature is useful for reducing component

count and wiring connections, for example, in isolated multi-

converter applications or for systems with a limited interfacing

capacity. Data readback is analogous to clocking a shift register.

A connection diagram example using two AD7985 devices is

shown in Figure 34, and the corresponding timing is given in

Figure 35.

When SDI and CNV are low, SDO is driven low. With SCK low,

a rising edge on CNV initiates a conversion, selects 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 AD7985 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 the data MSB

first, and 16 × 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 AD7985 devices in

the chain, provided that 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

AD7985

SDI

SDO

CNV

SCK

AD7985

SDI

SDO

CNV

SCK

TURBO

Figure 34. Chain Mode Without Busy Indicator Connection Diagram

TURBO = 0

SDOA = SDIB

DA15

DA14

DA13

SCK

tSSDISCK

tHSDISCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV

DA1

tSCK

tSCKL

tSCKH

DA0

SDIA = 0

SDOB

DB15

DB14

DB13

DA1

DB1DB0DA15

DA14

tHSDO

tDSDO

tQUIET

tHSCKCNV

DA0

Figure 35. Chain Mode Without Busy Indicator Serial Interface Timing

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