EVAL-AD7902SDZ datasheet(23/29 Pages) AD

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AD7902

Data Sheet

Rev. B | Page 22 of 28

CHAIN MODE

Chain Mode Without Busy Indicator

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

chain both ADCs within an AD7902 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.

See Figure 46 for a connection diagram example using both

ADCs in an AD7902. The corresponding timing is shown in

Figure 47.

When SDIx and CNVx are low, SDOx is driven low. With SCKx

low, a rising edge on CNVx initiates a conversion, selects chain

mode, and disables the busy indicator. In this mode, CNVx is

held high during the conversion phase and the subsequent data

readback. When the conversion is complete, the MSB is output

onto SDOx and the AD7902 enters the acquisition phase and

powers down. The remaining data bits stored in the internal

shift register are clocked by subsequent SCKx falling edges. For

each ADC, SDIx feeds the input of the internal shift register and

is clocked by the SCKx falling edge. Each ADC in the chain

outputs its data MSB first, and 16 × N clocks are required to

read back the N ADCs. The data is valid on both SCKx edges.

Although the rising edge can be used to capture the data, a

digital host using the SCKx falling edge allows a faster reading

rate and, consequently, more AD7902 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.

Figure 46. Chain Mode Without a Busy Indicator Connection Diagram

Figure 47. Chain Mode Without a Busy Indicator Serial Interface Timing

CONVERT

DATA IN

CLK

DIGITAL HOST

AD7902

ADC2

SDI2

SDO2

CNV2

SCK2

AD7902

ADC1

SDI1

SDO1

CNV1

SCK1

tCONV

tCYC

tSSDISCK

tSCKL

tSCK

tHSDISCK

tACQ

ACQUISITION

tSSDICNV

ACQUISITION

tSCKH

CONVERSION

SDO1 = SDI2

tHSDICNV

SCKx

CNVx

SDI1 = 0

SDO2

tEN

D115

D114

D113

D215

D214

D213

D21

D20

D115

D114

D10

D11

D10

tHSDO

tDSDO

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