AD7680 datasheet(21/24 Pages) AD

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AD7946

Rev. A | Page 21 of 24

CHAIN MODE, NO BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7946s 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 AD7946s is shown in

Figure 41, and the corresponding timing is given in Figure 42.

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 AD7946 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 14 × 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 AD7946s in the chain, provided the

digital host has an acceptable hold time. The maximum conver-

sion rate may be reduced due to the total readback time. For

instance, with a 3 ns digital host setup time and 3 V interface,

up to four AD7946s 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

AD7946

CNV

SCK

SDO

SDI

AD7946

Figure 41. Chain Mode, No BUSY Indicator Connection Diagram

SDOA = SDIB

DA13

DA12

DA11

SCK

262728

tSSDISCK

tHSDISCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV

DA1

tSCK

tSCKL

tSCKH

DA0

SDIA = 0

SDOB

DB13

DB12

DB11

DA1

DB1DB0DA13

DA12

tHSDO

tDSDO

tSSCKCNV

tHSCKCNV

DA0

Figure 42. Chain Mode, No BUSY Indicator Serial Interface Timing

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