AD79821 datasheet(22/24 Pages) AD

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AD7946

Rev. A | Page 22 of 24

CHAIN MODE WITH BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7946s on a

3-wire serial interface while providing a BUSY indicator. 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 three AD7946s is shown

in Figure 43, and the corresponding timing is given in Figure 44.

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

high, a rising edge on CNV initiates a conversion, selects the

chain mode, and enables the BUSY indicator feature. In this

mode, CNV is held high during the conversion phase and the

subsequent data readback. When all ADCs in the chain have

completed their conversions, the near-end ADC (ADC C in

Figure 43) SDO is driven high. This transition on SDO can

be used as a BUSY indicator to trigger the data readback

controlled by the digital host. The AD7946 then enters the

acquisition phase and powers down. The data bits stored in

the internal shift register are then clocked out, MSB first, 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 + 1 clocks are required to readback the N ADCs.

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. 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

to a single 3-wire port.

CLK

CONVERT

DATA IN

IRQ

DIGITAL HOST

CNV

SCK

SDO

SDI

AD7946

CNV

SCK

SDO

SDI

AD7946

CNV

SCK

SDO

SDI

AD7946

Figure 43. Chain Mode with BUSY Indicator Connection Diagram

SDOA = SDIB

DA13 DA12 DA11

SCK

tEN

CONVERSION

ACQUISITION

tCONV

tCYC

tACQ

ACQUISITION

CNV = SDIA

DA1

413

tSCK

tSCKH

tSCKL

DA0

SDOB = SDIC

DB13 DB12 DB11

DA1

DB1DB0DA13 DA12

tSSDISCK

tHSDISCK

tHSDO

tDSDO

SDOC

DC13 DC12 DC11

DA1DA0

DC1DC0DA12

DB1DB0DA13

DB13 DB12

tDSDOSDI

tSSCKCNV

tHSCKCNV

DA0

tDSDOSDI

Figure 44. Chain Mode with BUSY Indicator Serial Interface Timing

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