AD7944BCPZ datasheet(23/28 Pages) AD

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AD7944

Rev. 0 | Page 23 of 28

CHAIN MODE WITHOUT BUSY INDICATOR

This mode can be used to daisy-chain multiple AD7944 devices on

a 3-wire serial interface. It is available only in normal conversion

mode (TURBO is low). This feature is useful for reducing com-

ponent count and wiring connections, for example, in isolated

multiconverter applications or for systems with a limited inter-

facing capacity. Data readback is analogous to clocking a shift

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 AD7944 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 its data MSB

first, and 14 × N clocks are required to read back 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

AD7944 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

AD7944

SDI

SDO

CNV

SCK

AD7944

SDI

SDO

CNV

SCK

TURBO

TURBO = 0

SCK

Figure 34. Chain Mode Without Busy Indicator Connection Diagram

SDOA = SDIB

DA13

DA12

DA11

tSSDISCK

tHSDISCK

tEN

CONVER

ACQUISITION

SION

tCO

CNV

tCYC

tACQ

ACQUISITION

SDIA = 0

tSCK

tSCKL

tSCKH

DA1

DA0

SDOB

DB13

DB12

DB11

DA1

DB1DB0DA13

DA12

tHSDO

tDSDO

tQUIET

tHSCKCNV

DA0

Figure 35. Chain Mode Without Busy Indicator Serial Interface Timing

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