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AD7942

Rev. B | Page 21 of 2

Chain Mode Without Busy Indicator

This mode can be used to daisy-chain multiple AD7942s 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 AD7942s is shown in Figure 38 and the corresponding

timing diagram is given in Figure 39.

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 AD7942 enters the acquisition phase

and powers down. The remaining data bits stored in the inter-

nal shift register are then clocked by subsequent SCK falling

edges. For each ADC, SDI feeds the input of the internal shift

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 also using the SCK falling edge allows

a faster reading rate and consequently more AD7942s in the

chain, provided the digital host has an acceptable hold time.

The maximum conversion rate may be reduced due to the total

readback time. For instance, with a 5 ns digital host setup time

and 3 V interface, up to eight AD7942s running at a conversion

rate of 220 kSPS can be daisy-chained on a 3-wire port.

CNV

SCK

SDO

SDI

CLK

CONVERT

DATA IN

DIGITAL HOST

AD7942

CNV

SCK

SDO

SDI AD7942

Figure 38. Chain Mode Without 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 39. Chain Mode Without Busy Indicator, Serial Interface Timing

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