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Preliminary Technical Data

AD7658/AD7657/AD7656

Rev. PrI | Page 21 of 25

devices in Daisy Chain Mode.

The CS falling edge is used to frame the serial transfer from the

AD7658/AD7657/AD7656 devices, take the bus out of three-

state and clock out the MSB of the first conversion result. In the

example shown all twelve ADC channels were simultaneously

sampled. Two DOUT line are used to read the conversion

results in this example. CS frames a 96 SCLK transfer. During

the first 48 SCLK the conversion data is transferred from Device

#2 to Device #1. DOUT A on device #2 transfers conversion

data from V1, V2 and V5 into DCINA in device #1. DOUT B on

device #2 transfers conversion results from V3, V4 and V6 to

DCIN B in device #1. During the first 48 SCLK device #1

transfers data into the digital host, DOUTA on device #1

transfers conversion data from V1, V2 and V5. DOUTB on

device #1 transfers conversion data from V3, V4 and V6.

During the last 48 SCLKs device #2 will clock out zeros, device

#1 will shift the data it clocked in from device #2 during the

first 48 SCLKs into the digital host. This example could also

have been implemented using 3 x 32 SCLK individually framed

SCLK transfers or 6 x 16 SCLK individually framed SCLK

transfers provided DCEN remained high during the transfers.

Figure 14 shows the timing if two AD7656 were configured in

Daisy chain mode but operating with three DOUT lines. Again

assuming a simultaneous sampling of all 12 inputs occurred.

During the read operation the CS frames a 64 SCLK transfer.

During the first 32 SCLKs of this transfer the conversion results

from Device #1 are clocked into the digital host and the

conversion results from device #2 are clocked into device #1.

During the last 32 SCLKs of the transfer the conversion results

from device #2 are clocked out of device #1 and into the digital

host. Device #2 will clock out zeros.

Digital Host

CONVERT

SCLK

AD7656

CONVST

SCLK

DATA IN1

DATA IN2

DOUTA

DOUTB

DOUTA

DOUTB

DCINA

DCINB

DCEN =0

DCEN=1

DEVICE #1

DEVICE #2

Figure 12. Daisy-Chain Configuration

DEVICE#1DOUTA

MSB V1

LSB V1 MSB V2

LSB V2 MSB V5

LSB V5 MSB V1

LSB V1 MSB V2

LSB V5

MSB V1

LSB V1 MSB V2

LSB V2 MSB V5

LSB V5

tCONV

CONVST A/B/C

BUSY

SCLK

DEVICE#1DOUTB

MSB V3

LSB V3 MSB V4

LSB V4 MSB V6

LSB V6 MSB V3

LSB V3 MSB V4

LSB V6

DEVICE#2DOUTA

MSB V3

LSB V3 MSB V4

LSB V4 MSB V6

LSB V6

DEVICE#2DOUTB

t17

t18

t19

Figure 13. Daisy-Chain Serial Interface Timing with 2 DOUT lines

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