AD7765

Preliminary Technical Data

Rev. PrC | Page 14 of 21

DAISY CHAINING

Daisy chaining devices allows numerous devices to use the same

digital interface lines. This feature is especially useful for

reducing component count and wiring connections, e.g. in

isolated multi-converter applications or for systems with a

limited interfacing capacity. Data read-back is analogous to

clocking a shift register.

The block diagram in Figure 15 shows the way in which devices

must be connected in order to achieve daisy chain functionality.

Figure 15 shows four AD7765 devices daisy chained together

with a common MCLK signal applied, will work for both

decimate by 128 or 256 modes.

Reading Data in Daisychain Mode

The SDO line of AD7765 (A) provides the output data from the

chain of AD7765 converters. The last device in the chain

(AD7765(D) in Figure 15) will have its Serial Data In (SDI) pin

connected to ground. All the devices in the chain must use

common MCLK and SYNC signals.

To enable the daisy chain conversion process, apply a common

SYNC pulse to all devices (see synchronization of devices).

After applying a SYNC pulse to all the devices there is a delay of

TBD SCO periods before valid conversion data appears at the

output of the chain of devices. As shown in Figure 16 the first

conversion result is output from the device labeled AD7765(A).

This 32-bit conversion result is then followed by the conversion

results from the devices B,C and D respectively with all

conversion results output in an MSB first sequence. The signals

output from the daisy chain are the stream of conversion results

from the SDO pin of AD7765(A) and the FSO signal also

output by the first device in the chain (AD7765(A)).

The falling edge of FSO signals the MSB of the first conversion

output in the chain. FSO stays logic low throughout the 32 SCO

clock periods needed to output the AD7765(A) result and

thereafter goes logic high during the output of the conversion

results from the devices B,C, and D.

The maximum number of devices that can be daisy chained is

dependent on the decimation rate the user selects. The max

number of devices that can be daisy chained can be calculated

simply by dividing the chosen decimation rate by 32(the

number of bits that must be clocked out for each conversion).

Table 6 shows give the maximum number of chained devices for

each decimation rate.

Table 6 Maximum length of device chain for all decimation

rates

Decimation Rate

Maximum length of chain

x256

8

x128

4

MCLK

SDI

SDO

AD7765

(A)

(B)

(C)

(D)

SYNC

FSI

MCLK

MCLK

MCLK

SDO

MCLK

SDO

FSI

FSI

FSI

FSI

SDI

SDI

SDI

SDI

SYNC

SYNC

SYNC

AD7765

AD7765

AD7765

SDO

SYNC

FSO

Figure 15. Daisy Chaining 4xAD7765 devices in decimate by 128 mode using a 40Mhz MCLK signal.

AD7765 (A)

32-Bit O/P

AD7765 (B)

AD7765 (C)

AD7765 (D)

AD7765 (C)

AD7765 (D)

AD7765 (D)

AD7765 (C)

AD7765 (D)

AD7765 (B)

AD7765 (C)

AD7765 (D)

AD7765 (B)

AD7765 (C)

AD7765 (D)

AD7765 (A)

AD7765 (B)

32-Bit O/P

32-Bit O/P

32-Bit O/P

32-Bit O/P

32-Bit O/P

SCO

FSO (A)

SDO (A)

SDI (A) = SDO (B)

SDI (B) = SDO (C)

SDI (C) = SDO (D)

Figure 16. Daisychain mode, Data read timing diagram (for daisychain configuration shown in Figure 15).