Datasheet

AD7766
Rev. C | Page 18 of 24
DAISY CHAINING
Daisy chaining devices allows numerous devices to use the same
digital interface lines by cascading the outputs of multiple ADCs
on a single data line. This feature is especially useful for reduc-
ing 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 where data is clocked on the falling edge of SCLK.
The block diagram in Figure 36 shows how devices must be
connected to achieve daisy-chain functionality. The scheme
shown operates by passing the output data of the SDO pin of an
AD7766 device to the SDI input of the next AD7766 device in
the chain. The data then continues through the chain until it is
clocked onto the SDO pin of the first device in the chain.
READING DATA IN DAISY-CHAIN MODE
An example of a daisy chain of four AD7766 devices is shown in
Figure 36 and Figure 37. In the case illustrated in Figure 36, the
output of the AD7766 labeled A is the output of the full daisy
chain. The last device in the chain (the AD7766 labeled D) has
its serial data input (SDI) pin connected to ground. All the
devices in the chain must use common MCLK, SCLK,
CS
, and
SYNC
/
PD
signals.
To enable the daisy-chain conversion process, apply a common
SYNC
/
PD
pulse to all devices, synchronizing all the devices in
the chain (see the
section).
Power-Down, Reset, and Synchronization
After applying a
SYNC
/
PD
pulse to all the devices, there is a
delay (as listed in ) before valid conversion data appears
at the output of the chain of devices. As shown in , the
first conversion result is output from the AD7766 device labeled
A. This 24-bit conversion result is followed by the conversion
results from the devices labeled B, C, and D, with all conversion
results output in an MSB-first sequence. The stream of conversion
results is clocked through each device in the chain and is eventually
clocked onto the SDO pin of the AD7766 device labeled A. The
conversion results of all the devices in the chain must be clocked
onto the SDO pin of the final device in the chain while its
Tabl e 7
Figure 37
DRDY
signal is active low. This is illustrated in the examples shown
( and ), where the conversion results from
the devices labeled A, B, C, and D are clocked onto SDO (A)
during the time between the falling edge of
Figure 37 Figure 38
DRDY
(A) and the
rising edge of
DRDY
(A).
CHOOSING THE SCLK FREQUENCY
As shown in Figure 37, the number of SCLK falling edges that
occurs during the period when
DRDY
(A) is active low must
match the number of devices in the chain multiplied by 24 (the
number of bits that must be clocked through onto SDO (A) for
each device).
The period of SCLK (t
SCLK
) required for a known daisy-chain length
using a known common MCLK frequency must, therefore, be
established in advance. Note that the maximum SCLK frequency
is governed by t
8
and is specified in Table 3 for different V
DRIVE
voltages.
In the case where
CS
is tied logic low,
⎥
⎦
⎤
⎢
⎣
⎡
×
≤
K
t
t
READ
SCLK
24
(1)
where:
K is the number of AD7766/AD7766-1/AD7766-2 devices in
the chain.
t
SCLK
is the period of the SCLK.
t
READ
equals t
DRDY
− t
5
.
In the case where
CS
is used in the daisy-chain interface,
(
)
(
)
⎥
⎦
⎤
⎢
⎣
⎡
×
++−
≤
K
tttt
t
1376
READ
SCLK
24
(2)
where:
K is the number of AD7766/AD7766-1/AD7766-2 devices in
the chain.
t
SCLK
is the period of the SCLK.
t
READ
equals t
DRDY
− t
5
.
Note that the maximum value of SCLK is governed by t
8
and is
specified in Table 3 for different V
DRIVE
voltages.