REV. A
AD7675
–16–
In read-after-conversion mode, unlike in other modes, it should
be noted that the signal BUSY returns low after the 16 data bits
are pulsed out and not at the end of the conversion phase, which
results in a longer BUSY width.
In read-during-conversion mode, the serial clock and data toggle
at appropriate instances, which minimizes potential feedthrough
between digital activity and the critical conversion decisions.
To accommodate slow digital hosts, the serial clock can be
slowed down by using DIVSCLK.
SLAVE SERIAL INTERFACE
External Clock
The AD7675 is configured to accept an externally supplied
serial data clock on the SCLK Pin when the EXT/INT Pin is
held high. In this mode, several methods can be used to read the
data. The external serial clock is gated by CS and the data are
output when both CS and RD are low. Thus, depending on
CS, the data can be read after each conversion or during the
following conversion. The external clock can be either a con-
tinuous or discontinuous clock. A discontinuous clock can be
either normally high or normally low when inactive. Figure 19
and Figure 20 show the detailed timing diagrams of these meth-
ods. Usually, because the AD7675 has a longer acquisition
phase than the conversion phase, the data are read immediately
after conversion.
While the AD7675 is performing a bit decision, it is important
that voltage transients not occur on digital input/output pins or
degradation of the conversion result could occur. This is particu-
larly important during the second half of the conversion phase
because the AD7675 provides error-correction circuitry that can
correct for an improper bit decision made during the first half of
the conversion phase. For this reason, it is recommended that when
an external clock is being provided, it is a discontinuous clock
that is toggling only when BUSY is low or, more importantly,
that it does not transition during the latter half of BUSY high.
External Discontinuous Clock Data Read after Conversion
This mode is the most recommended of the serial slave modes.
Figure 19 shows the detailed timing diagrams of this method.
After a conversion is complete, indicated by BUSY returning
low, the result of this conversion can be read while both CS and
RD are low. The data is shifted out, MSB first, with 16 clock
pulses and is valid on both the rising and falling edge of the clock.
One of the advantages of this method is that the conversion
performance is not degraded because there are no voltage tran-
sient
on the digital interface during the conversion process.
Another advantage is the ability to read the data at any speed up
to 40 MHz, which accommodates both slow digital host inter-
face and the fastest serial reading.
Finally, in this mode only, the AD7675 provides a “daisy chain”
feature using the RDC/SDIN input pin for cascading multiple
converters together. This feature is useful for reducing compo-
nent count and wiring connections when it is desired as it is, for
instance, in isolated multiconverters applications.
An example of the concatenation of two devices is shown in
Figure 21. Simultaneous sampling is possible by using a com-
mon CNVST signal. It should be noted that the RDC/SDIN
input is latched on the opposite edge of SCLK of the one used
to shift out the data on SDOUT. Hence, the MSB of the
“upstream” converter just follows the LSB of the “downstream”
converter on the next SCLK cycle.
CS
SCLK
SDOUT
D15 D14 D1 D0D13
X15 X14 X13 X1 X0 Y15 Y14
BUSY
SDIN
INVSCLK = 0
X15 X14X
123 1415161718
EXT/INT = 1 RD = 0
t
35
t
36
t
37
t
31
t
32
t
34
t
16
t
33
Figure 19. Slave Serial Data Timing for Reading (Read after Convert)
