Datasheet

AD7985
Rev. A | Page 18 of 28
DATA READING OPTIONS
There are three different data reading options for the AD7985.
There is the option to read during conversion, to split the read
across acquisition and conversion (see Figure 28 and Figure 29),
and, in normal mode, to read during acquisition. The desired
SCK frequency largely determines which reading option to use.
Reading During Conversion, Fast Host (Turbo or
Normal Mode)
When reading during conversion (n), conversion results are for
the previous (n − 1) conversion. Reading should occur only up
to t
DATA
and, because this time is limited, the host must use a
fast SCK.
The required SCK frequency is calculated by
DATA
SCK
t
Edges
SCKNumber
f
__
≥
To determine the minimum SCK frequency, follow these
examples to read data from conversion (n − 1).
For turbo mode (2.5 MSPS):
Number_SCK_Edges = 16; t
DATA
= 190 ns
f
SCK
= 16/190 ns = 84.2 MHz
For normal mode (2.0 MSPS):
Number_SCK_Edges = 16; t
DATA
= 290 ns
f
SCK
= 16/290 ns = 55.2 MHz
The time between t
DATA
and t
CONV
is an I/O quiet time during
which digital activity should not occur, or sensitive bit decisions
may be corrupted.
Split-Reading, Any Speed Host (Turbo or Normal Mode)
To allow for a slower SCK, there is the option of a split read,
where data access starts at the current acquisition (n) and spans
into the conversion (n). Conversion results are for the previous
(n − 1) conversion.
Similar to reading during conversion, split-reading should
occur only up to t
DATA
. For the maximum throughput, the
only time restriction is that split-reading take place during the
t
ACQ
(minimum) + (t
DATA
− t
QUIET
) time. The time between the
falling edge of SCK and CNV rising is an acquisition quiet
time, t
QUIET
.
To determine how to split the read for a particular SCK frequency,
follow these examples to read data from conversion (n − 1).
For turbo mode (2.5 MSPS):
f
SCK
= 70 MHz; t
DATA
= 190 ns
Number_SCK_Edges = 70 MHz × 190 ns = 13.3
Thirteen bits are read during conversion (n), and three bits are
read during acquisition (n).
For normal mode (2.0 MSPS):
f
SCK
= 45 MHz; t
DATA
= 290 ns
Number_SCK_Edges = 45 MHz × 290 ns = 13.05
Thirteen bits are read during conversion (n), and three bits are
read during acquisition (n).
For slow throughputs, the time restriction is dictated by the
throughput required by the user; the host is free to run at any
speed. Similar to reading during acquisition, data access for
slow hosts must take place during the acquisition phase with
additional time into the conversion.
Note that data access spanning conversion requires the CNV
pin to be driven high to initiate a new conversion, and data
access is not allowed when CNV is high. Thus, the host must
perform two bursts of data access when using this method.
Reading During Acquisition, Any Speed Host (Turbo or
Normal Mode)
When reading during acquisition (n), conversion results are
for the previous (n − 1) conversion. Maximum throughput is
achievable in normal mode (2.0 MSPS); however, in turbo
mode, 2.5 MSPS throughput is not achievable.
For the maximum throughput, the only time restriction is that
reading take place during the t
ACQ
(minimum) time. For slow
throughputs, the time restriction is dictated by the throughput
required by the user; the host is free to run at any speed. Thus,
for slow hosts, data access must take place during the acquisi-
tion phase.