AD664
Figure 12. Preload First Rank Registers
Load and Update Multiple DAC Outputs
The following examples demonstrate two ways to update all
DAC outputs. The first method involves doing all data transfers
during one long CS low period. Note that in this case, shown in
Figure 13, LS returns high before CS goes high. Data hold time,
relative to an address change, is 70 ns. This updates the outputs
of all DACs simultaneously.
Figure 10a. Preload First Rank of a DAC
25؇C
MIN (ns)
TMIN to TMAX
MIN (ns)
SYMBOL
tLS
tLH
tCW
tDS
tDH
tAS
tAH
0
0
15
80
0
15
0
15
100
0
15
0
15
15
Figure 10b. Preload First Rank of a DAC Timing
This allows the user to “preload” the data to a DAC and strobe
it into the output latch at some future time. The user could do
this by reproducing the sequence of signals illustrated in the
next section.
Figure 13. Update All DAC Outputs
Update Second Rank of a DAC
The second method involves doing a CS assertion (low) and an
LS toggle separately for each DAC. It is basically a series of
preload operations (Figure 10). In this case, illustrated in Figure
14, two LS signals are shown. One, labeled LS, goes high before
CS returns high. This transfers the “new” input word to the
DAC outputs sequentially. The second LS signal, labeled Alter-
nate LS, stays low until CS returns high. Using this sequence
loads the first ranks with each “new” input word but doesn’t up-
date the DAC outputs. To then update all DAC outputs simul-
taneously would require the signals illustrated in Figure 11.
Assuming that a new input code had previously been placed into
the first rank of the input latches, the user can update the out-
put of the DAC by simply pulling CS low while keeping LS,
MS, TR, RD and RST high. Address data is not needed in this
case. In reality, all second ranks are being updated by this pro-
cedure, but only those which receive data different from that
already there would manifest a change. Updating the second
rank does not change the contents of the first rank.
Figure 11. Update Second Rank of a DAC
The same options that exist for individual DAC input loading
also exist for multiple DAC input loading. That is, the user can
choose to update the first and second ranks of the registers or
preload the first ranks and then update them at a future time.
Figure 14. Load and Update Multiple DACs
Preload Multiple First Rank Registers
SELECTING GAIN RANGE AND MODES (44-PIN
VERSIONS)
The first ranks of the DAC input registers may be preloaded
with new input data without disturbing the second rank data.
This is done by transferring the data into the first rank by bring-
ing CS low while LS is low. But CS must return high before LS.
This prevents the data from the first rank from getting into the
second rank. A simple second rank update cycle as shown in
Figure 11 would move the “preloaded” information to the
DACs.
The AD664’s mode select feature allows a user to configure the
gain ranges and output modes of each of the four DACs.
On-board switches take the place of up to eight external relays
that would normally be required to accomplish this task. The
switches are programmed by the mode select word entered via
the data I/O port. The mode select word is eight-bits wide and
D
REV.
–9–
ADI [ ADI ]
