AD664
Multiplying Mode Performance
Figure 6 illustrates the typical open-loop gain and phase perfor-
mance of the output amplifiers of the AD664.
the DAC operating mode data. All registers are double-buffered
to allow for simultaneous updating of all outputs. Register data
may be read back to verify the respective contents. The digital
port also allows transparent operation. Data from the input pins
can be sent directly through both ranks of latches to the DAC.
+20
GAIN
+15
+90
+45
0
+10
+5
0
PHASE
10k
100k
FREQUENCY – H
1M
z
Figure 6. Gain and Phase Performance of AD664 Outputs
Figure 8. Typical Output Noise
Crosstalk
Partial address decoding is performed by the DS0, DS1, QS0,
QS1 and QS2 address bits. QS0, QS1 and QS2 allow the 44-pin
versions of the AD664 to be addressed in 4-bit nibble, 8-bit byte
or 12-bit parallel words.
Crosstalk is a spurious signal on one DAC output caused by a
change in the output of one or more of the other DACs.
Crosstalk can be induced by capacitive, thermal or load current
induced feedthrough. Figure 7 shows typical crosstalk. DAC B
is set to output 0 volts. The outputs of DAC A, C and D switch
2 kΩ loads from 10 V to 0 V. The first disturbance in the output
of DAC B is caused by digital feedthrough from the input data
lows. The second disturbance is caused by analog feedthrough
from the other DAC outputs.
The RST pin provides a simple method to reset all output
voltages to zero. Its advantages are speed and low software
overhead.
INPUT DATA
In general, two types of data will be input to the registers of the
AD664, input code data and mode select data. Input code data
sets the DAC inputs while the mode select data sets the gain
and range of each DAC.
The versatile I/O port of the AD664 allows many different types
of data input schemes. For example, the input code for just one
of the DACs may be loaded and the output may or may not be
updated. Or, the input codes for all four DACs may be written,
and the outputs may or may not be updated.
The same applies for MODE SELECTION. The mode of just
one or many of the DACs may be rewritten and the user can
choose to immediately update the outputs or wait until a later
time to transfer the mode information to the outputs.
A user may also write both input code and mode information
into their respective first ranks and then update all second ranks
at once.
Figure 7. Output Crosstalk
Finally, transparent operation allows data to be transferred from
the inputs to the outputs using a single control line. This feature
is useful, for example, in a situation where one of the DACs is
used in an A/D converter. The SAR register could be connected
directly to a DAC by using the transparent mode of operation.
Another use for this feature would be during system calibration
where the endpoints of the transfer function of each DAC would
be measured. For example, if the full-scale voltages of each
DAC were to be measured, then by making all four DACs
transparent and putting all “1s” on the input port, all four
DACs would be at full-scale. This requires far less software
overhead than loading each register individually.
Output Noise
Wideband output noise is shown in Figure 8. This measurement
was made with a 7 MHz noise bandwidth, gain = 1 and all bits
on. The total rms noise is approximately one fifth the visual
peak-to-peak noise.
DIGITAL INTERFACE
As Table II shows, the AD664 makes a wide variety of operating
modes available to the user. These modes are accessed or pro-
grammed through the high speed digital port of the quad DAC.
On-board registers program and store the DAC input codes and
D
REV.
–7–
ADI [ ADI ]
