AD9764
REFERENCE OPERATION
The AD9764 contains an internal 1.20 V bandgap reference
that can be easily disabled and overridden by an external
reference. REFIO serves as either an
input
or
output,
depending
on whether the internal or external reference is selected. If
REFLO is tied to ACOM, as shown in Figure 22, the internal
reference is activated, and REFIO provides a 1.20 V output. In
this case, the internal reference
must
be compensated externally
with a ceramic chip capacitor of 0.1
µF
or greater from REFIO
to REFLO. Also, REFIO should be buffered with an external
amplifier having an input bias current less than 100 nA if any
additional loading is required.
+5V
0.1 F
OPTIONAL
EXTERNAL
REF BUFFER
provides several application benefits. The first benefit relates
directly to the power dissipation of the AD9764, which is pro-
portional to I
OUTFS
(refer to the Power Dissipation section). The
second benefit relates to the 20 dB adjustment, which is useful
for system gain control purposes.
The small signal bandwidth of the reference control amplifier is
approximately 1.4 MHz and can be reduced by connecting an
external capacitor between COMP1 and AVDD. The output of
the control amplifier, COMP1, is internally compensated via a
50 pF capacitor that limits the control amplifier small-signal
bandwidth and reduces its output impedance. Any additional
external capacitance further limits the bandwidth and acts as a
filter to reduce the noise contribution from the reference ampli-
fier. Figure 24 shows the relationship between the external
capacitor and the small signal –3 dB bandwidth of the refer-
ence amplifier. Since the –3 dB bandwidth corresponds to the
dominant pole, and hence the time constant, the settling time of
the control amplifier to a stepped reference input response can
be approximated.
1000
REFLO
+1.2V REF
REFIO
50pF
COMP1
AVDD
ADDITIONAL
LOAD
0.1 F
2k
FS ADJ
CURRENT
SOURCE
ARRAY
AD9764
Figure 22. Internal Reference Configuration
The internal reference can be disabled by connecting REFLO to
AVDD. In this case, an external reference may then be applied
to REFIO as shown in Figure 23. The external reference may
provide either a fixed reference voltage to enhance accuracy and
drift performance or a varying reference voltage for gain control.
Note that the 0.1
µF
compensation capacitor is not required
since the internal reference is disabled, and the high input im-
pedance (i.e., 1 MΩ) of REFIO minimizes any loading of the
external reference.
AVDD
0.1 F
BANDWIDTH – kHz
10
0.1
0.1
1
10
100
COMP1 CAPACITOR – nF
1000
Figure 24. External COMP1 Capacitor vs. –3 dB Bandwidth
AVDD
AVDD
V
REFIO
REFLO
+1.2V REF
50pF
REFIO
FS ADJ
R
SET
I
REF
=
V
REFIO
/R
SET
COMP1
EXTERNAL
REF
CURRENT
SOURCE
ARRAY
REFERENCE
CONTROL
AMPLIFIER
AD9764
The optimum distortion performance for any reconstructed
waveform is obtained with a 0.1
µF
external capacitor installed.
Thus, if I
REF
is fixed for an application, a 0.1
µF
ceramic chip
capacitor is recommended. Also, since the control amplifier is
optimized for low power operation, multiplying applications
requiring large signal swings should consider using an external
control amplifier to enhance the application’s overall large signal
multiplying bandwidth and/or distortion performance.
There are two methods in which I
REF
can be varied for a fixed
R
SET
. The first method is suitable for a single-supply system in
which the internal reference is disabled, and the common-mode
voltage of REFIO is varied over its compliance range of 1.25 V
to 0.10 V. REFIO can be driven by a single-supply amplifier or
DAC, thus allowing I
REF
to be varied for a fixed R
SET
. Since the
input impedance of REFIO is approximately 1 MΩ, a simple,
low cost R-2R ladder DAC configured in the voltage mode
topology may be used to control the gain. This circuit is shown
in Figure 25 using the AD7524 and an external 1.2 V reference,
the AD1580.
Figure 23. External Reference Configuration
REFERENCE CONTROL AMPLIFIER
The AD9764 also contains an internal control amplifier that is
used to regulate the DAC’s full-scale output current, I
OUTFS
.
The control amplifier is configured as a V-I converter, as shown
in Figure 23, such that its current output, I
REF
, is determined by
the ratio of the V
REFIO
and an external resistor, R
SET
, as stated
in Equation 4. I
REF
is copied over to the segmented current
sources with the proper scaling factor to set I
OUTFS
as stated in
Equation 3.
The control amplifier allows a wide (10:1) adjustment span of
I
OUTFS
over a 2 mA to 20 mA range by setting IREF between
62.5
µA
and 625
µA.
The wide adjustment span of I
OUTFS
–10–
REV. B
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