AD9763/AD9765/AD9767
The full-scale output current of each DAC is regulated by
separate reference control amplifiers and can be set from
2 mA to 20 mA via an external network connected to the full
scale adjust (FSADJ) pin. The external network, in combination
with both the reference control amplifier and voltage reference
(V
REFIO
) sets the reference current I
REF
, which is replicated to the
segmented current sources with the proper scaling factor. The
full-scale current (I
OUTFS
) is 32 × I
REF
.
Data Sheet
GAIN CONTROL MODE
The AD9763/AD9765/AD9767 has two gain control modes,
independent and master/slave. If the GAINCTRL terminal is low
(connected to ground), the full-scale currents of DAC1 and DAC2
are set separately using two different R
SET
resistors. One resistor
is connected to the FSADJ1 terminal, and the other resistor is
connected to the FSADJ2 terminal. This is independent mode.
If the GAINCTRL terminal is set high (connected to AVDD),
the full-scale currents of DAC1 and DAC2 are set to the same value
using one R
SET
resistor. In master/slave mode, full-scale current
for both DAC1 and DAC2 is set via the FSADJ1 terminal.
REFERENCE OPERATION
The AD9763/AD9765/AD9767 contain an internal 1.20 V band
gap reference. This can easily be overridden by a low noise external
reference with no effect on performance. REFIO serves as either
an input or output, depending on whether the internal or an
external reference is used. To use the internal reference, simply
decouple the REFIO pin to ACOM with a 0.1 μF capacitor. The
internal reference voltage is present at REFIO. If the voltage at
REFIO is used elsewhere in the circuit, an external buffer amplifier
with an input bias current of less than 100 nA should be used. An
example of the use of the internal reference is shown in Figure 59.
OPTIONAL
EXTERNAL
REFERENCE
BUFFER
GAINCTRL
AVDD
SETTING THE FULL-SCALE CURRENT
Both of the DACs in the AD9763/AD9765/AD9767 contain a
control amplifier that is used to regulate the full-scale output
current (I
OUTFS
). The control amplifier is configured as a V-I
converter, as shown in Figure 59, so that its current output (I
REF
)
is determined by the ratio of the V
REFIO
and an external resistor,
R
SET
.
I
REF
=
V
REFIO
/R
SET
The DAC full-scale current, I
OUTFS
, is an output current 32 times
larger than the reference current, I
REF
.
I
OUTFS
= 32 ×
I
REF
The control amplifier allows a wide (10:1) adjustment span of
I
OUTFS
from 2 mA to 20 mA by setting I
REF
between 62.5 μA and
625 μA. The wide adjustment range of I
OUTFS
provides several
benefits. The first relates directly to the power dissipation of the
AD9763/AD9765/AD9767, which is proportional to I
OUTFS
(refer to
the Power Dissipation section). The second relates to the 20 dB
adjustment, which is useful for system gain control purposes.
To ensure that the AD9763/AD9765/AD9767 performs properly,
connect a 22 nF capacitor and 256 Ω resistor network (shown in
and from the FSADJ2 terminal to ground.
1.2V
REF
ADDITIONAL
EXTERNAL
LOAD
REFIO
0.1µF
I
REF
R
SET
256Ω
22nF
FSADJ1/
FSADJ2
REFERENCE
SECTION
CURRENT
SOURCE
ARRAY
ACOM
00617-059
AD9763/
AD9765/
AD9767
Figure 59. Internal Reference Configuration
An external reference can be applied to REFIO as shown in
reference voltage to enhance accuracy and drift performance
or a varying reference voltage for gain control. The 0.1 μF
compensation capacitor is not required because the internal
reference is overridden and the relatively high input impedance
of REFIO minimizes any loading of the external reference.
GAINCTRL
AVDD
AVDD
EXTERNAL
REFERENCE
256Ω
I
REF
R
SET
22nF
1.2V
REF
REFIO
FSADJ1/
FSADJ2
REFERENCE
SECTION
CURRENT
SOURCE
ARRAY
ACOM
00617-060
AD9763/
AD9765/
AD9767
Figure 60. External Reference Configuration Gain Control Mode
Rev. G | Page 22 of 44
AD [ ANALOG DEVICES ]
