AD9767 Analog Devices, AD9767 Datasheet - Page 22

• Manufacturer Part Number: AD9767
• Manufacturer: Analog Devices

Specifications of AD9767

• Resolution (bits): 14bit
• Dac Update Rate: 125MSPS
• Dac Settling Time: 35ns
• Max Pos Supply (v): +5.5V
• Single-supply: Yes
• Dac Type: Current Out
• Dac Input Format: Par

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
segmented current sources with the proper scaling factor. The
full-scale current (I
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.
ADDITIONAL
An external reference can be applied to REFIO as shown in
Figure 60. The external reference can provide either a fixed
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.
EXTERNAL
REFIO
REFERENCE
LOAD
EXTERNAL
Figure 60. External Reference Configuration Gain Control Mode
REFERENCE
EXTERNAL
OPTIONAL
) sets the reference current I
BUFFER
AVDD
R
I
REF
SET
Figure 59. Internal Reference Configuration
0.1µF
OUTFS
I
REF
R
256Ω
SET
22nF
) is 32 × I
REFIO
FSADJ1/
FSADJ2
GAINCTRL
256Ω
22nF
1.2V
REF
REF
GAINCTRL
REFIO
FSADJ1/
FSADJ2
REF
1.2V
REF
.
, which is replicated to the
REFERENCE
AD9763/
AD9765/
AD9767
SECTION
REFERENCE
AD9763/
AD9765/
AD9767
SECTION
CURRENT
SOURCE
ARRAY
AVDD
CURRENT
SOURCE
ARRAY
ACOM
AVDD
ACOM
Rev. G | Page 22 of 44
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
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
for both DAC1 and DAC2 is set via the FSADJ1 terminal.
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
converter, as shown in Figure 59, so that its current output (I
is determined by the ratio of the V
R
The DAC full-scale current, I
larger than the reference current, I
The control amplifier allows a wide (10:1) adjustment span of
I
625 μA. The wide adjustment range of I
benefits. The first relates directly to the power dissipation of the
AD9763/AD9765/AD9767, which is proportional to I
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
Figure 59 and Figure 60) from the FSADJ1 terminal to ground
and from the FSADJ2 terminal to ground.
OUTFS
SET
.
I
I
REF
OUTFS
from 2 mA to 20 mA by setting I
= V
OUTFS
= 32 × I
SET
REFIO
). The control amplifier is configured as a V-I
resistor. In master/slave mode, full-scale current
/R
REF
SET
OUTFS
, is an output current 32 times
REFIO
REF
.
SET
REF
and an external resistor,
OUTFS
resistors. One resistor
between 62.5 μA and
provides several
Data Sheet
OUTFS
(refer to
REF
)