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AD7760
Rev. A | Page 29 of 36
SUPPLY DECOUPLING
Every supply pin must be connected to the appropriate supply
via a ferrite bead and decoupled to the correct ground pin with
a 100 nF, 0603 case size, X7R dielectric capacitor. There are two
exceptions to this:
Pin 12 (AVDD4) must have a 10 Ω resistor inserted between
the pin and a 10 nF decoupling capacitor, which is
connected to ground at Pin 9.
Pin 27 (AVDD2) does not require a separate decoupling
capacitor or a direct connection to the supply, but instead
is connected to Pin 14 via a 15 nH inductor.
ADDITIONAL DECOUPLING
There are two other decoupling pins on the AD7760—Pin 8
(DECAPA) and Pin 30 (DECAPB). Pin 8 should be decoupled
with a 100 nF capacitor, and Pin 30 requires a 33 pF capacitor.
REFERENCE VOLTAGE FILTERING
A low noise reference source, such as the
ADR431 (2.5 V) or
ADR434 (4.096 V), is suitable for use with the AD7760. The
reference voltage supplied to the AD7760 should be decoupled
The recommended scheme for the reference voltage supply is a
100 Ω series resistor connected to a 100 μF tantalum capacitor,
followed by a series resistor of 10 Ω, and finally a 10 nF capacitor
placed as close as possible to the VREF+ pin, decoupling this
capacitor to the associated ground pin, Pin 11.
7.5V
PIN 10
VOUT
2 V
IN
6
4
C15
10F
C9
100nF
R30
100
R17
10
+
C46
10nF
C11
100F
+
ADR434
GND
U3
04
97
5-
0
47
Figure 56. Reference Connection
DIFFERENTIAL AMPLIFIER COMPONENTS
The correct components for use around the on-chip differential
amplifier are detailed in
Table 8. Matching the components on
both sides of the differential amplifier is important to minimize
distortion of the signal applied to the amplifier. A tolerance of
0.1% or better is required for these components. Symmetrical
routing of the tracks on both sides of the differential amplifier
also assists in achieving the stated performance.
BIAS RESISTOR SELECTION
The AD7760 requires a resistor to be connected between the
RBIAS and AGND pins. The value of this resistor is dependent
on the reference voltage being applied to the device. The resistor
value should be selected to produce a current of 25 μA through
the resistor to ground. For a 2.5 V reference voltage, the correct
resistor value is 100 kΩ, and for a 4.096 V reference, the correct
resistor value is 160 kΩ.
LAYOUT CONSIDERATIONS
While using the correct components is essential to achieve
optimum performance, the correct layout is just as important.
contains the Gerber files for the AD7760 evaluation board.
These files should be used as a reference when designing any
system using the AD7760.
The location and orientation of some of the components
mentioned in previous sections of this data sheet are critical,
and particular attention must be paid to the components that
are located close to the AD7760. Locating these components
farther away from the device can have a direct impact on the
achievable maximum performance.
The use of ground planes should also be carefully considered.
To ensure that the return currents through the decoupling
capacitors are flowing to the correct ground pin, the ground side
of the capacitors should be as close as possible to the ground pin
associated with that supply. A ground plane should not be relied
on as the sole return path for decoupling capacitors because the
return current path using ground planes is not easily predictable.
EXPOSED PADDLE
The AD7760 64-lead TQFP employs a 6 mm × 6 mm exposed
paddle (see
Figure 59). The paddle reduces the thermal
resistance of the package by providing a path for heat energy to
flow between the package and the PCB and, in turn, increases
the heat transfer efficiency from the AD7760 package.
Connecting the exposed paddle to the AGND plane of the PCB
is essential in creating the conditions that allow the AD7760
package to perform to the highest specifications possible. The
exposed paddle should not be connected directly to any of the
ground pins on the AD7760 and should only be connected to
the analog ground plane. Best practice is to use multiple vias
connecting the exposed paddle to the AGND plane of the PCB.