AD7763 datasheet(23/32 Pages) AD

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AD7763

Rev. 0 | Page 23 of 32

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

• Pin 12 (AV

DD4

) must have a 10 Ω resistor inserted between

the pin and a 10 nF decoupling capacitor.

• Pin 27 (AV

DD2

) does not require a separate decoupling

capacitor or a direct connection to the supply; instead,

it is connected to Pin 14 via an 8.2 nH inductor.

The ferrite beads that are used to connect each supply pin to the

appropriate power supply should have a characteristic impedance

of 600 Ω to 1 MΩ at frequencies around 100 MHz, a dc impedance

of 1 Ω or less, and a rated current of 200 mA.

ADDITIONAL DECOUPLING

There are two other decoupling pins on the AD7763: 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 AD7763. The

reference voltage supplied to the AD7763 should be decoupled

and filtered, as shown in Figure 36.

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 decoupling capacitor very close to the VREF pin.

12V

PIN 10

VOUT

+VIN

C15

μF

100nF

C10

100nF

R30

100

R17

C46

10nF

C11

100

μF

ADR434

GND

Figure 36. Reference Connection

DIFFERENTIAL AMPLIFIER COMPONENTS

The correct components for use around the on-chip differential

amplifier are shown in Table 10. 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 stated performance.

EXPOSED PADDLE

The AD7763 64-lead TQFP_EP employs a 6 mm × 6 mm exposed

paddle (see Figure 39). The paddle reduces the thermal

resistance of the package by providing a path of low thermal

resistance to the PCB and, in turn, increases the heat transfer

efficiency from the AD7763 package. Soldering the exposed

paddle to the AGND plane of the PCB is fundamental in

creating the conditions that allow the AD7763 package to

perform to the highest specifications possible.

LAYOUT CONSIDERATIONS

While using the correct components is essential to achieve

optimum performance, the correct layout is just as important.

The Design Tools section of the AD7763 product page on

the Analog Devices website contains the Gerber files for the

AD7763 evaluation board. These files should be used as a

reference when designing any system using the AD7763.

The location and orientation of some of the components

mentioned in previous sections are critical, and particular

attention must be paid to the components that are located close

to the AD7763. Locating these components farther away from

the devices can have a direct impact on the maximum

performance achievable.

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 upon as the sole return path for decoupling capacitors,

because the return current path using ground planes is not

easily predicted.

AD7763 Datasheet(PDF) 23 Page - Analog Devices

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