AD7762 datasheet(19/28 Pages) AD

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AD7762

Rev. 0 | Page 19 of 28

SUPPLY DECOUPLING

DIFFERENTIAL AMPLIFIER COMPONENTS

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:

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 stated performance.

•

Pin 12 (AVDD4) must have a 10 Ω resistor inserted between

the pin and a 10 nF decoupling capacitor.

•

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.

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 AD7762 product page on

the Analog Devices website contains the gerber files for the

AD7762 evaluation board. These files should be used as a

reference when designing any system using the AD7762.

ADDITIONAL DECOUPLING

There are two other decoupling pins on the AD7762—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.

The location and orientation of some of the components

mentioned in previous sections is critical, and particular

attention must be paid to the components which are located

close to the AD7762. Locating these components further away

from the devices can have a direct impact on the maximum

performance achievable.

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 AD7762. The

reference voltage supplied to the AD7762 should be decoupled

and filtered, as shown in Figure 30.

The recommended scheme for the reference voltage supply

is a 100 Ω series resistor connected to a 100 μF tantalum

capacitor, followed by series resistor of 10 Ω, and finally a

10 nF decoupling capacitor very close to the VREF+ pin.

The use of ground planes also should 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 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.

+12V

PIN10

VOUT

+VIN

C15

μF

100nF

C10

100nF

R30

100

R17

C46

10nF

C11

100

μF

ADR434

GND

Figure 30. Reference Connection

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