MT-059 datasheet(14/17 Pages) AD

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AD8044

REV. B

–13–

Active Filters

Active filters at higher frequencies require wider bandwidth op

amps to work effectively. Excessive phase shift produced by

lower frequency op amps can significantly impact active filter

performance.

Figure 38 shows an example of a 2 MHz biquad bandwidth

filter that uses three op amps of an AD8044 package. Such

circuits are sometimes used in medical ultrasound systems to

lower the noise bandwidth of the analog signal before A/D

conversion.

VIN

50pF

AD8044

50pF

VOUT

Figure 38. 2 MHz Biquad Band-pass Filter Using AD8044

The frequency response of the circuit is shown in Figure 39.

FREQUENCY (Hz)

–10

10k

100M

100k

10M

–20

–30

–40

Figure 39. Frequency Response of 2 MHz Band-pass

Biquad Filter

Layout Considerations

The specified high speed performance of the AD8044 requires

careful attention to board layout and component selection.

Proper RF design techniques and low-pass parasitic component

selection are necessary.

The PCB should have a ground plane covering all unused por-

tions of the component side of the board to provide a low im-

pedance path. The ground plane should be removed from the

area near the input pins to reduce the stray capacitance.

Chip capacitors should be used for the supply bypassing. One

end should be connected to the ground plane and the other

within 1/8 inch of each power pin. An additional large (0.47

– 10

mF) tantalum electrolytic capacitor should be connected in

parallel, but not necessarily so close, to supply current for fast,

large signal changes at the output.

The feedback resistor should be located close to the inverting

input pin in order to keep the stray capacitance at this node to a

minimum. Capacitance variations of less than 1 pF at the invert-

ing input will significantly affect high speed performance.

Stripline design techniques should be used for long signal traces

(greater than about 1 inch). These should be designed with a

characteristic impedance of 50

W or 75 W and properly termi-

nated at each end.

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