Differential Input and Output Considerations

The CLC5506 typical application requires DC blocking ca-

pacitors for both inputs and outputs to main internal DC bias-

ing points.

The input impedance between the differential inputs (IN+,

IN-) is 200

Ω//0.5pF. Since the 0.5pF capacitance can be ne-

glected in the VHF band, a 1:4 impedance ratio balun can be

used to transform a 50

Ω source to the 200Ω differential in-

puts of CLC5506 for wide band design.

The CLC5506 has a pair of open collector differential outputs

(OUT+, OUT-). DC biasing is achieved through an RF induc-

tor. The RF inductor acts as a choke to block RF leakage and

interference. An external resistor across the differential out-

puts is used to set the output resistance of CLC5506. Wide-

band output matching to an unbalanced 50

Ω load can be

achieved by using a 1:n balun. A 1:4 impedance ratio balun

is used when a 200

Ω external resistor is used in a 50Ω sys-

tem.

Although the CLC5506 can be used as a single-ended de-

vice by grounding one of the inputs through a capacitor, the

noise figure would be severely degraded by 6dB.

The CLC5506 can also directly interface to balanced de-

vices, like SAW filters and ADCs. Narrowband design ex-

ample with ADC CLC5956 and SAW filter is provided below.

The component values of matching inductors and capacitors

depend on the actual input/output impedance of the SAW fil-

ter, ADC, PWB properties, layout and frequency band.

CLC5506 Evaluation Board

A proper printed circuit layout is essential for achieving high

frequency performance. To expedite evaluation, an as-

sembled and tested evaluation kit CLC5506PCASM is avail-

able for sale. See application note AN-1138 for technical de-

tails of evaluation kit. Order information and application note

is available on the Web at http://www.national.com

DS101050-20

FIGURE 4. Narrow band design example with balanced SAW filter and ADC

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