any stray parasitic impedance into a resonant circuit. By tun-

ing the resonant load, full gain can be achieved with a given

resistive load.

A typical tuned load is shown below in

Figure 3, where the

resonant frequency is tuned about 150 MHz.

The 1000

Ω load in this circuit can represent the input imped-

ance of the CLC5956 Analog to Digital converter. Actual val-

ues for the reactive components may vary slightly to account

for board and device parasitic elements.

Typical Application

Although the CLC5526 can be used as a general purpose

digital variable gain amplifier, it was specifically designed to

provide the variable gain function in National’s Diversity Re-

ceiver Chipset. In this application, the CLC5526 drives a

tuned BPF and the CLC5956 Analog to Digital converter.

Digitized IF data is downsampled and tuned with the

CLC5902 dual digital tuner which also provides the AGC

control function. AGC data is fed back to the CLC5526. The

CLC5956 differential input impedance is 1000

Ω, so with the

tuned load, full gain of the CLC5526 is achieved.

Figure 4

shows the block diagram of the Diversity Receiver Chipset

application.

Figure 5 shows the SINAD vs Input Power of the

diversity receiver chipset. For input power levels ranging

from 0 dB to −110 dB, the chip set provides a signal to noise

ratio in excess of the 9 dB required for a typical GSM system.

Layout Considerations

A proper printed circuit layout is essential for achieving high

frequency performance. National Semiconductor provides

evaluation boards for the CLC5526, which include input and

output transformers for impedance matching and single to

differential signal conversion.

Supply bypassing is required for best performance. Provide

a 6.8 µF Tantalum and 0.1 µF ceramic capacitor as close as

possible to the supply pin.

In addition, a 100 pF ceramic capacitor should be placed be-

tween the COMP pin (pin 9) and the system ground. This will

filter high frequency noise from the common-mode level.

Ceramic coupling capacitors should be used to AC couple

both the input and output. Actual values will depend upon the

signal frequency.

DS015016-19

FIGURE 3. CLC5526 Driving a Tuned Load

DS015016-20

FIGURE 4. Diversity Receiver Chipset Block Diagram

DS015016-21

FIGURE 5. Diversity Receiver Chipset

SINAD vs Input Power

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