ADL5380ACPZ-R7 datasheet(26/36 Pages) AD

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ADL5380

Rev. 0 | Page 26 of 36

Figure 86 and Figure 87 show the excellent image rejection

capabilities of the ADL5380 for low IF applications, such as

W-CDMA. The ADL5380 exhibits image rejection greater than

45 dB over a broad frequency range.

400

800

1200

1600 2000

2400

2800

3200 3600

4000

RF FREQUENCY (MHz)

2.5MHz LOW IF

5MHz LOW IF

7MHz LOW IF

Figure 86. Low Band and Midband Image Rejection vs. RF Frequency for a

W-CDMA Signal, IF = 2.5 MHz, 5 MHz, and 7.5 MHz

5000

5200

5400

5600

5800

6000

RF FREQUENCY (MHz)

2.5MHz LOW IF

5MHz LOW IF

7MHz LOW IF

Figure 87. High Band Image Rejection vs. RF Frequency for a W-CDMA Signal,

IF = 2.5 MHz, 5 MHz, and 7.5 MHz

EXAMPLE BASEBAND INTERFACE

In most direct-conversion receiver designs, it is desirable to

select a wanted carrier within a specified band. The desired

channel can be demodulated by tuning the LO to the appropriate

carrier frequency. If the desired RF band contains multiple

carriers of interest, the adjacent carriers are also down converted to

a lower IF frequency. These adjacent carriers can be problematic if

they are large relative to the wanted carrier because they can

overdrive the baseband signal detection circuitry. As a result, it

is often necessary to insert a filter to provide sufficient rejection

of the adjacent carriers.

It is necessary to consider the overall source and load impedance

presented by the ADL5380 and ADC input when designing the

filter network. The differential baseband output impedance of

the ADL5380 is 50 Ω. The ADL5380 is designed to drive a high

impedance ADC input. It may be desirable to terminate the

ADC input down to lower impedance by using a terminating

resistor, such as 500 Ω. The terminating resistor helps to better

define the input impedance at the ADC input at the cost of a

slightly reduced gain (see the Circuit Description section for

details on the emitter-follower output loading effects).

The order and type of filter network depends on the desired high

frequency rejection required, pass-band ripple, and group delay.

Filter design tables provide outlines for various filter types and

orders, illustrating the normalized inductor and capacitor values

for a 1 Hz cutoff frequency and 1 Ω load. After scaling the

normalized prototype element values by the actual desired

cut-off frequency and load impedance, the series reactance

elements are halved to realize the final balanced filter network

component values.

As an example, a second-order Butterworth, low-pass filter design

is shown in Figure 88 where the differential load impedance is

500 Ω and the source impedance of the ADL5380 is 50 Ω. The

normalized series inductor value for the 10-to-1, load-to-source

impedance ratio is 0.074 H, and the normalized shunt capacitor

is 14.814 F. For a 10.9 MHz cutoff frequency, the single-ended

equivalent circuit consists of a 0.54 μH series inductor followed

by a 433 pF shunt capacitor.

The balanced configuration is realized as the 0.54 μH inductor

is split in half to realize the network shown in Figure 88.

433pF

RS = 50Ω

RL= 500Ω

0.54µH

0.27µH

433pF

BALANCED

CONFIGURATION

DENORMALIZED

SINGLE-ENDED

EQUIVALENT

RS = 50Ω

= 0.1

RL= 500Ω

LN = 0.074H

14.814F

NORMALIZED

SINGLE-ENDED

CONFIGURATION

= 25Ω

= 250Ω

fC = 10.9MHz

fC = 1Hz

Figure 88. Second-Order Butterworth, Low-Pass Filter Design Example

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