ADL5505 View Datasheet(PDF) - Analog Devices

Part Name

Description

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ADL5505

450 MHz to 6000 MHz TruPwr Detector

Analog Devices 

ADL5505

APPLICATIONS INFORMATION

BASIC CONNECTIONS

Figure 32 shows the basic connections for the ADL5505. The

device is powered by a single supply between 2.5 V and 3.3 V,

with a quiescent current of 1.8 mA. The VPOS pin is decoupled

using 100 pF and 0.1 µF capacitors.

Placing a single 75 Ω resistor at the RF input provides a

broadband match of 50 Ω. More precise resistive or reactive

matches can be applied for narrow frequency band use (see

the RF Input Interfacing section).

The ac residual can be reduced further by increasing the output

capacitance, COUT. The combination of the internal 100 Ω ouutpt

resistance and COUT produces a low-pass filter to reduce output

ripple of the VRMS output (see the Selecting the Output Low-

Pass Filter section for more details).

+VS = 2.5V TO 3.3V

1

VPOS

VRMS 4

0.1µF

100pF

ADL5505

ROUT

VRMS

COUT

RFIN

2 RFIN

R10

75Ω

COMM 3

Figure 32. Basic Connections for ADL5505

RF INPUT INTERFACING

The input impedance of the ADL5505 decreases with increasing

frequency in both its resistive and capacitive components (see

Figure 9). The resistive component varies from 370 Ω at 900 MHz

to about 245 Ω at 2600 MHz.

A number of options exist for input matching. For operation

at multiple frequencies, a 75 Ω shunt to ground, as shown in

Figure 33, provides the best overall match. For use at a single

frequency, a resistive or a reactive match can be used. By plotting

the input impedance on a Smith Chart, the best value for a

resistive match can be calculated. (Both input impedance and

input capacitance can vary by up to ±20% around their nominal

values.) Where VSWR is critical, the match can be improved

with a series inductor placed before the shunt component.

RF TRANSMISSION LINE

DIRECTIONAL

COUPLER

50Ω

ATTN

RFIN

75Ω

ADL5505

Figure 33. Input Interfacing to Directional Coupler

Resistive Tap RF Input

Figure 34 shows a technique for coupling the input signal into

the ADL5505 that can be applicable when the input signal is

much larger than the input range of the ADL5505. A series

resistor combines with the input impedance of the ADL5505

to attenuate the input signal. Because this series resistor forms

a divider with the frequency-dependent input impedance, the

apparent gain changes greatly with frequency. However, this

method has the advantage of very little power being tapped off

in RF power transmission applications. If the resistor is large

compared with the impedance of the transmission line, the

VSWR of the system is relatively unaffected.

RF TRANSMISSION LINE

RSERIES

RFIN

ADL5505

Figure 34. Attenuating the Input Signal

The resistive tap or series resistance, RSERIES, can be expressed as

RSERIES = RIN (1 − 10ATTN/20)/(10ATTN/20)

(1)

where:

RIN is the input impedance of RFIN.

ATTN is the desired attenuation factor in decibels.

For example, if a power amplifier with a maximum output power

of 28 dBm is matched to the ADL5505 input at 5 dBm, then a

−23 dB attenuation factor is required. At 900 MHz, the input

resistance, RIN, is 370 Ω.

RSERIES = (370 Ω)(1 − 10−23/20)/(10−23/20) = 4856 Ω

(2)

Thus, for an attenuation of −23 dB, a series resistance of apprxo-

imately 4.87 kΩ (the nearest available standard resistor value)

is needed.

Rev. A | Page 14 of 20

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