ADL5502ACBZ-P2 datasheet(16/28 Pages) AD

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ADL5502

Rev. A | Page 16 of 28

APPLICATIONS INFORMATION

BASIC CONNECTIONS

Figure 44 shows the basic connections for the ADL5502. The

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

with a quiescent current of 3 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 rms averaging can be augmented by placing additional

capacitance at CFLTR. The ac residual can be reduced further by

increasing the output capacitance, COUT. The combination of the

internal 100 Ω output resistance and COUT produce a low-pass

filter to reduce output ripple of the VRMS output (see the

Selecting the Square-Domain Filter and Output Low-Pass Filter

section for more details).

RFIN

CFLTR

+VS 2.5V TO 3.3V

CONTROL

(HIGH RESET;

LOW PEAK HOLD)

VPEAK

VRMS

ROUT

0.1µF

0.1pF

COUT

FLTR

VPOS

RFIN

VRMS

PEAK

CNTL

COMM

ENBL

ADL5502

R10

75Ω

Figure 44. Basic Connections for ADL5502

To measure the peak of a waveform, the control line (CNTL)

must be temporally set to high (reset mode for >1 μs) and then set

back to low (peak-hold mode). This allows the ADL5502 to be

initialized to a known state. When setting the device to measure

peak, peak-hold mode should be toggled for a period in which

the input rms power and CF is not likely to change.

If the ADL5502 is in peak-hold mode and the CF changes from

high to low or the input power changes from high to low, a

faulty peak measurement is reported. The ADL5502 simply

keeps reporting the highest peak that occurred when the peak-

hold mode was activated and the input power or the CF was

high. Unless CNTL is reset, the PEAK output does not reflect

the new peak in the signal.

RF INPUT INTERFACING

The input impedance of the ADL5502 decreases with increasing

frequency in both its resistive and capacitive components (see

Figure 34). The resistive component varies from 330 Ω at 900 MHz

to about 240 Ω at 1900 MHz.

A number of options exist for input matching. For operation

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

Figure 45, 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 prior to the shunt component.

ADL5502

RFIN

RF TRANSMISSION LINE

50Ω

DIRECTIONAL

COUPLER

75Ω

ATTN

Figure 45. Input Interfacing to Directional Coupler

Resistive Tap RF Input

Figure 46 shows a technique for coupling the input signal into

the ADL5502 that can be applicable where the input signal is

much larger than the input range of the ADL5502. A series

resistor combines with the input impedance of the ADL5502

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 to impedance of the transmission line, the VSWR of

the system is relatively unaffected.

ADL5502

RFIN

RF TRANSMISSION LINE

RSERIES

Figure 46. 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 dB.

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

of 28 dBm is matched to the ADL5502 input at 5 dBm, then a

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

resistance, RIN, is 330 Ω.

RSERIES

= (330 Ω)(1 − 10−23/20)/(10−23/20) = 4330 Ω

(2)

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

approximately 4.33 kΩ is needed.

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