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ADL5501

Rev. B | Page 19 of

INPUT COUPLING USING A SERIES RESISTOR

Figure 43 shows a technique for coupling the input signal into

the ADL5501 that can be applicable where the input signal is

much larger than the input range of the ADL5501. A series

resistor combines with the input impedance of the ADL5501

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 the impedance of the transmission line, the VSWR

of the system is relatively unaffected.

ADL5501

RFIN

RSERIES

Figure 43. Attenuating the Input Signal

The resistive tap or series resistance, RSERIES, can 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 ADL5501 input at +5 dBm, then

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

resistance, RIN, is 55 Ω.

RSERIES = (55 Ω) (1 − 10−23/20)/(10−23/20) = 722 Ω

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

approximately 722 Ω is needed.

MULTIPLE RF INPUTS

Figure 44 shows a technique for combining multiple RF input

signals to the ADL5501. Some applications can share a single

detector for multiple bands. Three 16.5 Ω resistors in a T-network

combine the three 50 Ω terminations (including the ADL5501).

The broadband resistive combiner ensures that each port of the

T-network sees a 50 Ω termination. Because there are only 6 dB

of isolation from one port of the combiner to the other ports,

only one band should be active at a time.

ADL5501

RFIN

BAND 1

50Ω

BAND 2

DIRECTIONAL

COUPLER

16.5Ω

50Ω

16.5Ω

DIRECTIONAL

COUPLER

Figure 44. Combining Multiple RF Input Signals

SELECTING THE SQUARE-DOMAIN FILTER AND

OUTPUT LOW-PASS FILTER

The internal filter capacitor of the ADL5501 provides averaging

in the square domain but leaves some residual ac on the output.

Signals with high peak-to-average ratios, such as W-CDMA or

CDMA2000, can produce ac-residual levels on the ADL5501

dc output. To reduce the effects of these low frequency components

in the waveforms, some additional filtering is required.

The square-domain filter capacitance of the ADL5501 can be

augmented by connecting a capacitor between Pin 2 (FLTR) and

Pin 1 (VPOS). In addition, the output of the ADL5501 can be

filtered directly by placing a capacitor between VRMS (Pin 6)

and ground. The combination of the on-chip, 100 Ω output

series resistance and the external shunt capacitor forms a low-

pass filter to reduce the residual ac.

Table 4 shows the effects of several capacitor values for various

communications standards with high peak-to-average ratios

along with the residual ripple at the output, in peak-to-peak and

rms volts. Note that large load capacitances increase the turn-on

and pulse response times (see Figure 30, Figure 32, Figure 33,

Figure 35, Figure 37, and Figure 38). For more information on

the effects of the filter capacitances on the response, see the

Power Consumption, Enable, and Power-On/Power-Off

Response Time section.

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