adl5505 Analog Devices, Inc., adl5505 Datasheet - Page 14

• Manufacturer Part Number: adl5505
• Description: 450 Mhz To 6000 Mhz Trupwr Detector Adl5505
• Manufacturer: Analog Devices, Inc.

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, C
resistance and C
ripple of the VRMS output (see the Selecting the Output Low-
Pass Filter section for more details).
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.
0.1µF
+V
S
RFIN
= 2.5V TO 3.3V
DIRECTIONAL
100pF
RF TRANSMISSION LINE
Figure 33. Input Interfacing to Directional Coupler
COUPLER
OUT
Figure 32. Basic Connections for ADL5505
R10
75Ω
OUT
. The combination of the internal 100 Ω output
produces a low-pass filter to reduce output
1
2
VPOS
RFIN
ADL5505
ATTN
50Ω
COMM
VRMS
75Ω
4
3
ADL5505
R
RFIN
OUT
C
OUT
VRMS
Rev. 0 | Page 14 of 20
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.
The resistive tap or series resistance, R
where:
R
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, R
Thus, for an attenuation of −23 dB, a series resistance of approx-
imately 4.87 kΩ (the nearest available standard resistor value)
is needed.
IN
is the input impedance of RFIN.
R
R
SERIES
SERIES
= R
= (370 Ω)(1 − 10
IN
, is 370 Ω.
IN
RF TRANSMISSION LINE
Figure 34. Attenuating the Input Signal
(1 − 10
ATTN/20
R
−23/20
SERIES
)/(10
)/(10
ATTN/20
ADL5505
RFIN
−23/20
SERIES
)
) = 4856 Ω
, can be expressed as
(1)
(2)