adl5504 Analog Devices, Inc., adl5504 Datasheet - Page 14

• Manufacturer Part Number: adl5504
• Description: 450 Mhz To 6000 Mhz Trupwr Detector
• Manufacturer: Analog Devices, Inc.

ADL5504
APPLICATIONS INFORMATION
BASIC CONNECTIONS
Figure 34 shows the basic connections for the ADL5504. 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 rms averaging can be augmented by placing additional
capacitance at C
increasing the output capacitance, C
the internal 100 Ω output resistance and C
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).
0.1µF
RF INPUT INTERFACING
The input impedance of the ADL5504 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 240 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 35, 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.
100pF
DIRECTIONAL
RFIN
RF TRANSMISSION LINE
Figure 35. Input Interfacing to Directional Coupler
COUPLER
Figure 34. Basic Connections for ADL5504
FLTR
C
+V
FLTR
R10
75Ω
. The ac residual can be reduced further by
S
= 2.5V TO 3.3V
1
2
3
FLTR
VPOS
RFIN
ATTN
ADL5504
50Ω
COMM
VRMS
ENBL
OUT
75Ω
. The combination of
6
5
4
ADL5504
RFIN
OUT
R
OUT
produces a low-
C
OUT
VRMS
Rev. 0 | Page 14 of 24
Resistive Tap RF Input
Figure 36 shows a technique for coupling the input signal into
the ADL5504 that can be applicable when the input signal is
much larger than the input range of the ADL5504. A series
resistor combines with the input impedance of the ADL5504
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 ADL5504 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
approximately 4.87 kΩ is needed.
IN
is the input resistance of RFIN.
R
R
SERIES
SERIES
= R
= (370 Ω)(1 − 10
IN
, is 370 Ω.
IN
RF TRANSMISSION LINE
Figure 36. Attenuating the Input Signal
(1 − 10
ATTN/20
R
−23/20
SERIES
)/(10
)/(10
ATTN/20
ADL5504
RFIN
−23/20
SERIES
)
) = 4870 Ω
, can be expressed as
(1)
(2)