Data Sheet

ADL5506

Rev. A | Page 19 of 23

Input Coupling Options

The internal 5 pF coupling capacitor of the ADL5506, along

with the low frequency input impedance of 1.7 kΩ, gives a high-

pass input corner frequency of approximately 19 MHz. This sets

the minimum operating frequency. Figure 46 to Figure 48 show

three options for input coupling. A broadband resistive match

can be implemented by connecting a shunt resistor to ground at

RFIN (see Figure 46). This 52.3 Ω resistor (other values can also

be used to select different overall input impedances) combines

with the input impedance of the AD5506 to give a broadband

input impedance of 50 Ω. While the input resistance and

±20% from device to device, the dominance of the external

shunt resistor means that the variation in the overall input

impedance is close to the tolerance of the external resistor.

Achieve better return loss by placing the 52.3 Ω shunt resistor

as near the device under test (DUT) as possible.

A reactive match can also be implemented, as shown in Figure 47.

This is not recommended at low frequencies because device

tolerances dramatically vary the quality of the match due to the

large input resistance. For low frequencies, the option shown in

Figure 46 or Figure 48 is recommended.

In Figure 47, the matching components are drawn as general

reactances. Depending on the frequency, the input impedance

at that frequency and the availability of standard value components,

either a capacitor or an inductor, is used. As in the previous

case, the input impedance at a particular frequency is plotted on

a Smith Chart and matching components are chosen (Shunt or

Series L, or Shunt or Series C) to move the impedance to the center

of the chart. Matching components for specific frequencies can be

calculated using the Smith Chart (see Figure 17). Table 4 outlines

the input impedances for some commonly used frequencies.

The impedance matching characteristics of a reactive matching

network provide voltage gain ahead of the ADL5506, which

increases device sensitivity (see Table 4). The voltage gain is

calculated by

R1

R2

Gain

Voltage

dB

10

log

20



where:

R2 is the input impedance of the ADL5506.

R1 is the source impedance to which the ADL5506 is being

matched.

Note that this gain is only achieved for a perfect match. Component

tolerances and the use of standard values tend to reduce gain.

Figure 46. Broadband Resistive Method for Input Coupling

Figure 47. Narrow-Band Reactive Method for Input Coupling

Figure 48. Series Attenuation Method for Input Coupling

Figure 48 shows a third method for coupling the input signal

into the ADL5506 in applications where the input signal is

larger than the input range of the log amp. A series resistor,

connected to the RF source, combines with the input impedance of

the ADL5506 to resistively divide the input signal being applied

to the input. This has the advantage of very little power being

tapped off in RF power transmission applications.

Table 4. Input Impedance with 52.3 Ω Shunt for Select

Frequency

Frequency

Impedance Ω

(GHz)

Real

Imaginary

(Series)

0.05

+0.0023

−0.031

50.14 − j3.10

0.1

−0.014

−0.033

48.48 − j3.21

0.9

−0.144

+0.007

37.37 − j0.51

1.9

−0.052

+0.282

38.66 − j23.73

2.2

+0.074

+0.329

45.89 − j34.09

2.5

+0.233

+0.312

61.85 − j45.48

3.0

+0.467

+0.096

131.91 − j32.64

3.5

+0.394

−0.305

81.58 − j66.25

Table 5. Raw Input Impedance for Select Frequency

Frequency

Impedance Ω

(GHz)

Real

Imaginary

(Series)

0.1

+0.838

−0.251

131.9 − j281.59

0.9

−0.206

+0.714

11.41 − j36.33

1.9

−0.571

+0.397

9.84 + j15.11

2.2

−0.284

+0.639

12.42 + j31.05

2.5

+0.077

+0.699

18.87 − j52.17

3.0

+0.564

+0.407

72.70 + j114.52

3.5

+0.602

−0.099

186.70 − j58.55

RFIN

ADL5506

ADL5506

50Ω SOURCE

RFIN

X2

X1

50Ω

RFIN

STRIPLINE

ADL5506