AD8361

Data Sheet

Rev. F | Page 14 of 22

Figure 43. Input Coupling/Matching Options, Broadband Resistor Match

Figure 44. Input Coupling/Matching Options, Series Inductor Match

Figure 45. Input Coupling/Matching Options, Narrowband Reactive Match

Figure 46. Input Coupling/Matching Options, Attenuating the Input Signal

Table 5. Recommended Component Values for Resistive or

Inductive Input Matching (Figure 43 and Figure 44)

Frequency

Matching Component

100 MHz

63.4 Ω Shunt

800 MHz

75 Ω Shunt

900 MHz

75 Ω Shunt

1800 MHz

150 Ω Shunt or 4.7 nH Series

1900 MHz

150 Ω Shunt or 4.7 nH Series

2500 MHz

150 Ω Shunt or 2.7 nH Series

Alternatively, a reactive match can be implemented using a shunt

inductor to ground and a series capacitor, as shown in Figure 45. A

method for hand calculating the appropriate matching components

is shown on page 12 of the AD8306 data sheet.

especially at high frequencies. As a result, a stray capacitance as

small as 1 pF can significantly degrade the quality of the match.

The main advantage of a reactive match is the increase in

sensitivity that results from the input voltage being gained up

(by the square root of the impedance ratio) by the matching

network. Table 6 shows the recommended values for reactive

matching.

Table 6. Recommended Values for a Reactive Input

Matching (Figure 45)

Frequency (MHz)

100

16

180

800

2

15

900

2

12

1800

1.5

4.7

1900

1.5

4.7

2500

1.5

3.3

Input Coupling Using a Series Resistor

Figure 46 shows a technique for coupling the input signal into

the AD8361 that may be applicable where the input signal is

much larger than the input range of the AD8361. A series

resistor combines with the input impedance of the AD8361 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 transmission line’s impedance, then the VSWR

of the system is relatively unaffected.

Figure 47. Input Impedance vs. Frequency, Supply 3 V, SOT-23

Selecting the Filter Capacitor

The AD8361’s internal 27 pF filter capacitor is connected in

parallel with an internal resistance that varies with signal level

from 2 kΩ for small signals to 500 Ω for large signals. The

resulting low-pass corner frequency between 3 MHz and

12 MHz provides adequate filtering for all frequencies above

240 MHz (i.e., 10 times the frequency at the output of the

squarer, which is twice the input frequency). However, signals

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

signals, and low frequency components require additional

filtering. TDMA signals, such as GSM, PDC, or PHS, have a

peak-to average ratio that is close to that of a sinusoid, and the

internal filter is adequate.

AD8361

RFIN

RFIN

AD8361

RFIN

RFIN

AD8361

RFIN

RFIN

AD8361

RFIN

RFIN

FREQUENCY (MHz)

200

0

500

100

0

250

150

50

1000

1500

2000

2500

3000

3500

0.2

0.5

0.8

1.1

1.4

1.7