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AD8313

Rev. D | Page 17 of 24

LMATCH

CIN

RIN

AD8313

Ω

Ω SOURCE

Figure 35. Narrow-Band Reactive Match

Typically, the AD8313 needs to be matched to 50 Ω. The input

impedance of the AD8313 at 100 MHz can be read from the

Smith chart (Figure 26) and corresponds to a resistive input

impedance of 900 Ω in parallel with a capacitance of 1.1 pF.

To make the matching process simpler, the AD8313 input cap-

acitance, CIN, can be temporarily removed from the calculation

by adding a virtual shunt inductor (L2), which resonates away

CIN (Figure 36). This inductor is factored back into the calculation

later. This allows the main calculation to be based on a simple

resistive-to-resistive match, that is, 50 Ω to 900 Ω.

The resonant frequency is defined by the equation

therefore,

CIN

CMATCH =

(C1

× C2)

(C1 + C2)

RIN

AD8313

Ω

Ω SOURCE

TEMPORARY

INDUCTANCE

LMATCH =

(C1

× C2)

(C1 + C2)

Figure 36. Input Matching Example

With CIN and L2 temporarily out of the picture, the focus is now

on matching a 50 Ω source resistance to a (purely resistive) load

of 900 Ω and calculating values for CMATCH and L1. When

MATCH

the input looks purely resistive at a frequency given by

MHz

100

MATCH

Solving for CMATCH gives

MATCH

Solving for L1 gives

337

2 0

Because L1 and L2 are parallel, they can be combined to give the

final value for LMATCH, that is,

294

L MATCH

C1 and C2 can be chosen in a number of ways. First, C2 can be

set to a large value, for example, 1000 pF, so that it appears as an

RF short. C1 would then be set equal to the calculated value of

CMATCH. Alternatively, C1 and C2 can each be set to twice CMATCH

so that the total series capacitance is equal to CMATCH. By making

C1 and C2 slightly unequal (that is, select C2 to be about 10%

less than C1) but keeping their series value the same, the ampli-

tude of the signals on INHI and INLO can be equalized so that

the AD8313 is driven in a more balanced manner. Any of the

options detailed above can be used provided that the combined

series value of C1 and C2, that is, C1 × C2/(C1 + C2) is equal to

CMATCH.

In all cases, the values of CMATCH and LMATCH must be chosen

from standard values. At this point, these values need now be

installed on the board and measured for performance at

100 MHz. Because of board and layout parasitics, the component

values from the preceding example had to be tuned to the final

values of CMATCH = 8.9 pF and LMATCH = 270 nH as shown in

Table 4.

Assuming a lossless matching network and noting conservation

of power, the impedance transformation from RS to RIN (50 Ω to

900 Ω) has an associated voltage gain given by

log

Gain

Because the AD8313 input responds to voltage and not to true

power, the voltage gain of the matching network increases the

effective input low-end power sensitivity by this amount. Thus,

in this case, the dynamic range is shifted downward, that is, the

12.6 dB voltage gain shifts the 0 dBm to −65 dBm input range

downward to −12.6 dBm to −77.6 dBm. However, because of

network losses, this gain is not be fully realized in practice.

Refer to Figure 33 and Figure 34 for an example of practical

attainable voltage gains.

Table 4 shows recommended values for the inductor and cap-

acitors in Figure 35 for some selected RF frequencies in addition

to the associated theoretical voltage gain. These values for a

reactive match are optimal for the board layout detailed as

Figure 45.

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