Data Sheet

AD8313

APPLICATIONS INFORMATION

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Rev. F | 16 of 23

beneficial, since it requires larger coupling capacitors. The two 680

pF input coupling capacitors set the high-pass corner frequency of

the network at 9.4 MHz.

Figure 32. A Simple Broadband Resistive Input Termination

The high-pass corner frequency can be set higher according to the

equation

f3 db= 12×π×C×50

(7)

where:

C=C1×C2C1×C2

(8)

In high frequency applications, the use of a transformer, balun,

or matching network is advantageous. The impedance matching

characteristics of these networks provide what is essentially a gain

stage before the AD8313 that increases the device sensitivity. This

gain effect is explored in the following matching example.

Figure 33 and Figure 34 show device performance under these

three input conditions at 900 MHz and 1.9 GHz.

While the 900 MHz case clearly shows the effect of input matching

by realigning the intercept as expected, little improvement is seen

at 1.9 GHz. Clearly, if no improvement in sensitivity is required, a

simple 50 Ω termination may be the best choice for a given design

based on ease of use and cost of components.

Figure 33. Comparison of Terminated, Matched, and Balanced Input Drive at

900 MHz

Figure 34. Comparison of Terminated, Matched, and Balanced Input Drive at

1.9 GHz

NARROW-BAND LC MATCHING EXAMPLE AT

100 MHZ

While numerous software programs provide an easy way to calcu-

late the values of matching components, a clear understanding of

the calculations involved is valuable. A low frequency (100 MHz)

value has been used for this example because of the deleterious

board effects at higher frequencies. RF layout simulation software is

useful when board design at higher frequencies is required.

A narrow-band LC match can be implemented either as a series-

inductance/shunt-capacitance or as a series-capacitance/ shunt-in-

ductance. However, the concurrent requirement that the AD8313

inputs, INHI and INLO, be ac-coupled, makes a series-capaci-

tance/shunt-inductance type match more appropriate (Figure 35).

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 capaci-

tance, 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

ω= 1L2×CIN

(9)