AD8313

–11–

REV. B

This should be arranged to increase the gain of the PA. The

Input Coupling

The signal may be coupled to the AD8313 in a variety of ways.

In all cases, there must not be a dc path from the input pins to

ground. Some of the possibilities include: dual input coupling

capacitors, a flux-linked transformer, a printed-circuit balun,

direct drive from a directional coupler, or a narrow-band imped-

ance matching network.

Figure 30 shows a simple broadband resistive match. A termina-

tion resistor of 53.6

Ω combines with the internal input imped-

ance of the AD8313 to give an overall resistive input impedance

of approximately 50

Ω. The termination resistor should prefer-

ably be placed directly across the input pins, INHI to INLO,

where it serves to lower the possible deleterious effects of dc

offset voltages on the low end of the dynamic range. At low

frequencies, this may not be quite as attractive, since it necessi-

tates the use of larger coupling capacitors. The two 680 pF

input coupling capacitors set the high-pass corner frequency of

the network at 9.4 MHz.

53.6

C1

680pF

C2

680pF

AD8313

50

50

SOURCE

Figure 30. A Simple Broadband Resistive Input Termination

The high pass corner frequency can be set higher according to

the equation:

f

C

dB

3

1

250

=

×× ×

π

CC

CC

=

×

+

12

12

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

or matching network is advantageous. The impedance match-

ing characteristics of these networks provide what is essentially a

gain stage before the AD8313 that increases the device sensitiv-

ity. This gain effect is further explored in the following match-

ing example.

Figures 31 and 32 show device performance under these three

input conditions at 900 MHz and 1900 MHz.

While the 900 MHz case clearly shows the effect of input

matching by realigning the intercept as expected, little improve-

ment is seen at 1.9 GHz. Clearly, if no improvement in sensitiv-

ity 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.

INPUT AMPLITUDE – dBm

–80

–70

–60

–50

–40

–30

–20

–10

3

2

1

0

–1

–2

–3

TERMINATED

DR = 66dB

–90

10

0

BALANCED

MATCHED

BALANCED

DR = 71dB

MATCHED

DR = 69dB

Figure 31. Comparison of Terminated, Matched and

Balanced Input Drive at 900 MHz

INPUT AMPLITUDE – dBm

–80

–70

–60

–50

–40

–30

–20

–10

0

3

2

1

0

–1

–2

–3

–90

10

TERMINATED

DR = 75dB

BALANCED

BALANCED

DR = 75dB

MATCHED

DR = 73dB

MATCHED

TERMINATED

Figure 32. Comparison of Terminated, Matched and

Balanced Input Drive at 1900 MHz

A Narrow-Band LC Matching Example at 100 MHz

While numerous software programs are available that allow the

values of matching components to be easily calculated, a clear

understanding of the calculations involved is valuable. A low

frequency (100 MHz) value has been used for this exercise

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-inductance. However, the concurrent requirement that the

AD8313 inputs, INHI and INLO, be ac-coupled, makes a

series-capacitance/shunt-inductance type match more appropri-

ate (see Figure 33).

C1

C2

AD8313

50

50

SOURCE

Figure 33. Narrow-Band Reactive Match