AD8313

Rev. D | Page 16 of 24

INPUT COUPLING

The signal can 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

impedance matching network.

Figure 32 shows a simple broadband resistive match. A

termination resistor of 53.6 Ω combines with the internal input

impedance of the AD8313 to give an overall resistive input

impedance of approximately 50 Ω. It is preferable to place the

termination resistor directly across the input pins, INHI to

INLO, where it lowers 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 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.

53.6

Ω

C2

680pF

C1

680pF

AD8313

50

Ω

50

Ω SOURCE

Figure 32. A Simple Broadband Resistive Input Termination

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

the equation

50

2

1

3

×

×

π

×

=

C

f

dB

where:

C2

C1

C2

C1

C

×

×

=

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.

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 33. 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 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

calculate the values of matching components, a clear under-

standing 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-inductance. However, the concurrent requirement that

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

series-capacitance/shunt-inductance type match more

appropriate (Figure 35).