AD8313
3
2
INPUT COUPLING
BALANCED
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.
TERMINATED
DR = 66dB
1
MATCHED
0
BALANCED
–1
–2
–3
DR = 71dB
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.
MATCHED
DR = 69dB
–90 –80 –70 –60 –50 –40 –30 –20 –10
INPUT AMPLITUDE (dBm)
0
10
Figure 33. Comparison of Terminated, Matched, and Balanced
Input Drive at 900 MHz
3
TERMINATED
DR = 75dB
2
C1
680pF
50Ω SOURCE
50Ω
AD8313
MATCHED
1
TERMINATED
R
MATCH
53.6Ω
C2
C
R
IN
IN
680pF
0
MATCHED
DR = 73dB
BALANCED
–1
–2
–3
Figure 32. A Simple Broadband Resistive Input Termination
BALANCED
DR = 75dB
The high-pass corner frequency can be set higher according to
the equation
0
–90 –80 –70 –60 –50 –40 –30 –20 –10
INPUT AMPLITUDE (dBm)
10
1
f3 dB
where:
C =
=
2 × π × C × 50
Figure 34. Comparison of Terminated, Matched, and Balanced
Input Drive at 1.9 GHz
NARROW-BAND LC MATCHING EXAMPLE
AT 100 MHz
C1× C2
C1× C2
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.
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.
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).
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.
Rev. D | Page 16 of 24
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