Datasheet

AD8307
Rev. D | Page 18 of 24
INPUT MATCHING
Where higher sensitivity is required, an input matching network
is valuable. Using a transformer to achieve the impedance
transformation also eliminates the need for coupling capacitors,
which lowers the offset voltage generated directly at the input,
and balances the drives to Pin INP and Pin INM. The choice of
turns ratio depends somewhat on the frequency. At frequencies
below 50 MHz, the reactance of the input capacitance is much
higher than the real part of the input impedance. In this frequency
range, a turns ratio of about 1:4.8 lowers the input impedance to
50  while raising the input voltage, thus lowering the effect of
the short-circuit noise voltage by the same factor. There is a
small contribution from the input noise current, so the total
noise is reduced by a lesser factor. The intercept is also lowered
by the turns ratio; for a 50  match, it is reduced by 20 log
10
(4.8) or 13.6 dB.
NARROW-BAND MATCHING
Transformer coupling is useful in broadband applications. How-
ever, a magnetically coupled transformer may not be convenient in
some situations. At high frequencies, it is often preferable to use
a narrow-band matching network, as shown in Figure 35.
Using a narrow-band matching network has several advantages.
The same voltage gain is achieved, providing increased sensitivity,
but a measure of selectivity is also introduced. The component
count is low: two capacitors and an inexpensive chip inductor.
Further, by making these capacitors unequal, the amplitudes at
Pin INP and Pin INM can be equalized when driving from a
single-sided source, that is, the network also serves as a balun.
Figure 36 shows the response for a center frequency of 100 MHz.
Note the very high attenuation at low frequencies. The high fre-
quency attenuation is due to the input capacitance of the log amp.
C2
C1
OUTPUT
25mV/dB
01082-035
AD8307
50Ω INPUT
–88dBm TO
+3dBm
NC
INP VPS ENB INT
INM COM OFS OUT
NC
NC = NO CONNECT
Z
IN
= 50Ω
8765
234
1
4.7Ω
0.1µ
F
L
M
V
P
, 2.7V TO 5.5
V
AT ~8mA
Figure 35. High Frequency Input Matching Network
14
4
–1
3
2
1
0
INPUT
GAIN
9
8
7
6
5
13
12
11
10
01082-036
DECIBELS
FREQUENCY (MHz)
60 150140130120110100908070
Figure 36. Response of 100 MHz Matching Network
Table 4 provides solutions for a variety of center frequencies (f
C
)
and matching impedances (Z
IN
) of nominally 50  and 100 .
The unequal capacitor values were chosen to provide a well-
balanced differential drive and to allow better centering of the
frequency response peak when using standard value components,
which generally results in a Z
IN
that is not exact. The full AD8307
HF input impedance and the inductor losses are included in the
modeling.
Table 4. Narrow-Band Matching Values
f
C
(MHz) Z
IN
(Ω) C1 (pF) C2 (pF) L
M
(nH) Voltage Gain (dB)
10 45 160 150 3300 13.3
20 44 82 75 1600 13.4
50 46 30 27 680 13.4
100 50 15 13 330 13.4
150 57 10 8.2 220 13.2
200 57 7.5 6.8 150 12.8
250 50 6.2 5.6 100 12.3
500 54 3.9 3.3 39 10.9
10 103 100 91 5600 10.4
20 102 51 43 2700 10.4
50 99 22 18 1000 10.6
100 98 11 9.1 430 10.5
150 101 7.5 6.2 260 10.3
200 95 5.6 4.7 180 10.3
250 92 4.3 3.9 130 9.9
500 114 2.2 2.0 47 6.8