AD8307
THEORY OF OPERATION
The AD8307 has very high gain and a bandwidth from dc to
over 1 GHz, at which frequency the gain of the main path is still
over ꢀ0 dB. Consequently, it is susceptible to all signals within
this very broad frequency range that find their way to the input
terminals. It is important to remember that these are
indistinguishable from the wanted signal, and has the effect of
raising the apparent noise floor (that is, lowering the useful
dynamic range). For example, while the signal of interest can be
an IF of 50 MHz, any of the following could easily be larger
than the IF signal at the lower extremities of its dynamic range:
ꢀ0 Hz hum (picked up due to poor grounding techniques);
spurious coupling (from a digital clock source on the same PC
board); local radio stations; and so on.
the log amp side of the coupling capacitors; in the former case,
smaller capacitors can be used for a given frequency range; in
the latter case, the effective RIN is lowered directly at the log
amp inputs.
Figure 33 shows the output versus the input level, in dBm, when
driven from a terminated 50 Ω generator, for sine inputs at
10 MHz, 100 MHz, and 500 MHz; Figure 34 shows the typical
logarithmic conformance under the same conditions. Note that
+10 dBm corresponds to a sine amplitude of 1 V, equivalent to
an rms power of 10 mW in a 50 Ω termination. However, if the
termination resistor is omitted, the input power is negligible.
The use of dBm to define input level therefore needs to be
considered carefully in connection with the AD8307.
3.0
Careful shielding is essential. A ground plane should be used to
provide a low impedance connection to the common pin,
COM, for the decoupling capacitor(s) used at VPS, and as the
output ground. It is inadvisable to assume that the ground plane
is an equipotential. Neither of the inputs should be ac-coupled
directly to the ground plane, but should be kept separate from
it, being returned instead to the low associated with the source.
This can mean isolating the low side of an input connector with
a small resistance to the ground plane.
2.5
10MHz
2.0
1.5
100MHz
1.0
BASIC CONNECTIONS
500MHz
0.5
Figure 32 shows the simple connections suitable for many
applications. The inputs are ac coupled by C1 and C2, which
should have the same value, say, CC. The coupling time constant
is RIN CC/2, thus forming a high-pass corner with a 3 dB
attenuation at fHP = 1/(pRINCC ). In high frequency applications,
0
–80 –70 –60 –50 –40 –30 –20 –10
INPUT LEVEL (dBm)
0
10
20
Figure 33. Log Response at 10 MHz, 100 MHz, and 500 MHz
5
f
HP should be as large as possible in order to minimize the
4
coupling of unwanted low frequency signals. Conversely, in low
frequency applications, a simple RC network forming a low-
pass filter should be added at the input for the same reason. For
the case where the generator is not terminated, the signal range
should be expressed in terms of the voltage response, and
extends from −85 dBV to +ꢀ dBV.
3
2
500MHz
1
0
10MHz
–1
–2
0.1µF
100MHz
4.7Ω
V , 2.7V TO 5.5V
P
AT ~8mA
C1 = C
–3
–4
–5
C
8
NC
5
7
6
INPUT
–75dBm TO
+16dBm
INP VPS ENB INT
R
≈
IN
–80 –70 –60 –50 –40 –30 –20 –10
INPUT LEVEL (dBm)
0
10
20
R
T
AD8307
INM COM OFS OUT
1.1kΩ
Figure 34. Logarithmic Law Conformance at 10 MHz, 100 MHz, and 500 MHz
1
2
3
4
NC
OUTPUT
25mV/dB
INPUT MATCHING
C2 = C
C
NC = NO CONNECT
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
Figure 32. Basic Connections
Where it is necessary to terminate the source at a low impedance,
the resistor RT should be added, with allowance for the shunting
effect of the basic 1.1 kΩ input resistance (RIN) of the AD8307.
For example, to terminate a 50 Ω source, a 52.3 Ω 1% tolerance
resistor should be used. This can be placed on the input side or
Rev. C | Page ±7 of 24
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