AD8307
USING TH E AD 8307
Figure 31 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 32 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. But if the termi-
nation resistor is omitted, the input power is negligible. T he use
of dBm to define input level therefore needs to be considered
carefully in connection with the AD8307.
T he 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 60 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 quite indis-
tinguishable from the “wanted” signal, and will have the effect
of raising the apparent noise floor (that is, lowering the useful
dynamic range). For example, while the signal of interest may
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:
60 Hz hum, picked up due to poor grounding techniques; spuri-
ous coupling from a digital clock source on the same PC board;
local radio stations; etc.
3
2.5
10MHz
2
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, however, and neither of the inputs should be ac-
coupled directly to the ground plane, but kept separate from it,
being returned instead to the low associated with the source.
T his may mean isolating the low side of an input connector with
a small resistance to the ground plane.
1.5
100MHz
1
500MHz
0.5
0
–80 –70 –60 –50 –40 –30 –20 –10
0
10
20
Basic Connections
INPUT LEVEL – dBm
Figure 30 shows the simple connections suitable for many appli-
cations. T he inputs are ac-coupled by C1 and C2, which should
have the same value, say, CC. T he coupling time-constant is RIN
CC/2, thus forming a high pass corner with a 3 dB attenuation at
fHP = 1/(p RIN CC ). In high frequency applications, fHP should
be as large as possible, in order to minimize the 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 +6 dBV.
Figure 31. Log Response at 10 MHz, 100 MHz and 500 MHz
5
4
3
2
500MHz
1
0
10MHz
–1
–2
–3
–4
–5
100MHz
0.1F
4.7⍀
V , 2.7V – 5.5V
P
AT 8mA
C1 = C
C
NC
–80 –70 –60 –50
–30
INPUT LEVEL – dBm
–10
0
10
20
–40
–20
INP VPS ENB INT
R
INϷ
INPUT
–75dBm TO
+16dBm
R
AD8307
Figure 32. Logarithm ic Law Conform ance at 10 MHz,
100 MHz and 500 MHz
T
1.1k⍀
INM COM OFS OUT
OUTPUT
25mV/dB
NC
Input Matching
C2 = C
C
Where higher sensitivity is required, an input matching network
is valuable. Using a transformer to achieve the impedance trans-
formation also eliminates the need for coupling capacitors,
lowers the offset voltage generated directly at the input, and
balances the drives to INP and INM. T he choice of turns ratio
will depend 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 will lower the input imped-
ance to 50 Ω while raising the input voltage, and thus lowering
the effect of the short circuit noise voltage by the same factor.
T here will be a small contribution from the input noise current,
so the total noise will be reduced by a somewhat smaller factor.
T he intercept will also be lowered by the turns ratio; for a
50 Ω match, it will be reduced by 20 log10 (4.8) or 13.6 dB.
NC = NO CONNECT
Figure 30. Basic Connections
Where it is necessary to terminate the source at a low imped-
ance, 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. T his may be placed on the
input side or the log-amp side of the coupling capacitors; in the
former case, smaller capacitors can be used for a given fre-
quency range; in the latter case, the effective RIN is lowered
directly at the log-amp inputs.
REV. A
–13–
ADI [ ADI ]
