AD8307 Low Cost DC—500 MHz, 92 dB Logarithmic Amplifier Data ...

AD8307
USING THE AD8307
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 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 indistinguishable
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; spurious
coupling from a digital clock source on the same PC board; local
radio stations; and so on.
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 may mean isolating the low side of an input connector with
a small resistance to the ground plane.
Basic Connections
Figure 12 shows the simple connections suitable for many applications.
The inputs are ac-coupled by C1 and C2, which should
have the same value, say, C C . The coupling time constant is
R IN C C /2, thus forming a high-pass corner with a 3 dB attenuation
at f HP = 1/(pR IN C C ). In high frequency applications, f HP 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.
INPUT
–75dBm TO
+16dBm
4.7
C1 = C C
NC
R
INP VPS ENB INT
IN
R T AD8307
1.1k
INM COM OFS OUT
C2 = C C
0.1F
NC
NC = NO CONNECT
V P , 2.7V–5.5V
AT 8mA
OUTPUT
25mV/dB
Figure 12. Basic Connections
Where it is necessary to terminate the source at a low impedance,
the resistor R T should be added, with allowance for the
shunting effect of the basic 1.1 kΩ input resistance (R IN ) of the
AD8307. For example, to terminate a 50 Ω source, a 52.3 Ω 1%
tolerance resistor should be used. This 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
frequency range; in the latter case, the effective R IN is lowered
directly at the log-amp inputs.
Figure 13 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 14 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 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.
OUTPUT VOLTAGE (V)
3.0
2.5
2.0
1.5
1.0
0.5
500MHz
10MHz
100MHz
0
–80 –70 –60 –50 –40 –30 –20 –10 0 10 20
INPUT LEVEL (dBm)
Figure 13. Log Response at 10 MHz, 100 MHz, and 500 MHz
ERROR (dB)
5
4
3
2
1
0
–1
–2
–3
–4
10MHz
500MHz
100MHz
–5
–80 –70 –60 –50 –40 –30 –20 –10 0 10 20
INPUT LEVEL (dBm)
Figure 14. Logarithmic Law Conformance at 10 MHz,
100 MHz, and 500 MHz
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,
lowers the offset voltage generated directly at the input, and
balances the drives to INP and INM. The 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 impedance to
50 Ω while raising the input voltage, thus lowering the effect of
the short circuit noise voltage by the same factor. There will be
a small contribution from the input noise current, so the total
noise will be reduced by a somewhat smaller factor. The intercept
will also be lowered by the turns ratio; for a 50 Ω match, it will
be reduced by 20 log 10 (4.8) or 13.6 dB.
REV. B
–13–