AD8307AN Analog Devices Inc, AD8307AN Datasheet - Page 17

• Manufacturer Part Number: AD8307AN
• Description: IC LOGARITHMIC AMP 8-DIP
• Manufacturer: Analog Devices Inc
• Type: Logarithmic Amplifierr

Specifications of AD8307AN

• Rohs Status: RoHS non-compliant
• Applications: Receiver Signal Strength Indication (RSSI)
• Mounting Type: Through Hole
• Package / Case: 8-DIP (0.300", 7.62mm)

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 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
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:
60 Hz hum (picked up due to poor grounding techniques),
spurious coupling (from a digital clock source on the same PC
board), and local radio stations, for example.
Careful shielding is essential. A ground plane should be used to
provide a low impedance connection to the common pin, COM,
for the decoupling capacitors used at VPS, and as the output
ground. It is inadvisable to assume that the ground plane is
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.
BASIC CONNECTIONS
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, for example, C
constant is R
3 dB attenuation at f
tions, f
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 should extend
from −85 dBV to +6 dBV.
–75dBm TO
HP
+16dBm
should be as large as possible to minimize the coupling
INPUT
IN
NC = NO CONNECT
C
R
C
T
/2, thus forming a high-pass corner with a
HP
Figure 32. Basic Connections
= 1/(pR
1.1kΩ
R
IN
C1 = C
C2 = C
≈
INM COM OFS OUT
0.1µF
INP VPS ENB INT
IN
C
C
8
1
C
C
AD8307
7
2
). In high frequency applica-
4.7Ω
NC
6
3
NC
C
5
4
. The coupling time
V
AT ~8mA
OUTPUT
25mV/dB
P
, 2.7V TO 5.5V
Rev. D | Page 17 of 24
Where it is necessary to terminate the source at a low impedance,
the resistor R
effect of the basic 1.1 kΩ input resistance (R
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
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
amp inputs.
Figure 33 shows the output vs. 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.
Figure 34. Logarithmic Law Conformance at 10 MHz, 100 MHz, and 500 MHz
Figure 33. Log Response at 10 MHz, 100 MHz, and 500 MHz
3.0
2.5
2.0
1.5
1.0
0.5
–1
–2
–3
–4
–5
5
4
3
2
1
0
0
–80
–80
10MHz
T
–70
should be added, with allowance for the shunting
–70
500MHz
–60
–60
100MHz
–50
–50
INPUT LEVEL (dBm)
INPUT LEVEL (dBm)
–40
–40
IN
500MHz
is lowered directly at the log
–30
–30
10MHz
100MHz
–20
–20
–10
–10
IN
) of the AD8307.
0
0
AD8307
10
10
20
20