AD603 Просмотр технического описания (PDF) - Analog Devices
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AD603 Datasheet PDF : 25 Pages
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AD603
Data Sheet
APPLICATIONS INFORMATION
A LOW NOISE AGC AMPLIFIER
Figure 49 shows the ease with which the AD603 can be
connected as an AGC amplifier. The circuit illustrates many of
the points previously discussed: it uses few parts, has linear-in-
dB gain, operates from a single supply, uses two cascaded amplifiers
in sequential gain mode for maximum SNR, and an external
resistor programs each gain of the amplifier. It also uses a
simple temperature-compensated detector.
The circuit operates from a single 10 V supply. Resistors R1, R2,
R3, and R4 bias the common pins of A1 and A2 at 5 V. The
common pin is a low impedance point and must have a low
impedance path to ground, provided here by the 100 μF tantalum
capacitors and the 0.1 μF ceramic capacitors.
The cascaded amplifiers operate in sequential gain. Here, the
offset voltage between Pin 2 (GNEG) of A1 and A2 is 1.05 V
(42.14 dB × 25 mV/dB), provided by a voltage divider consisting of
Resistors R5, R6, and R7. Using standard values, the offset is not
exact, but it is not critical for this application.
The gain of both A1 and A2 is programmed by Resistors R13
and R14, respectively, to be about 42 dB; therefore, the maximum
gain of the circuit is twice that, or 84 dB. The gain control range
can be shifted up by as much as 20 dB by appropriate choices of
R13 and R14.
The circuit operates as follows:
A1 and A2 are cascaded.
Capacitor C1 and the 100 Ω of resistance at the input of A1
form a time constant of 10 μs.
C2 blocks the small dc offset voltage at the output of A1
(which might otherwise saturate A2 at its maximum gain)
and introduces a high-pass corner at about 16 kHz,
eliminating low frequency noise.
A half-wave detector is used, based on Q1 and R8. The current
into capacitor, CAV, is the difference between the collector
current of Q2 (biased to be 300 μA at 300 K, 27°C) and the
collector current of Q1, which increases with the amplitude
of the output signal.
The automatic gain control voltage, VAGC, is the time integral
of this error current. For VAGC (and thus the gain) to remain
insensitive to short-term amplitude fluctuations in the output
signal, the rectified current in Q1 must, on average, exactly
balance the current in Q2. If the output of A2 is too small to
do this, VAGC increases, causing the gain to increase until Q1
conducts sufficiently.
Consider the case where R8 is zero and the output voltage VOUT
is a square wave at, for example, 455 kHz, which is well above
the corner frequency of the control loop.
10V
J1
R T1
100Ω
C32 + C4
100µF 0.1µF
C7
0.1µF
10V
C1
0.1µF
10V
R1
2.49kΩ
8
3
6
A1
AD603
4
1
R2
2.49kΩ
R13
2.49kΩ
5
7
2
C2
0 .1µF
C8
0.1µF
10V
R3
2.49kΩ
THIS CAPACITOR SETS
AGC TIME CONSTANT
VAGC
10V
8
3
6
A2
AD603
4
1
CAV
R1 4 0.1µF
2.49kΩ
5
7
2
C52 + C6
100µF 0.1µF
R4
2.49kΩ
AGC LINE
R9
1.54kΩ
Q2
2N3906
Q1
2N3904
R8
806Ω
R5
5.49kΩ
5.5V
1V OFFSET FOR
SEQUENTIAL GAIN
R6
1.05kΩ
R7
3.48kΩ
10V
6.5V
1 RT PR OVI D ES A 5 0Ω IN PU T I MPED A N C E.
2C3 AND C5 ARE TANTALUM.
Figure 49. A Low Noise AGC Amplifier
R10
1.24kΩ
C11
0 .1µF
R11
3.83kΩ
5V
R12
4.99kΩ
C10
0 .1µF
C9
0 .1µF
J2
Rev. K | Page 18 of 24
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