SBOS275F – JUNE 2003 – REVISED DECEMBER 2010
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RANGE-FINDING TGC AMPLIFIER
The block diagram in
illustrates the
fundamental configuration common to pulse-echo
range finding systems. A photodiode preamp
provides an initial gain stage to the photodiode.
20W
OPA657
VCA810
20kW
20W
C
F
V
C
-V
B
0
t
V
C
ADC
and DSP
charging the holding capacitor. This charge drives the
capacitor voltage in a positive direction, reducing the
amplifier gain. R
3
and the C
H
largely determine the
attack time of this AGC correction. Between gain
corrections, resistor R
1
charges the capacitor in a
negative direction, increasing the amplifier gain. R
1
,
R
2
, and C
H
determine the release time of this action.
Resistor R
2
forms a voltage divider with R
1
, limiting
the maximum negative voltage developed on C
H
. This
limit prevents input overload of the VCA810 gain
control circuit.
shows the AGC response for the values
shown in
V
IN
2mV to 2V
RSSI 100kHz
Port
VCA810
V
C
R
3
1kW HP5082
OPA820
R
1
50kW
R
2
50kW
C
H
0.1
m
F
V
O
V
OUT PEAK
= V
R
R
4
100W
V
R
C
C
47pF
0.1 VDC
-2V
Time-Gain Compensated Control Voltage
Figure 31. Typical Range-Finding Application
The control voltage V
C
varies the amplifier gain for a
basic signal-processing requirement: compensation
for distance attenuation effects, sometimes called
time-gain
compensation
(TGC).
Time-gain
compensation increases the amplifier gain as the
signal moves through the air to compensate for signal
attenuation. For this purpose, a ramp signal applied
to the VCA810 gain control input linearly increases
the dB gain of the VCA810 with time.
V-
Figure 32. 60dB Input Range AGC
0.15
0.10
0.05
0
-
0.05
-
0.10
-
0.15
-
0.20
Time (5
m
s/div)
V
IN
= 10mV
PP
V
IN
= 1V
PP
V
IN
= 100mV
PP
WIDE-RANGE AGC AMPLIFIER
The voltage-controlled gain feature of the VCA810
makes this amplifier ideal for precision AGC
applications with control ranges as large as 60dB.
The AGC circuit of
adds an op amp and
diode for amplitude detection, a hold capacitor to
store the control voltage and resistors R
1
through R
3
that determine attack and release times. Resistor R
4
and capacitor C
C
phase-compensate the AGC
feedback loop. The op amp compares the positive
peaks of output V
O
with a dc reference voltage, V
R
.
Whenever a V
O
peak exceeds V
R
, the
output swings positive, forward-biasing the diode and
Output Voltage (50mV/div)
Figure 33. AGC Output Voltage for 100kHz
Sinewave at 10mV
PP
, 100mV
PP
, and 1V
PP
12
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