Datasheet
VCA810
V
C
20W
20W
ADC
and DSP
V
C
t
0
-2V
Time-Gain Compensated Control Voltage
OPA657
-V
B
20kW
C
F
VCA810
C
H
0.1 Fm
V-
V
IN
OPA820
V
R
V = V
OUT PEAK R
V
O
R
3
1kW
HP5082
R
1
50kW
2mV to 2V
100kHz
RSSI
Port
R
2
50kW
C
C
47pF
0.1 VDC
R
4
100W
V
C
Time (5 s/div)m
V = 10mV
IN PP
V = 100mV
IN PP
V = 1V
IN PP
0.15
0.10
0.05
0
0.05
0.10
0.15
0.20-
-
-
-
Output Voltage (50mV/div)
VCA810
SBOS275F –JUNE 2003–REVISED DECEMBER 2010
www.ti.com
RANGE-FINDING TGC AMPLIFIER charging the holding capacitor. This charge drives the
capacitor voltage in a positive direction, reducing the
The block diagram in Figure 31 illustrates the
amplifier gain. R
3
and the C
H
largely determine the
fundamental configuration common to pulse-echo
attack time of this AGC correction. Between gain
range finding systems. A photodiode preamp
corrections, resistor R
1
charges the capacitor in a
provides an initial gain stage to the photodiode.
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.
Figure 33 shows the AGC response for the values
shown in Figure 32.
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
Figure 32. 60dB Input Range AGC
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.
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 Figure 32 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
.
Figure 33. AGC Output Voltage for 100kHz
Whenever a V
O
peak exceeds V
R
, the OPA820
Sinewave at 10mV
PP
, 100mV
PP
, and 1V
PP
output swings positive, forward-biasing the diode and
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