Datasheet

OPA656
R
2
300
Ω
R
E
OTA
V
IN
R
B
V
OUT
H
CL
C
HOLD
•
Current Control
•
Non−Inverting
OPA615
100
Ω
2
3
4
7
11
10
12
100
Ω
V
REF
100
Ω
R
1
300
Ω
SOTA
C
B
Hold /Track
50
Ω
100
Ω
150
Ω
100
Ω
OTA
2
12
3
4
300
Ω
50
Ω
C
HOLD
22pF
V
IN
10
11
7
300
Ω
V
OUT
SOTA
OPA615
+2.5
+1.5
+0.5
−
0.5
−
1.5
−
2.5
1MHz SAMPLE−AND−HOLD OF A 100kHz SINEWAVE
Time (1
µ
s/div)
Output Voltage (V)
5
4
3
2
1
0
Hold−and−Track Signal (V)
OPA615
www.ti.com ...................................................................................................................................... SBOS299E –FEBRUARY 2004–REVISED SEPTEMBER 2009
CLAMPED VIDEO/RF AMPLIFIER
The external capacitor (C
HOLD
) allows for a wide
range of flexibility. By choosing small values, the
Another circuit example for the preamplifier and the
circuit can be optimized for a short clamping period or
clamp circuit is shown in Figure 44. The preamplifier
with high values for a low droop rate. Another
uses the wideband, low noise OPA656, again
advantage of this circuit is that small clamp peaks at
configured in a gain of +2V/V. Here, the OPA656 has
the output of the switching comparator are integrated
a typical bandwidth of 200MHz with a settling time of
and do not cause glitches in the signal path.
about 21ns (0.02%) and offers a low bias current
JFET input stage.
SAMPLE-AND-HOLD AMPLIFIER
With a control propagation delay of 2.5ns and
730MHz bandwidth, the OPA615 can be used
advantageously in a high-speed sample-and-hold
amplifier. Figure 45 illustrates this configuration.
Figure 44. Clamped Video/RF Amplifier
Figure 45. Sample-and-Hold Amplifier
The video signal passes through the capacitor C
B
,
To illustrate how the digitization is realized in the
blocking the DC component. To restore the DC level
Figure 45 circuit, Figure 46 shows a 100kHz
to the desired baseline, the OPA615 is used. The
sinewave being sampled at a rate of 1MHz. The
inverting input (pin 11) is connected to a reference
output signal used here is the I
OUT
output driving a
voltage. During the high time of the clamp pulse, the
50Ω load.
switching comparator (SOTA) will compare the output
of the op amp to the reference level. Any voltage
difference between those pins will result in an output
current that either charges or discharges the hold
capacitor, C
HOLD
. This charge creates a voltage
across the capacitor, which is buffered by the OTA.
Multiplied by the transconductance, the voltage will
cause a current flow in the collector, C, terminal of
the OTA. This current will level-shift the OPA656 up
to the point where its output voltage is equal to the
reference voltage. This level-shift also closes the
control loop. Because of the buffer, the voltage
across the C
HOLD
stays constant and maintains the
baseline correction during the off-time of the clamp
pulse.
Figure 46. 1MHz Sample-and-Hold of a 100kHz
Sine Wave
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