Datasheet

OPA698
20
SBOS258D
www.ti.com
OPA698
V
L
V
H
Open
Open
V
L
V
H
V
OUT
R
3
402Ω
R
4
75Ω
OPA698
R
1
75Ω
R
2
402Ω
V
IN
–0.2V
FIGURE 19. Sync Stripper Circuit.
ORDERING LITERATURE
PRODUCT PACKAGE NUMBER NUMBER
OPA698ID SO-8 DEM-OPA-SO-1A SBOU009
TABLE I. Demonstration Fixture.
DC RESTORER
Figure 18 shows a DC restore circuit using the OPA698 and
OPA660. The buffer element of the OPA660 is used to buffer
the input signal while the transconductance element is used
to restore the DC level after the decoupling capacitor C
1
. The
DC level is set using R
1
and R
2
. The OPA698 is configured
at a gain of 2 to compensate for the 75Ω series into a 75Ω
load. The OPA698 also limits the output to ground.
VIDEO SYNC STRIPPER
Figure 19 shows a sync stripper using two OPA698 output-
limiting op amps. One OPA698 is configured as a limiting
inverting comparator. Referred to the input, the negative
excursions lower than –0.2V are clipped to ground, and all
excursions greater than –0.2V generate an output voltage
set by the default limiting value (–3.5V). The second OPA698
is using this waveform to effectively remove the sync pulse
from the video signal.
DESIGN-IN TOOLS
DEMONSTRATION FIXTURE
A printed circuit board (PCB) is available to assist in the initial
evaluation of circuit performance using the OPA698. The
fixture is offered free of charge as an unpopulated PCB,
delivered with user's guide. The summary information for this
fixture is shown in Table I.
FIGURE 18. DC Restore to Ground.
This demonstration fixture can be requested at the Texas
Instruments web site (www.ti.com) through the OPA698
product folder.
OPERATING SUGGESTIONS
THEORY OF OPERATION
The OPA698 is a voltage-feedback op amp that is unity-gain
stable. The output voltage is limited to a range set by the
voltage on the limiter pins (5 and 8). When the input tries to
overdrive the output, the limiters take control of the output
buffer. This action from the limiters avoids saturating any part
of the signal path, giving quick overdrive recovery and
excellent limiter accuracy at any signal gain. The limiters
have a very sharp transition from the linear region of opera-
tion to output limiting. This transition allows the limiter volt-
ages to be set very near (< 100mV) the desired signal range.
The distortion performance is also very good near the limiter
voltages.
OPA698
V
O
75Ω
Load
8
5
V
L
V
H
= Open
402Ω
R
2
1kΩ
R
3
200Ω
D
1
402Ω
20Ω
V
IN
200Ω
75Ω
R
Q
250Ω
R
1
19.6kΩ
D
2
C
1
20µF
6
1
C
E
B3
CCII
2
U1
5
+1
U1
U1 = OPA660
R
Q
= 250Ω (sets I
Q
for U1)
D
1
, D
2
= 1N4148