OPA361AIDCKT Datasheet, PDF (10/17 Pages) Texas Instruments – 3V VIDEO AMPLIFIER with Internal Gain and Filter in SC70

OPA361
SBOS334A − SEPTEMBER 2005 − REVISED JANUARY 2006
INTERNAL LEVEL SHIFT
Many common video DACs embedded in digital media
processors, like the new OMAP242x processors, operate
on a single supply (no negative supply). Typically, the
lowest point of the sync pulse output by these video DACs
is close to 0V. With a 0V input, the output of a common
single-supply op amp saturates at a voltage > 0V. This
effect would clip the sync pulse, and therefore degrade the
video signal integrity. The OPA361 employs an internal
level shift circuit to avoid clipping. The input signal is
typically shifted by approximately 11mV. This shift is well
within the linear output voltage range of the OPA361 with
a standard 150Ω video load.
Output Swing Capability
Figure 7 shows the true output swing capability of the
OPA361 by taking the tip of the input sync pulse to a
slightly negative voltage. Even when the output sync tip is
at 3mV, the output after the 75Ω series termination still
shows no clipping of the sync pulse.
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voltage level is pulled LOW if the TV (or other video
equipment) is connected, or HIGH if nothing is connected.
A GPIO in the processor can be used to read this logic level
and decide if a video load is connected. Figure 8 shows a
scope plot with the TV disconnected and Figure 9 shows
a scope plot with the TV connected; the upper line in both
figures is the disable pulse. Figure 10 shows a circuit
drawing using the TV-detect signal to disable or enable the
OPA361.
OPA361 Disable Pulse
OPA361 Ouput
Pulled HIGH;
No Video Load
Connected
∆: 1.84V
@: 1.84V
Equilization Pulses
1st Vertical Sync Pulse
Vertical Sync Pulses
∆: 305mV
@: 3.00mV
Figure 8. Output of OPA361 Pulled Up To 1.8V
During Disable: TV Disconnected
OPA361 Disable Pulse
Figure 7. No Clipping of the Sync Pulse
TV-Detect Function
The TV-detection feature of the OPA361 works in
conjunction with the OMAP242x (or other processors) to
detect if a television is connected to the video output of the
device. In order to detect a TV load, the OPA361 is briefly
turned off, ideally during the first vertical sync pulse. For
the detection, a simple pull-up resistor to the processor
logic supply is used on the output of the OPA361. The
OPA361 Ouput Pulled LOW
Due to Video Load
Equilization Pulses
1st Vertical Sync Pulse
Vertical Sync Pulses
Figure 9. Output of OPA361 Pulled Down:
TV Connected.
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