OPA358 datasheet(10/19 Pages) TI2

10 / 19 page

OPA361

SBOS334A − SEPTEMBER 2005 − REVISED JANUARY 2006

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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 150

Ω 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 75

Ω series termination still

shows no clipping of the sync pulse.

∆:

305mV

3.00mV

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

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.

∆:

1.84V

OPA361 Disable Pulse

Equilization Pulses

OPA361 Ouput

Pulled HIGH;

No Video Load

Connected

Vertical Sync Pulses

1st Vertical Sync Pulse

Figure 8. Output of OPA361 Pulled Up To 1.8V

During Disable: TV Disconnected

OPA361 Disable Pulse

Equilization Pulses

OPA361 Ouput Pulled LOW

Due to Video Load

Vertical Sync Pulses

1st Vertical Sync Pulse

Figure 9. Output of OPA361 Pulled Down:

TV Connected.

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