The TV output function of the mobile device outputs a composite video signal, which is the most commonly used video signal and can be received by almost any TV. In addition, portable devices such as cell phones or portable media players require a way to convert digital video signals into analog signals and convert them into NTSC or PAL composite video signals. This way you can see the signal on an external TV. In addition, the analog signal needs to be amplified and impedance matched to a cable with a characteristic impedance of 75 ohms. This composite video signal contains all the information needed to recover the video at the receiving end, including line sync and field sync, luminance and chrominance signals. The driving force is enhanced, and the resulting image is more stable and more beautiful.
Video filter/driver
In addition to reconstructing the filter, a video driver is required to amplify the video signal and drive a 75 ohm coaxial cable. The amplifier needs 6dB of gain to accommodate dual termination loads. The IA171 integrated video filter/driver solution combines a reconstruction filter with a low impedance video driver. The device operates from 2.8V to 5.5V. Compared to typical 2nd and 3rd order passive solutions, 4th order filters provide better image quality. This filter/driver is preferably driven directly by a DC-coupled DAC output, but also operates with an AC-coupled input. The device's input common-mode range is 1.2Vpp referenced to ground. Its output can drive a single 75 ohm coaxial cable (150 ohm) load with AC or DC coupling. DC coupled outputs do not require expensive output coupling capacitors. If the output is AC-coupled, the SAG correction circuit can be used to reduce the capacitance and physical size of the AC output coupling capacitor while still producing acceptable field tilt.
SAG correction
Traditionally, if the video application is AC-coupled, a very large output coupling capacitor (between 220F and 1000F) would be required. The SAG correction circuit provides superior performance with a small output coupling capacitor, eliminating the need for large coupling capacitors. The usual output circuit (220F at 150 ohm load) creates a single pole (-3dB) at 5Hz. Reducing this capacitance can cause excessive phase shifts, causing the video field to tilt, thereby preventing proper recovery of the sync signal.
The SAG correction circuit in the IA171 has a small peak value and therefore provides phase response compensation that can significantly reduce the tilt of the video field. This SAG correction, the output does not use coupling capacitors, which are much smaller than the volume and cost of the capacitors required by other circuits, saving peripheral components.
Field of application
Surveillance cameras, matrix, DVR, optical transceivers, splitters, mobile phones, PMP, GPS, car DVD, car TV, TV set-top boxes, portable DVD, digital cameras.
