Nikkei Electronics Asia -- February 2007
Monthly Special
Video Signal Interfacing via AC, DC Coupling

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Jan 26, 2007 18:25 Nikkei Electronics Asia
Video filter, driver, and switch matrix products can have various input/output coupling and clamping configurations. Choices include input AC or DC coupling, output AC or DC coupling, and various input clamping configurations. An example is the classic sync strip and pulse DC restore circuit, or alternatively, the continuous time clamp/bias circuit. Each type of coupling/clamping implementation has specific advantages and disadvantages for a specific application.

AC Coupled Input
It is common in video and graphics systems to AC couple the analog video input signals into a given device. This allows the receiving device to set its own optimum DC bias level on the device side of the capacitor independently of the driving signalˇÇs DC bias level. For example, a receiving device of an analog-to-digital converter may wish to set the clamping level or blanking level of the video signal equal to the internal ADC code zero voltage, regardless of the driving signals absolute DC level. Another example would be in a purely analog system where the receiving device may wish to set the analog signalˇÇs common mode level around VCC/2 to optimize its headroom in processing the signal. The receiving device can also match the clamped level to a predetermined DC reference voltage allowing for a consistent and stable DC output voltage. Also, by blocking DC the receiving device protects itself from any potentially damaging DC current flow.

Sync Strip, DC Restore
A classic way to implement DC restore for any AC-coupled video input signal is by slicing through the embedded sync portion of the video signal and creating a digital pulse. This pulse can be used to trigger a charge pump circuit. Each time a sync signal is detected, the charge pump circuit is enabled, charging or discharging the input capacitor proportional to an on-chip error voltage in a closed-loop system. The system converges on the bias voltage over the first several lines and thereafter maintains or corrects for small errors on each successive line. Some devices use this type of clamping DC restore system. The circuit is triggered from the channel that includes the embedded sync, usually a Y, G, or CV channel. The chrominance channels (Pb, Pr, or C) are also pumped to the appropriate DC levels during the triggered event. This implementation has several advantages and also some inherent disadvantages.

One advantage of this type of clamping is that it can be triggered by the horizontal sync event. This allows updates and corrections in the bias level for every line, even during a non-active portion of the video. Since the DC reference level can be generated from an on-chip band gap voltage, the system can lock to a known DC level. This will allow the DC output levels of the device to be accurately controlled and remain very flat over the temperature and voltage variations of the system, which is an advantage when using the DC output coupling option. In addition, it allows for an external digital separated sync input which triggers the clamping event.

Clamp/Bias Input Circuitry
Some video/driver/switch products offer clamp/bias circuitry on the input of the chip to set the DC bias for AC-coupled applications. The circuit default is set to the clamp mode and by using a 7.5M¦¸ resistor to VCC the device can be set to the bias mode. In clamp mode the circuit clamps the lowest level (usually the bottom of the sync tip) of the incoming video signal to a predetermined on-chip reference voltage level. The clamp circuit is not triggered by a sync tip event, but rather by a continuous time circuit that will clamp the lowest level of the input at a predetermined DC level and will prevent the signal from falling below this level.

In bias mode, the input is biased to the mid-scale reference voltage level through an on-chip high impedance source. The clamp referred to in this section operates differently than the pulsed DC restore circuit described previously. There is no internal sync stripper and pulsed charge pump circuit, and there is no closed loop system monitoring the output level. This has the advantage of reducing on-chip circuitry which translates to less die area and thus a lower cost device. It also has the advantage of being independent of predetermined timing and formats since no sync stripper and pulse generator are necessary, allowing more compatibility with unsupported video and/or graphics formats. These devices can also be driven with a DC-coupled input that may be advantageous in certain applications such as a known DC input drive. Limitations when compared to the closed loop pulsed DC restore approach include the fact that output DC voltages may vary with system temperature and supply voltage variations.

DC-Coupled Input
Some other filter/driver devices are designed specifically for DC-coupled input applications. The intent is that these devices will be driven by an input that is single ended, ground referenced, and DC-coupled. An example would be a standard current mode output of a video/graphics DAC. These common DAC devices use the doubly terminated 75¦¸ load (37.5¦¸) as the load for the current stirring DAC to develop the output voltage. Therefore, the DAC output in this type of system has a known DC level that is ground referenced.

The limitations are that the input signal must be at a known DC level and biased with a voltage swing in the range of 0 to 1.3V DC. There is no feedback control on the absolute DC level of the output voltage and it may vary with system temperature and supply voltage.

AC-Coupled Output
The most common approach used to feed a video signal to a visual media device is to AC couple the signal. This allows the receiving device to set the common mode level on its input, independent of the incoming video signal DC level. A 75¦¸ series resistor should be placed as close to the device output as possible. This will help isolate the down stream parasitics from the output of the device and provide for optimal signal conditions. The AC coupling capacitor should be 200µF, minimum. This is the smallest coupling capacitor that can be utilized and still achieve acceptable field tilt. Most applications have more stringent field tilt requirements and use a 470µF or 1,000µF as coupling capacitors.

DC Coupled Output
The most direct approach used to feed a video signal to a visual media device is to DC couple the signal. This eliminates the need for a coupling capacitor and allows for a tilt free signal to be sent to the media device. One disadvantage to this approach is that the receiving device will need to know the incoming DC levels so it can process the video signal properly. This will work for a system designed to handle known DC levels but may cause a problem with systems which expect the common mode level at a different reference point.

The video filter/driver and switch matrix products have a variety of possible input/output signal coupling and clamping configurations. System designers have the choice of input/output AC or DC coupling. The choices for clamping options include the classic sync strip and pulsed DC restore circuit, the continuous time clamped/bias circuit, or DC coupling. Each type of implementation has specific advantages and limitations for a given application, which needs to be taken into consideration by the system designer.

by Earl Schreyer,
Duane Sorlie,
Fairchild Semiconductor