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AN-6041 PCB Layout Considerations for Video Filter / Drivers
Power
The bulk capacitor (Tantalum or Electrolytic) should be placed reasonably close to the device. If used, a linear regulator for analog VCC should be close to the power area of the device. Use separate analog and digital power planes.
Analog GND and Digital GND
The ground plane is the most important layer in the PCB layout; it greatly affects the performance of analog components and signals. Proper layout of the ground plane keeps the board noise level within acceptable margins. Avoid long current loops, especially when mixing analog and digital signals. The best way to achieve this is to partition analog and digital ground very carefully and clearly so that signal and return current paths can be localized in their sections. If analog and digital circuitry is partitioned well, there is no need to split the ground. In most cases, a single solid ground plane is the best choice because it keeps ground potential lower between every ground point and helps reduce EMI. In a complex digital intensive design, it may be difficult to keep the analog area free from digital return current. In that case, there may be some benefit from cutting ground between the digital and analog and tying the two together under the device. Avoid any traces running across the split.
Decoupling Capacitors
Placement of bypass capacitors is important to maintain proper function. Every supply pin should connect to a ceramic decoupling capacitor. The distance from the device pin should be no greater than 0.1 inches, as shown in Figure 1. Place high-frequency decoupling capacitors as close to the device power supply pin as possible; without series vias between the capacitor and the device pin. This is normally done for the smallest capacitor, closest to the supply pin. Board space does not always allow for all bypass capacitors to be on the same plane; second and third capacitors may need vias to connect to the power supply pin.
Input Interface
Figure 2 shows a typical AC-coupled input configuration for driving the filter/driver. In this configuration, use a 0.1F ceramic capacitor to AC couple the input signal. The coupling capacitor and the input termination resistor at the input to the filter/driver should be placed close to the input pin for optimal signal integrity. If the input signal does not go below ground, the clamp is inactive; but if the input signal goes below ground, the clamp circuitry sets the bottom of the sync tip (or lowest voltage) to just below ground. The input level set by the clamp, combined with the internal DC offset, keeps the output signal within acceptable range. This clamp feature allows the input to be directly driven (DC-coupled) by a ground-referenced DAC output.
Figure 1.
Decoupling Capacitor Placement
Input Rterm 0.1F
Clamp/ Bias
LPF
Buf
Termination & Coupling close to device input
Figure 2.
Typical AC-Coupled Input Configuration for Driving the Filter / Driver
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(c) 2006 Fairchild Semiconductor Corporation Rev. 1.0.1 * 12/7/06
AN-6041
APPLICATION NOTE
Output Interface
To obtain the highest quality output signal, the series termination resistor must be placed as close to the device output pin as possible. This greatly reduces the parasitic capacitance and inductance effect on the output of the driver. The distance from device pin to the series termination resistor should be no greater than 0.1 inches, as shown in Figure 3. Figure 4 is the schematic representation of a video filter/driver used in a system as the output driver to a media device. Figure 4 shows the composite video signal terminated by the media device and the S-video output terminations open. It is very critical, in this case, to have the series termination resistors close to the output pin of the device to minimize the effects of parasitic capacitance on the filter output driver, which may show up as noise on the CV output.
Figure 3.
Proper Termination Resistor Placement
Fairchild Filter Driver 75-Ohm Clamp/ Bias
+ Buf Series Termination Resistor 0.1 inches from DUT pin
42
LPF
Filter/Driver set for gain of 2X
S-Video
31
75-Ohm
Summer
+
RCA JACK
Coax ADC
Series Termination Resistor 0.1 inches from DUT pin
75-Ohm
Media
Fairchild Filter Driver 75-Ohm Clamp/ Bias
+ Buf Series Termination Resistor 0.1 inches from DUT pin
LPF
Filter/Driver set for gain of 2X
Figure 4.
Schematic Representation of a Video Filter / Driver in a System
(c) 2006 Fairchild Semiconductor Corporation Rev. 1.0.1 * 12/7/06
www.fairchildsemi.com 2
AN-6041
APPLICATION NOTE
Printed Circuit Board (PCB) Layers
It is best to use, as a minimum, a four-layer PCB. Assign one inner layer to the dedicated signal ground plane and one inner layer to the single or split power plane, as shown in Figure 5.
Figure 5.
Recommended PCB Layers
Layout Considerations
General layout and supply bypassing play major roles in high-frequency performance and thermal characteristics. Fairchild offers a demonstration board for each product to guide layout and aid device evaluation. The demo boards are four-layer boards with full power and ground planes. Following this layout configuration provides the optimum performance and thermal characteristics for the device. For the best results, follow the steps and recommended routing rules listed below. dissipation. When designing a system board, determine how much power each device dissipates. Make sure devices of high power are not placed in the same location, such as directly above (top plane) and below (bottom plane) each other on the PCB.
PCB Thermal Layout Considerations
Understand the system power requirements and environmental conditions. Maximize thermal performance of the PCB. Consider using 70m of copper for high-power designs. Make the PCB as thin as possible by reducing FR4 thickness. Use vias in power pad to tie adjacent layers together. Remember that baseline temperature is a function of board area, not copper thickness. Modeling techniques can provide a first-order approximation.
Recommended Routing/Layout Rules
Do not run analog and digital signals in parallel. Use separate analog and digital power planes to supply power. Traces should run on top of the ground plane at all times. No trace should run over ground/power splits. Avoid routing at 90-degree angles. Minimize clock and video data trace length differences.
Thermal Considerations
Since the interior of most systems, such as set-top boxes, TVs, and DVD players are at +70C; adequate heat sink must be provided for the device package for heat
(c) 2006 Fairchild Semiconductor Corporation Rev. 1.0.1 * 12/7/06
www.fairchildsemi.com 3
AN-6041
APPLICATION NOTE
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DISCLAIMER FAIRCHILD SEMICONDUCTOR RESERVES THE RIGHT TO MAKE CHANGES WITHOUT FURTHER NOTICE TO ANY PRODUCTS HEREIN TO IMPROVE RELIABILITY, FUNCTION, OR DESIGN. FAIRCHILD DOES NOT ASSUME ANY LIABILITY ARISING OUT OF THE APPLICATION OR USE OF ANY PRODUCT OR CIRCUIT DESCRIBED HEREIN; NEITHER DOES IT CONVEY ANY LICENSE UNDER ITS PATENT RIGHTS, NOR THE RIGHTS OF OTHERS. LIFE SUPPORT POLICY FAIRCHILD'S PRODUCTS ARE NOT AUTHORIZED FOR USE AS CRITICAL COMPONENTS IN LIFE SUPPORT DEVICES OR SYSTEMS WITHOUT THE EXPRESS WRITTEN APPROVAL OF THE PRESIDENT OF FAIRCHILD SEMICONDUCTOR CORPORATION. As used herein: 1. Life support devices or systems are devices or systems which, (a) are intended for surgical implant into the body, or (b) support or sustain life, or (c) whose failure to perform when properly used in accordance with instructions for use provided in the labeling, can be reasonably expected to result in significant injury to the user. 2. A critical component is any component of a life support device or system whose failure to perform can be reasonably expected to cause the failure of the life support device or system, or to affect its safety or effectiveness.
(c) 2006 Fairchild Semiconductor Corporation Rev. 1.0.1 * 12/7/06
www.fairchildsemi.com 4

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