DC Restoration Circuit for AC Coupled Video Drivers

“The rear view camera provides an unobstructed view of the area behind the car. These cameras are often used in automotive applications to enable drivers to back up safely. When installing a rear view camera, the video source signal and video driver circuit are typically AC coupled to provide DC blocking to the device. The DC level of the video signal represents the Display level of black, and this level must be constant to accommodate the video processing circuitry.

“

By Christopher John Gozon Analog Devices

Introduction

The rear view camera provides an unobstructed view of the area behind the car. These cameras are often used in automotive applications to enable drivers to back up safely. When installing a rear view camera, the video source signal and video driver circuit are typically AC coupled to provide DC blocking to the device. The DC level of the video signal represents the Display level of black, and this level must be constant to accommodate the video processing circuitry.

This application note describes a method to restore the correct DC level of a composite video signal.

Figure 1. Schematic of DC Restoration Circuit Using the ADA4433-1

Composite Video Signal Description

A composite video signal, also known as Color, Video, Blanking, and Sync (CVBS), is one of the most complex waveforms in electronics. The luminance information, color information, and sync signal combine to form a composite video signal. Figure 2 shows a typical waveform of an all-white National Television System Committee (NTSC) composite video signal.

Figure 2. All-white NTSC composite video waveform

In FIG. 2, the video signal corresponds to one horizontal scanning line. A scan line consists of an active video part and a horizontal blanking part. The active video part contains the luminance information (luminance) and color information (chrominance) of the picture. Luminance is the instantaneous amplitude of the signal, and chrominance is a sine wave. To identify the colors of the picture (yellow, cyan, green, magenta, red, and blue), compare the difference between the phase of the sine wave and the phase of the color sync. Chroma is applied to the luminance signal. This composite signal is shown in Figure 3, which shows the scan lines of the TV’s color bars.

Figure 3. Scan lines of TV color bars

The magnitude of the active video portion corresponds to the amount of color (saturation), and the phase difference between chrominance and burst represents the hue of the color. The horizontal blanking portion contains the horizontal sync pulse and the burst (called the trailing edge) that follows the rising edge of the sync pulse.

AC coupling

video source through the output capacitor (C_S) for AC coupling. This method protects the device from potential damage, such as accidental shorting of the battery or power connection due to DC current.

C_SStores the average value of the voltage signal. Video content affects the average voltage signal of the video waveform, for example, the black level varies with scene brightness. Depending on the video content, such as NTSC or Phase Alternating Line (PAL), a time constant must be considered because C_SCombined with the high input impedance of the amplifier, the resulting filter has a time constant of τ = R × C_Sthe cutoff frequency (f_C) is calculated as follows:

where R is the input impedance of the amplifier.

C_Sand R-values ​​are variables that can cause a drop in signal amplitude.

A dip is the change in brightness from the left edge to the right edge of the video, which causes a temporary flickering or fading visible on the display. The drop must be lower than human perception. To minimize droop, the time constant of the AC-coupled circuit must be as short as possible. To compensate for short time constants, one or more capacitors can be added to the circuit. Therefore, the value of the capacitor must be properly adjusted. When choosing the right capacitor, low leakage is more important than the equivalent series resistance (ESR) specification.

To properly set the input common-mode level, a clamp and bias circuit is required for the filter or driver input.

DC Recovery Using Schottky Diodes

DC content is lost when AC-coupling the output of video equipment. To recover the DC bias level, the AC-coupled output of the video source must be clamped to the reference DC voltage using a DC recovery or clamp circuit. One way to minimize power requirements when using AC coupling is to use a Schottky diode for DC clamping, as shown in Figure 1. The use of Schottky diodes in this application has significant advantages over ordinary silicon or germanium diodes. Schottky diodes have low forward voltage and fast recovery or switching times. Radio frequency (RF) Schottky diodes have low turn-on voltages. A diode with a 30 mV turn-on voltage provides satisfactory results in single-supply applications.

When the load on the coupling capacitor is high, such as the ADA4432-1 input impedance buffer, the average voltage on the coupling capacitor penetrates upward along the forward voltage. A low-voltage Schottky diode provides sync-side clamping, as shown in Figure 4. When the cathode voltage of the diode is above zero, the diode acts as a one-way switch. Therefore, the lowest part of the signal is forced to the reference voltage, the voltage level of the Schottky diode anode. The value of the coupling capacitor also depends on the terminating resistor. If the 75 Ω termination resistor is in parallel with the Schottky diode, the coupling capacitance must be large (~100 μF or more). If the termination resistance is high, a coupling capacitor of 1 μF or less is suitable. A disadvantage of this circuit is that there is a slight loss on the sync side due to diode conduction current. Therefore, low leakage current is more important than high ESR in the system.

The ADA4432-1 operates at 3.3 V and the amplifier output has a gain of 2. Therefore, a typical 1 V pp NTSC composite signal has the following design margins:

in:
V_OHis a high voltage output.
V_OLis a low voltage output.

Figure 4. Clamping the Composite Signal

in conclusion

When a video source is AC-coupled to the receiver, restoring the proper DC level of the composite signal is essential to provide the correct brightness of the transmitted video signal. Therefore, it is important to consider the correct capacitance value to avoid voltage dips and use low voltage Schottky diodes to reduce synchronous terminal losses. The DC clamp also prevents the sync signal from drifting when changing scenes because the diode provides a constant reference voltage to the sync terminal.