Low Cost Differential Video Receiver Using the ADA4851 Amplifier and the ADV7180 Video Decoder
Figure 1. Low Cost Differential Video Receiver Using the ADA4851-1
(Simplified Schematic: Decoupling and All Connections Not Shown)
The ADA4851 family of amplifiers along with the ADV7180 video decoder are all automotive qualified which makes both products ideal for auto-vision applications. Low power, low cost, high speed, and fast settling make these amplifiers well suited for many video applications where these requirements are very important. Figure 1 shows only a single amplifier for simplicity; the amplifier circuit can be repeated for each input as many times as needed.
This configuration is a standard four-resistor difference amplifier optimized for composite video frequencies, with a differential-to-single-ended gain of 1. The amplifier only amplifies the difference between the two inputs, while eliminating the common-mode noise between the incoming signals and allowing the reconstruction of the original signal. Using low value, high accuracy resistors and a high CMRR amplifier provides the exceptional performance.
A critical design consideration that is important to note about this architecture is its dependence on resistor matching for the common-mode rejection (CMR) of this circuit design. The CMR can be calculated using the following formula:
where KR is a single resistor tolerance in fractional form (for example, 1% = 0.01). It is assumed that the amplifier has significantly higher CMR performance. This formula shows that if 54 dB or better CMR is desired, then R1, R2, R3, and R4 must have a matching tolerance of 0.1% or better.
To prevent the incoming signal from violating the amplifier input and output voltage range, it is necessary to add a dc offset. This is conveniently done by connecting R4 to the 1.8 V analog supply voltage (AVDD_1.8V) also used by the ADV7180, which helps keep the cost low. To prevent supply noise from coupling into the video signal stream, be sure to have adequate bypassing on this supply (AVDD_1.8V), as shown in Figure 1.
Some automotive applications require ac-coupled inputs to protect the input to the amplifier from a short to a voltage higher than its supply voltage. The input impedance of 1 kΩ (R2 and R3) with the 22 μF capacitors (C1 and C2) sets a high-pass corner frequency pole at approximately 7 Hz. If a lower frequency is desired, increasing the capacitor value decreases the corner frequency proportionally. For example, using 47 μF capacitors results in a 3.4 Hz high-pass corner frequency.
R5 and R6 form a typical voltage divider network that is required to keep the input video signal within the allowed range of the ADC, 0 V to 1 V. This circuit should be placed before all analog inputs to the ADV7180. If the amplitude of the analog video signal is too high, clipping may occur, resulting in visual artifacts. The voltage divider impedance of can be scaled to help reduce the output power of the ADA4851. For high impedance dividers, care must be taken to keep the ADA4851 and the ADV7180 very close together, on the PCB, to reduce the trace parasitic capacitance.
A function that is often overlooked but very important is buffering. For example, many automotive customers use low cost amplifiers to protect more expensive and complex devices, such as video decoders and encoders. Amplifiers with ac-coupled inputs, as shown in Figure 1, help protect such devices from overvoltage and ESD damage.
The ADV7180 automatically detects and converts standard analog baseband television signals compatible with worldwide NSTC, PAL, and SECAM standards into 4:2:2 component video data compatible with the 8-bit ITU-R.656 interface standards. The accurate 10-bit analog-to-digital conversion provides professional quality video performance for consumer applications with true 8-bit data resolution. Three analog video input channels accept standard composite, S-Video, or component video signals, supporting a wide range of consumer video sources. AGC and clamp-restore circuitry allow an input video signal peak-to-peak range of up to 1.0 V.
The circuit must be constructed on a multilayer PC board with a large area ground plane. Proper layout, grounding, and decoupling techniques must be used to achieve optimum performance (see MT-031 Tutorial and MT-101 Tutorial).
There are a few options for supplying the dc bias voltage for the ADA4851-1. The ADR12x family of voltage references has a wide range of values that can be used. Another method is to add one more resistor to form a voltage divider in conjunction with R4. Be sure that the parallel combination of R4 and the new R5 matches the value of the RF feedback resistor.
If better CMR performance at high frequencies and higher input impedance are desired, the ADA4851-1 can be replaced with the AD8130 differential line receiver.
|ADV7180||10-Bit, 4× Oversampling SDTV Video Decoder||
|ADA4851-1||Low Cost, High Speed, Rail-to-Rail, Output Op Amp (Single)||