Low-Cost Differential Video Receiver Using the ADA4851 Amplifier and ADV7180 Video Decoder

Figure 1. Low-cost differential receiver using the ADA4851.

Circuit Description

The ADA4851-1 is a single (dual or quad amplifiers are also available in a single package), 130 MHz, low power, low cost, high speed, voltage feedback, rail-to-rail output amplifier that operates from a +3 V to +5 V supply. In Figure 1, the ADA4851-1 is configured as a low cost differential to single-ended receiver to receive a video signal. This is a standard 4-resistor difference amplifier optimized for composite video signals with a differential to single-ended gain of 1. The amplifier simply amplifies the difference between the two inputs while canceling the common-mode noise between the input signals to reconstruct the original signal. The use of low value, high precision resistors and a high common-mode rejection ratio (CMRR) amplifier provides excellent performance.

A key design factor worth mentioning about this architecture is the dependence of common-mode rejection (CMR) on resistor matching. CMR can be calculated using the following formula:

In this equation, the "Kr" term is the tolerance of a single resistor expressed as a decimal (e.g., 1% = 0.01, etc.), and it is assumed that the amplifier has high CMR performance. This equation shows that if 54dB or better CMR is desired, the matching tolerance of R1, R2, R3, and R4 must be 0.1% or better.

To prevent the input signal from exceeding the input and output voltage range, it is necessary to add a dc bias. This is accomplished by connecting R4 to the 1.8V analog supply voltage (AVDD 1.8V), which is also used by the ADV7180 to help keep costs low. To prevent power supply noise from coupling into the video signal stream, be sure to adequately bypass this supply (AVDD 1.8V), as shown in Figure 1.

Some automotive applications require AC-coupled inputs to protect the amplifier inputs from shorting to voltages greater than its supply voltage. The input impedance of 1 kΩ (R2 and R3) and the 22 μF capacitors (C1 and C2) set the high-pass corner frequency pole to approximately 7 Hz. If lower frequencies are required, increasing the capacitor values ​​will proportionally reduce the corner frequency. For example, with 47 μF capacitors, the high-pass corner frequency is 3.4 Hz.

The ADV7180 automatically detects standard analog baseband TV signals compatible with global NSTC, PAL, and SECAM standards and converts them to 4:2:2 component video data compatible with the 8-bit ITUR 656 interface standard. Accurate 10-bit analog-to-digital conversion can provide 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 to support a wide range of consumer video sources. AGC and clamp-reset circuits allow input video signals to have a peak-to-peak range of up to 1.0 V.

Common changes

There are several ways to provide a dc bias voltage for the ADA4851. For example, you can use the ADR12X series of references with various voltage values. Another method is to add a second resistor to form a voltage divider with R4, taking care to match the parallel combination of R4 and the new resistor R5 to the value of the feedback resistor Rf.

If better CMR performance and high input impedance are required, the AD8130 can be used instead of the ADA4851.

[Attachment: Low-Cost Differential Video Receiver Using the ADA4851 Amplifier and ADV7180 Video Decoder]