The circuit shown in Figure 1 effectively removes background noise from video signals in automotive and industrial applications (Figure 3). This special design removes more than 1000 times more common-mode noise and is much simpler than other op amp-based topologies. The design uses a high CMRR differential amplifier that requires only two resistors to set the differential gain (a gain of 2 in this example). To achieve unity gain, no additional components are required. Single-supply operation usually requires output offset control so that the signal is not clipped. The dual-input structure of U1 (LT6552) makes this easy (see VDCADJ in Figure 1).

The circuit shown in Figure 1 effectively removes background noise from video signals in automotive and industrial applications (Figure 3). This special design removes more than 1000 times more common-mode noise and is much simpler than other op amp-based topologies. The design uses a high CMRR differential amplifier that requires only two resistors to set the differential gain (a gain of 2 in this example). To achieve unity gain, no additional components are required. Single-supply operation usually requires output offset control so that the signal is not clipped. The dual-input structure of U1 (LT6552) makes this easy (see VDCADJ in Figure 1).

  Eliminate background noise from video signals in applications with high CMRR differential amplifiers

Figure 1. Noise Rejection Sense Amplifier with DC Regulation

Eliminate background noise from video signals in applications with high CMRR differential amplifiers

Figure 2, Combined Twisted Pair Video Line Receiver, Cable Equalizer, and Display Driver

One specific use of this circuit is to carry closed-circuit television (CCTV) signals over twisted pair cable, which replaces the more traditional shielded coaxial cable. Not only is twisted pair easy to get this obvious benefit, it also provides far more signal per cable and lower termination costs than coaxial cable solutions. Of course, twisted pair will add common mode noise, but coaxial cable will not.

Eliminate background noise from video signals in applications with high CMRR differential amplifiers

Figure 3, Multipulse video transmitted over 1000 feet of CAT5 twisted pair

Figure 3 shows a combination of twisted pair line receiver, cable equalizer, and coaxial driver. The differential termination resistance of the input pair is 110W, while the output rear termination resistance is 75W. The circuit accepts a 1VP-P differential input and provides a single-ended 1VP-P voltage to the input of a 75W Display or video capture system.

The nominal gain is 2.0, set via a 1kW feedback resistor, and is only suitable for short cable runs (up to 300′). The additional RC network provides three selectable wear leveling (EQ) methods for different CAT5 run lengths. The slightly underbalanced performance is not noticeable on the monitor, while the overbalanced performance is very distinct. One of four options is available for different cables up to approximately 1300′ distance. The fixed network shown can be replaced by a continuously adjustable EQ circuit.

Figure 3 shows the response to a multi-pulse video test circle, the upper trace is the TP+ input after 1000′ of cable, and the lower trace is the recovery output on the load, both in native (receiver) ground potential as the reference. The LT6552’s CMRR of ≥75dB over the entire video bandwidth (DC to 4MHz) completely eliminates the spurious pickup clearly visible in the upper trace, which includes AM RF signals (≈1MHz). The 1000′ equalization network accurately corrects for cable attenuation, resulting in a near-perfect video response.

The circuit operates from supply voltages as low as 2.5V/~1.7V (assuming an AC-coupled video source), but ±5V is given to maximize the available common-mode input range. The input is returned to ground through a 10kW resistor to ensure proper circuit bias when the input is disconnected.

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