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Figure 1. Noise suppression sense amplifier with DC regulation.
The circuit shown in Figure 1 effectively removes background noise from video signals in automotive and industrial applications (Figure 3). This special design eliminates more than 1000 times the 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 (in this case, the gain is 2). For unity gain, no additional components are required. Single-supply operation typically requires output offset control so that the signal is not clipped. The dual input structure of U1 (LT6552) makes this easy (see V DCADJ in Figure 1).
Figure 2. Combined twisted pair video line receiver, cable equalizer, and display driver.
One specialized use of such circuits is to pass closed-circuit television (CCTV) signals over twisted-pair cables, which replaces the more traditional shielded coaxial cables. Twisted pair not only has this obvious benefit, but it also provides much more signal per cable and lower terminal costs compared to coaxial cable solutions. Of course, twisted pair will add common mode noise, while coaxial cable will not.
Figure 3. Multi-pulse video transmitted over a 1000 foot CAT5 twisted pair
Figure 3 shows a combination of a twisted pair line receiver, cable equalizer, and coaxial driver. The differential termination resistance of the input pair is 110W, and the output termination termination resistance is 75W. The circuit accepts a 1V PP differential input and provides a single-ended 1V PP voltage to the input of a 75W display or video capture system.
The nominal gain is 2.0, set by a 1kW feedback resistor and is only suitable for short-distance cables (up to 300'). The additional RC network provides three selectable loss equalization (EQ) methods for different CAT5 run lengths. A slightly insufficient balance is not obvious on the display, but the performance of over-balance is very clear. One of the four options is available for different cables up to approximately 1300' distance. A continuously adjustable EQ circuit can be used instead of the fixed network shown.
Figure 3 shows the response to a multi-pulse video test circle with the upper trace being the TP+ input after the 1000' cable and the lower trace being the recovery output on the load, both of which are native (receiver) The ground potential is the benchmark. The LTRR's CMRR of ≥75dB over the entire video bandwidth (DC to 4MHz) completely eliminates the spurs that are clearly visible in the upper trace, including the AM RF signal (≈1MHz). The 1000' equalization network accurately corrects cable attenuation, resulting in a near-perfect video response.
The circuit operates with a supply voltage as low as 2.5V/~1.7V (assuming an AC-coupled video source), but ±5V is given to maximize the existing common-mode input range. The input is returned to ground through a 10 kW resistor to ensure proper circuit bias when the input is disconnected.
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