The circuit shown in Figure 1 is effective in removing ground noise from video signals in vehicular and industrial applications, as shown in Figure 3. This particular design kills common-mode noise by over a factor of 1000 and is far simpler than other op-amp-based topologies. The design is a high CMRR difference amplifier where only two resistors are required to set the difference gain (Gain = 2 in this example). For unity-gain, no additional components are required. Single supply operation usually requires a means of output offset control so that signals aren't clipped. The dual input structure of U1 (an LT6552) makes this easy (see VDCADJ in Figure 1).
Figure 1: Noise rejecting sense amplifier with DC adjust.
One specific application of this circuit is for the delivery of closed circuit TV (CCTV) signals over twisted pair which picks up common mode noise where coax doesn't.
Figure 2: All-in-one twisted-pair video line receiver, cable equalizer, and display driver.
Figure 3 shows a combination twisted-pair line receiver, cable equalizer, and coax driver. The input wire pair is differentially terminated with 110Ω, and the output is back terminated with 75. The circuit accepts 1VP-P differential input and delivers single-ended 1VP-P to a 75Ω display or video capture system input.
Figure 3: Multi-burst video sent through 1000 ft. of CAT5 twisted-pair.
The nominal gain is 2.0, set by the 1kΩ feedback resistors, and this alone is satisfactory for short cable runs (up to 300′) The additional RC networks provide three selectable loss equalizations (EQ) for use with various run lengths of CAT5. Slight under-equalization is not very noticeable on-screen, while over-equalization is rather apparent. One of the four selections is acceptable for various cable runs up to about 1300′. A continuously adjustable EQ circuit could be used instead of the fixed networks shown.
Figure 3 shows the response to a multiburst video test pattern, where the upper trace is the TP+ input after 1000′ of cable and the lower trace is the recovered output at the load, both with respect to local (receiver) ground. The LT6552 CMRR of ≥ 75dB across the video band (DC to 4MHz) completely eliminates the stray pickup evident in the upper trace, which includes AM radio signals (≈ 1MHz). The 1000′ equalization network accurately corrects the cable rolloff to produce a nearly perfect video response.
This circuit operates with supplies as low as 2.5V/-1.7V (assuming an AC-coupled video source), but 5V is shown in order to maximize the available common-mode input range. The inputs are returned to ground through 10kW resistors so that appropriate circuit bias is assured during input disconnections.