Amplifier removes common-mode noise on RGB differential-video-transmission line

Comprising four twisted pairs within a durable external sheath, Category 5 network cable offers a common and cost effective choice for transmitting component-video signals. Three of the pairs can carry RGB video signals, and the fourth pair carries audio, synchronization, and other transmissions. Unfortunately, Category 5 cable lacks shielding, and thus it’s somewhat vulnerable to common-mode coupling that induces equal voltages in each of the cable’s conductors. As a first line of defense against commonmode problems, you can configure RGB signals as differential voltages, but any voltage difference between the ground references of the twisted-pairs’ drivers and receivers results in a common-mode signal on each of the received lines.

Common-mode-noise voltages limit transmission quality of video signals. This Design Idea shows how you can use a single operational amplifier to minimize common-mode signals’ effects on differential-component-video receivers. In Figure 1, the receiver circuits’ ground terminals (in red) show that the ground-reference voltages of each of the RGB differential signals differ from those present at the drivers. To maintain signal quality and minimize ref lections, each video-signal twisted-pair transmission line terminates in 100Ω. For example, resistors R35 and R37 terminate the R+ line, and R36 and R38 terminate the R– line. Meanwhile, the G and B termination circuits are identical. Any common-mode voltage on the R-signal pair appears at the junction of R37 and R38 and across R39.



To create a common-mode cancellation voltage, operational amplifier IC1 sums and inverts the signals on all three or four signal line pairs. For example, adding the R+ and R– signals cancels their differential-voltage components and doubles the common-mode voltage that each line contributes. Capacitors C1 and C2 provide ac coupling for the circuit’s input and output, respectively. The output from IC1 applies a commonmode bias voltage through a matched pair of 30kΩ resistors, R42 and R43, to the R+ and R– receiver network. Close tolerances for R42 and R43 ensure that the differential voltages delivered at ROUT+ and ROUT– closely balance with respect to the inputs’ common-mode voltage. Capacitors C11 and C12 provide equalization to boost the differential-video signal’s higher frequency components.

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Before applying cancellation, the signals at the circuit’s outputs ROUT+ and ROUT– would appear as: ROUT+ =VDIFF/2+VCM, and ROUT– =–VDIFF/2+ VCM, where VDIFF represents the desired differential signal, and VCM exists with respect to the circuit’s local ground. After applying cancellation, the output signals appear as: ROUT+=+VDIFF/2+VCM–VCMS = +VDIFF/2, and ROUT–=–VDIFF/2+ VCM–VCMS=–VDIF/2, where VCMS represents the summed and inverted common-mode voltage at IC1’s output.

Figure 2 shows a representative 1V peak received signal that’s on the R+ line (yellow trace) and an accompanying 2V peak common-mode signal (pink trace). Figure 3 shows the circuit’s common-mode-cancellation abilities. Although the differential signal (yellow) remains unchanged, the common-mode signal (pink) exhibits an 80%, 14dB reduction. Any mismatch between the time delay and the summed analog signal, which the passive input network and IC1, respectively, produce, prevents complete cancellation. Also, for best performance, the common-mode signal must not exceed IC1’s common-mode input-voltage rating. In addition, IC1, an Intersil ISL55001, must exhibit unity-gain stability over a wide bandwidth and an excellent slew-rate response and, for best results, must operate at relatively high-power-supply voltages for good linearity. Use 10μF, nonpolarized input- and output-coupling capacitors to accommodate extremely low frequency common-mode voltages. Ensure adequate bypassing for IC1’s power-supply terminals for all frequencies of interest.