Tamara Papalias
EDN
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 common-mode 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 reflections, each video-signal twisted-pair transmission line terminates in 100 Ω. 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.
 |
||
| Figure 1. | A common-mode-cleanup circuit reduces noise pickup on unshielded Category 5 differential-video signals. |
|
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 common-mode bias voltage through a matched pair of 30-kΩ 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.
Before applying cancellation, the signals at the circuit's outputs ROUT+ and ROUT– would appear as:
and
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:
and
where VCMS represents the summed and inverted common-mode voltage at IC1's output.
 |
||
| Figure 2. | The common-mode signal (yellow trace) heavily influences the video signal (pink trace). |
|
Figure 2 shows a representative 1 V peak received signal that's on the R+ line (yellow trace) and an accompanying 2 V 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%, 14-dB 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 10-µF, 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.
 |
||
| Figure 3. | Adding the common-mode-reduction circuit in Figure 1 significantly reduces the amount of common-mode voltage on the video signal. |
|
