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Volume 44 – July 2010

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High-Speed, Current-Feedback Amplifier Drives and Equalizes Up to 100-m VGA Cables

By Charly El-Khoury

In classrooms, lecture halls, and conference rooms, PCs are connected to projectors through VGA cables to transmit
red-green-blue (RGB) video signals. The average cable length depends on the room size and ceiling height, but most cables are shorter than 100 m. This article shows how the ADA4858-3 triple high-speed current-feedback op amp with integrated charge pump (see Appendix) can drive and equalize up to 100 m of VGA cable. This convenient, inexpensive, easy-to-implement solution—added between the PC and the cable—requires only a few passive components and a single 3.3-V to 5-V supply that can be generated from a USB port.

Driving and Equalizing a 45-m VGA Cable
Figure 1 shows one channel of a VGA cable equalizer based on the ADA4858-3 amplifier. Three channels are required for a complete RGB equalizer. The 150-Ωload resistor represents the 75-Ωterminated cable and its impedance-matching drive resistor.

Figure 1. Schematic for 45-m VGA cable equalizer (single channel).

Figure 2 shows the large-signal frequency response of a 45-m VGA cable, the equalizer, and the equalizer/cable combination. In addition to the 6-dB attenuation inherent in the impedance-matched cable drive, the VGA cable has a 0.6-dB loss for frequencies lower than 1 MHz and an 8-dB loss at 100 MHz. To restore the signal strength, the equalizer must deliver 6.6-dB gain at low frequency and 14-dB gain at 100 MHz to boost the original signal by 6 dB for RGB video applications. The cable/equalizer combination shows a 100:1 improvement in 1-dB flatness, from 1.6 MHz unequalized to 160 MHz with equalization.

Figure 2. Large-signal frequency response (45-m VGA).

Equalization also improves the transient response, as shown in Figure 3. The high and low frequencies are restored, providing a sharper image without the smearing caused by the cable.

Figure 3. Transient response before and after equalization (45-m VGA).

The transfer function of this circuit is given by Equation 1. The magnitude is given by Equation 2.

  (1)
  (2)

Driving and Equalizing a 105-m VGA Cable
Figure 4 shows the schematic for driving a 105-m cable. This length was chosen because it is close to the maximum equalization of which the ADA4858-3 is capable. The schematic is similar to Figure 1, except for the addition of the RZCZ feedback network that creates a pole to reduce the value of RF at the higher frequencies.

Figure 4. Schematic for 105-m VGA cable equalizer (single channel).

Figure 5 shows the large-signal frequency response of the 105-m cable, the corresponding equalizer, and the combination of the two. The −3-dB bandwidth of the cable is about 2 MHz before equalization and 90 MHz after equalization; the −1-dB bandwidth has improved from 0.7 kHz to 75 MHz.

Figure 5. Large-signal frequency response (105-m VGA).

Figure 6 shows the transient response. Both high- and low frequencies have been restored. With more tweaking, better flatness between 1 MHz and 10 MHz could have been achieved for even better fidelity to the input signal.

Figure 6. Transient response before and after equalization (105-m VGA).

Figure 7 shows the schematic for all three channels (R, G, B), including all of the components required for a standalone solution. A mini USB port powers the overall system. R4, R5, and R6 are chosen to match the characteristic impedance of the cable.

Figure 7. Complete board schematic showing all three equalization channels.

Conclusion
This article describes how to use the ADA4858-3 triple video driver to drive and equalize up to 100 m of VGA cable when transmitting RGB video. Two examples based on 45-m and 105-m cables are shown, but the solution can be scaled to accommodate various cable lengths. Convenient, inexpensive, and easy to implement, it combines the ADA4858-3, a few passive components, and a single 3.3-V to 5-V supply, which can be generated from a USB port.

Appendix
The ADA4858-3 triple current-feedback op amp draws only 42 mA of total quiescent current—including the charge pump. To further reduce the power consumption, a power-down feature lowers the total supply current to 2.5 mA when the amplifier is not being used; the charge pump, which eliminates the need for negative supplies, can still power external components in this mode. The ADA4858-3's wide input common-mode voltage range extends from 1.8 V below ground to 1.2 V below the positive rail (in 5-V operation). The 600 MHz bandwidth and 600 V/µs slew rate make it well suited for many high-speed applications, and the 0.1-dB flatness at frequencies up to 85 MHz (G = 2, 150-Ωload) make it well suited for professional- and consumer video. In addition, current-feedback amplifiers avoid the gain-bandwidth limitation of voltage-feedback amplifiers.

The on-chip charge pump creates a negative supply whose voltage depends on the positive supply voltage. With a 5-V positive supply, the charge pump generates a −3-V negative supply with 150 mA output current; with a 3.3-V supply, the charge pump generates a −2-V negative supply with 45 mA output current. External capacitors, C1 and C2, should have capacitance between 1 µF and 4 µF, with low ESR and low ESL, and should be placed as closely as possible to the ADA4858-3. C1 is connected between C1_a and C1_b; C2 is connected between CPO and ground. (return to text)

Figure A. Functional block diagram.

Author

Charly El-Khoury [charly.el-khoury@analog.com] is an applications engineer in the High Speed Amplifier Group. He has worked at ADI since graduating with a master's in ECE from Worcester Polytechnic Institute (WPI) in 2006.
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