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High-Speed, Current-Feedback Amplifier Drives and Equalizes Up to 100-m VGA Cables
In classrooms, lecture
halls, and conference rooms, PCs are connected to projectors through VGA
cables to transmit
Driving and Equalizing
a 45-m VGA Cable
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.
Driving and Equalizing
a 105-m VGA Cable
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.
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.
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