Twisted-pair lines, such as telephone lines, are low cost and widely available alternatives to coax cable, but they introduce the difficult job of overcoming higher attenuation. This attenuation increases with the line length as well as increasing frequency. While terminating the line with the proper impedance is necessary to minimize reflections, active circuitry is imperative to overcome the attenuation. In addition, specialized applications such as ADSL/HDSL add stringent requirements for very low distortion amplification. Also, noise coupling into both conductors of the twisted-pair line necessitates circuitry with high common-mode rejection capabilities. A complete twisted-pair driver and receiver system meeting these strict requirements is presented here.

Three high-bandwidth amplifiers can be configured to cancel common-mode offsets and provide sufficient filtering and gain. While the common-mode rejection degrades badly over frequency with discrete amplifier circuits, better matching, lower distortion and few offsets will be observed in a monolithic solution.

Transmit circuitry must convert the single-ended video signal into a differential signal (or accept a differential signal) and be capable of driving the nominal 100 ohm impedance of twisted-pair cable. A high frequency amplifier, the EL2140, is configured (see Fig. 1) as the transmitter. The gain of the driver is given by this equation:

Gain (driver) = (R₁ + R₂+ R₃) / R₂

Typically, R₁ is chosen to equal R₃ for balance and simplicity, although the equation holds for any values. The optimum value for R₂ is 400 ohms with this amplifier while larger values of R₂ result in peaking in the frequency response, unless the voltage gain is sufficiently large. Values of R₂ smaller than 400 ohms only increase signal distortion and power dissipation. R_M1 and R_M2 are 50-ohm resistors to back match the 100-ohm transmission line.

Conversely, the receiver takes the differential signal and outputs it in single-ended form. The EL2142 is configured as a compensated line receiver (see Fig. 1, again.) R_M3 is selected at a value of 100 ohms to properly terminate the cable and minimize any power loss due to reflections. At low frequencies, R₄ and R₅ set the gain:

Gain (receiver) = (R₄ + R₅) / R₄

Additionally, R₄ should be approximately 200 ohms (half the resistance between the two feedback terminals of the driver) and little is gained by using a smaller value, as in the driver circuitry. Overall, R_M3, R₄, and R₅ are the only passive components necessary to configure this receiver. However, it may be helpful to add a simple compensating network, like the one composed of R₆ and C₁, to counteract the attenuation caused by the line at higher frequencies. Line loss (in dB) increases in proportion to the square root of frequency. A single pole-zero combination is a simple way to approximate the gain enhancement needed at higher frequencies. The zero and pole locations can be chosen to best fit your particular application (see frequency response in Fig. 2.)

A complete twisted-pair driver/receiver system has been presented. Both the EL2140 and the EL2142 have typical differential phase errors of 0.16^o and typical differential gain errors of 0.24%. This precision, in addition to the very high bandwidth, enables this system to readily handle the challenges of ADSL/HDSL transmission and reception.

Analog Main | Product of the Week | Columns | Editorial | Tech Notes

		Click here to get your listing up.