Datasheet EL2002, EL2003 (Elantec) - 7

EL2003, EL2033 Driving Cables
There are at least three ways to use the EL2003 and EL2033 to drive cables, as shown in the adjacent figure. The most obvious is to directly connect the cable to the output of the buffer. This results in a gain determined by the output resistance of the EL2003 or EL2033 and the characteristic impedance of the cable, assuming it is properly terminated. For RG-58 into 50Ω the gain is about -1dB, exclusive of cable losses. For optimum response and minimum reflections it is important for the cable to be properly
Top Trace is without Snubber.
terminated.
Bottom Trace is with Snubber Circuit. DRIVING A PURE CAPACITANCE
Double termination of a cable is the cleanest way to drive it since reflections are absorbed on both ends of the cable.
Inductive Loads
The cable source resistor is equal to the characteristic The EL2003 and EL2033 can drive small motors, solenoids, impedance of the cable less the output resistance of the LDTs and other inductive loads. Foldback current limiting is EL2003 and EL2033. The gain is -6dB exclusive of the cable NOT used in the EL2003 or EL2033 and current limiting into attenuation. an inductive load does NOT in and of itself cause spikes or Back matching is the last and most interesting way to drive a kickbacks. However, if the EL2003 or EL2033 is in current cable. The cable source resistor is again the characteristic limit and the input voltage is changing quickly (i.e., a impedance less the output resistance of the EL2003 and squarewave) the inductive load can kick the output beyond EL2033; the termination resistance is now much greater the supply voltage. Motors are also able to generate than the cable impedance. The gain is 0dB and DC levels kickbacks when the EL2003 or EL2033 is in current limit. waste no power. To prevent damage to the EL2003 and EL2033 when the An additional EL2003 or EL2033 make a good receiver at output kicks beyond the supplies it is recommended that the terminating end. Because an unterminated cable looks catch diodes be placed from each supply to the output. like a resonant circuit, the receiving EL2003 or EL2033
Reverse Isolation
should have an isolating resistor in series with its input to The EL2003 and EL2033 have excellent output to input prevent oscillations when the cable is not connected to the isolation over a wide frequency range. This characteristic is driver. Of course if the cable is always connected to the back very important when the buffer is used to drive signals match, no resistor is necessary. between different equipment over cables. Often the cable is WARNING: ONE END OF A CABLE MUST BE PROPERLY not perfect or the termination is improper and reflections TERMINATED. If neither end is terminated in the cable occur that act like a signal source at the output of the buffer. characteristic impedance, the cable will have standing waves Worst case the cable is connected to a source instead of that appear as resonances in the frequency response. The where it is supposed to go. In both situations the buffer must resonant frequencies are a function of the cable length and keep these signals from its input. The following curve shows even relatively short cables can cause problems at the reverse isolation of the EL2003 and EL2033 verses frequencies as low as 1MHz. Longer cables should be frequency for various source resistors. terminated on both ends. 7