7

Inductive Loads

The EL2003 and EL2033 can drive small motors, solenoids,

LDTs and other inductive loads. Foldback current limiting is

NOT used in the EL2003 or EL2033 and current limiting into

an inductive load does NOT in and of itself cause spikes or

kickbacks. However, if the EL2003 or EL2033 is in current

limit and the input voltage is changing quickly (i.e., a

squarewave) the inductive load can kick the output beyond

the supply voltage. Motors are also able to generate

kickbacks when the EL2003 or EL2033 is in current limit.

To prevent damage to the EL2003 and EL2033 when the

output kicks beyond the supplies it is recommended that

catch diodes be placed from each supply to the output.

Reverse Isolation

The EL2003 and EL2033 have excellent output to input

isolation over a wide frequency range. This characteristic is

very important when the buffer is used to drive signals

between different equipment over cables. Often the cable is

not perfect or the termination is improper and reflections

occur that act like a signal source at the output of the buffer.

Worst case the cable is connected to a source instead of

where it is supposed to go. In both situations the buffer must

keep these signals from its input. The following curve shows

the reverse isolation of the EL2003 and EL2033 verses

frequency for various source resistors.

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 50

Ω the gain is about -1dB, exclusive of

cable losses. For optimum response and minimum

reflections it is important for the cable to be properly

terminated.

Double termination of a cable is the cleanest way to drive it

since reflections are absorbed on both ends of the cable.

The cable source resistor is equal to the characteristic

impedance of the cable less the output resistance of the

EL2003 and EL2033. The gain is -6dB exclusive of the cable

attenuation.

Back matching is the last and most interesting way to drive a

cable. The cable source resistor is again the characteristic

impedance less the output resistance of the EL2003 and

EL2033; the termination resistance is now much greater

than the cable impedance. The gain is 0dB and DC levels

waste no power.

An additional EL2003 or EL2033 make a good receiver at

the terminating end. Because an unterminated cable looks

like a resonant circuit, the receiving EL2003 or EL2033

should have an isolating resistor in series with its input to

prevent oscillations when the cable is not connected to the

driver. Of course if the cable is always connected to the back

match, no resistor is necessary.

WARNING: ONE END OF A CABLE MUST BE PROPERLY

TERMINATED. If neither end is terminated in the cable

characteristic impedance, the cable will have standing waves

that appear as resonances in the frequency response. The

resonant frequencies are a function of the cable length and

even relatively short cables can cause problems at

frequencies as low as 1MHz. Longer cables should be

terminated on both ends.

DRIVING A PURE CAPACITANCE

Top Trace is without Snubber.

Bottom Trace is with Snubber Circuit.

EL2003, EL2033