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50 Ohm TTL Driver

The motivation for this project arose from the desire to trigger a Q-Drive Pockels Cell for use in an undergraduate optics research laboratory. The purpose of the Pockels Cell is not the focus of this write up, however if more details are wanted please feel free to contact me. Unlike a majority of TTL devices, this driver terminates the incoming 0-5 Volt TTL signal into a nominal 50 Ohm load and the manufacturer claims this is because it was designed to be used with a pulse generator, not necessarily a computer generated TTL signal. However, if one demands the utmost control over their experiment, all electronic variables must be controlled by a central computer so that the operator may synchronize operation to the highest attainable resolution. Therefore, this Pockels Cell, sitting near the heart of the experiment must be accurately controlled at the whim of the experimenter. Since an indequate TTL signal will produce a jitter effect at the output of the Pockels Cell, a "high current" driver must be constructed to quench this thirst. The 74HC240 3-state inverting logic buffer was chosen due to its fast 9ns rise time to produce sharp edges and its ability to supply enough current. Standard TTL: 16mA

The General Case

Using http://home.broadpark.no/~pbrakken/lab3ee/avgrensa/74xx.htm as a reference, the 74xx family has these general pertinant specifications:
  • For a logic 1 input: 2.0 Volt at 40 mA
  • For a logic 1 output: 2.4 Volt at 400 mA

So as long as the incoming TTL signal source can source a square wave at >2.0 Volts and 40 mA, then the 74hc240 will trigger and source an output of at least 2.4 Volts at 400 mA.

As a simple approximation for our situation, we are driving these signals into a 50 Ohm load so using Ohm's law and assuming a best/worst case of 5 Volts:

  • $$I = \frac{V}{R}$$
  • $$I = \frac{5 \ \textrm{Volts}}{50 \ \textrm{Ohms}}$$
  • $$I = 0.100 \ \textrm{A} = 100 \ \textrm{mA}$$

This means that we will need to connect three of the inputs together to source a total of 1200 mA. Wait, what?

In our Case