Output impedance of a fast logic driver

The output impedance of a fast logic circuit driver is an important quantity to know, since when it is being used with a transmission line or being interfaced to another circuit the impedance matching becomes critical. If the impedance is mismatched then ringing occurs and if very severe can even damage the device. It appears that a good way to get a feel for the output impedance of the driver is to use the VOH, IOH and VOL, IOL numbers for the output driver. So to check for the output impedance in the high state use the following: VCC – VOH/IOH and for the low state use VOL/IOL. We recently did this on the Fairchild tiny logic driver NC7SV17 and got 50 Ohms for both levels. Please visit the Signal Processing Group Inc., website for more interesting articles and data.

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