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S11 of a TTL input - RF Cafe Forums

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Roger

Post subject: S11 of a TTL input Posted: Fri Sep 02, 2005 9:16 am

Does anyone know how to figure the input return loss of a digital circuit like TTL? I want to feed a 50 ohm sinewave into a 74LS series TTL chip and need to know how to make it "look like" 50 ohms. What about CMOS? I'm not an RF type, but remember impedance matching is important. Thanks guys.

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Kirt Blattenberger

Post subject: Posted: Fri Sep 02, 2005 1:41 pm

Site Admin

Joined: Sun Aug 03, 2003 2:02 pm

Posts: 308

Location: Erie, PA

Greetings Roger:

The input resistance of most logic devices is listed on their datasheets, but none are anywhere near 50 ohms (>1k for older TTL, >1M for CMOS). The best bet would be to use a simple opamp buffer where you can set the input impedance with a 50 ohm resistor and drive the logic circuit with that. You can also use it to adjust the amplitude as necessary.

Otherwise, you might be able to get away with a poor impedance mismatch as long as your output is stable and the 50 ohm source isn't affected by the TTL input.

The "proper" engineering solution is to use a buffer/driver circuit.

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- Kirt Blattenberger

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Guest

Post subject: S11 of 74LSPosted: Fri Sep 02, 2005 5:13 pm

There's a second problem. 74xx integrated circuits are known for being picky about the rise and fall times of their input signals. So unless you're using a 74xx14 Schmitt Trigger, a sine wave input probably won't have the required rise and fall times. Result: "flaky", unreliable operation.

The answer is to use a comparator, with the input terminated in 50 Ohms like Kirt said, or to use an L pad and a Schmitt Trigger such as the 74LS14 or 74HC14. Design of that pad is a bit tricky, and highly dependent on the specific logic family. OpAmps are generally not a good choice because of slew-rate limitations.

Good Luck!

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