Post subject: S11 of a TTL input Posted: Fri Sep 02, 2005
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.
Post subject: Posted: Fri Sep 02, 2005
Joined: Sun Aug 03, 2003 2:02
Location: Erie, PA
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
- Kirt Blattenberger
Cafe Progenitor & Webmaster
Post subject: S11 of 74LSPosted: Fri Sep 02, 2005 5:13 pm
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
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.