The importance of proper termination

RE: The importance of proper termination

Jesus Alonso — Tue, 11 Nov 2014 07:39:42 GMT

I think it is important to clarify at which frequencies this effect matters. I know it is important for high frequency circuits (e.g. radio circuits and high speed data buses) and when considering electromagnetic emissions (for CE/FCC compatibility tests), but for low frequency and small signal (e.g. audio aimplifiers), I don't think this impedance adaptation is desired (in fact usually ensuring a high input impedance is usually important).

RE: The importance of proper termination

Tim Claycomb — Tue, 11 Nov 2014 03:14:15 GMT

Thank you all for your comments and questions!

Otto,

No, placing a capacitor in series with the 50 ohm resistor will not have the

same effect. The capacitor changes the impedance of the termination. The

impedance of a capacitor changes with frequency and therefore as the input

frequency changes so will the termination impedance. You can see the effects

of placing a capacitor in series with a resistor using the free TINA-TI

simulation tool found here. http://www.ti.com/tool/tina-ti

jstoehrer,

Yes you are correct, with a 50 ohm termination the amplitude should be half.

I wanted to emphasize the ringing of the input signal and not how the

amplitude changes. Therefore I used a larger amplitude for the input signal

when using a 50 ohm termination.

Hope this helps!

-Tim Claycomb

RE: The importance of proper termination

MKARGER — Mon, 10 Nov 2014 23:35:57 GMT

jstoehrer, I suspect that the author changed the load impedance setting on the function generator from High-Z in Figure 3 to 50 Ohms in Figure 5. The function generator automatically adjusts its output so that you view a full-scale waveform on the scope.

RE: The importance of proper termination

jstoehrer — Mon, 10 Nov 2014 07:41:56 GMT

With 50 ohm termination, the amplitude should drop to half the value of Zi equal to 1MΩ.

Why is it always the same in your screen shots?

RE: The importance of proper termination

Gabor Endredi — Sun, 09 Nov 2014 15:18:51 GMT

High Timothy!

Thank you for your nice article. It is always interesting when at an unexpected occasion it turns out that the electromagnetism propagates. I would like to add a small clarification. Not (only) the lab equipment must be terminated but the cable between the lab equipment and the DUT. If you vary the length of the poorly terminated cable you will see that the frequency of the ringing changes.

Thank you again,

Gábor

RE: The importance of proper termination

Otto Hunt — Sun, 09 Nov 2014 15:07:37 GMT

50 ohm terminations use significant power. Would placing a capacitor in series with the terminating resistor have a similar effect?