Digital signal overshoot and ringing

[rakeshm55]

Hi,
I am working on LVCMOS 1.8V digital logic.... My clock signal has a rise time of 2.5ns..... I observe a 11% overshoot while probing....
clock frequency is 1MHz....

I have already place a 33ohms series resistor.... Please let me know whether this amount of overshoot is ok....What is the tolerable limit for overshoot??.... How to know this??....

Please advice...

[rx8pilot]

It is very possible the probe and technique you are using is actually causing the overshoot. Before you worry too much, can you describe how you are probing the signal, where and how are you referencing GND.

[rakeshm55]

I was just connecting the oscilloscope ground lead to the PCB ground.... This ground is abt 50mm away from the probing resistor....
By probe point is on the 33ohms resistor.....

[FrankBuss]

I was just connecting the oscilloscope ground lead to the PCB ground.... This ground is abt 50mm away from the probing resistor....
By probe point is on the 33ohms resistor.....

The long ground lead might be a problem, use a shorter ground connection, as you can see in the first answer here:
http://electronics.stackexchange.com/questions/134673/weird-oscilloscope-readings-with-a-digital-signal-ws2812
Most probes comes with something like this.

[Jeroen3]

You'll need that springy clip-on thingy to poke in a via nearby for ground.

Also make sure the probe compensation is adjusted with your tongue at the right angle.

[tggzzz]

Hi,
I am working on LVCMOS 1.8V digital logic.... My clock signal has a rise time of 2.5ns..... I observe a 11% overshoot while probing....
clock frequency is 1MHz....

I have already place a 33ohms series resistor.... Please let me know whether this amount of overshoot is ok....What is the tolerable limit for overshoot??.... How to know this??....

Please advice...

It would help if you would quote your scope's bandwidth, the probe tip capacitance, and the approximate ringing frequency.

For practical techniques, see https://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/
For theoretical reasons and better probing techniques, see https://entertaininghacks.wordpress.com/library-2/scope-probe-reference-material/

Glad to see you quote the risetime; as I'm sure you are aware, the period/frequency is irrelevant.

[timb]

Have at look at the first two sections of Linear AN47: http://cds.linear.com/docs/en/application-note/an47fa.pdf

Along with The Rogue's Gallery in AN72: http://cds.linear.com/docs/en/application-note/an72f.pdf

(Yes, those app notes are about op-amps and comparators, but they both extensively cover proper probing techniques and contain a lot of good information that's directly applicable to what you're trying to measure.)

[T3sl4co1l]

Layout? Probe type and technique?  Instrument specs?

Tim

[Neilm]

I think that Dave has done a video on this, but I can't find it ATM.

[AndyC_772]

Also make sure the probe compensation is adjusted with your tongue at the right angle.

Probe compensation has an effect on an entirely different frequency range. A typical 'probe comp' output on a scope has a frequency of 1kHz and quite slow rise/fall times.

Although the effect looks similar on the screen, ringing on a digital signal is observed with a time base at least 1000x faster.

Try adjusting your probe compensation while measuring the ringing on a fast digital edge... the shape of the waveform doesn't change, it just scales up and down.

[AndyC_772]

I am working on LVCMOS 1.8V digital logic.... My clock signal has a rise time of 2.5ns..... I observe a 11% overshoot while probing....
clock frequency is 1MHz....

Clock frequency isn't relevant, it's only the edge rate (ie. V/ns, rather than number of transitions/sec) which influences ringing.

Almost certainly, what you're seeing is an artefact of how you're probing the signal, rather than the signal's true behaviour. To really see something approximating the true behaviour, there's really no substitute for an active probe with very short signal and ground pins.

Here's a superbly detailed document from Tektronix on the subject of scope probing, well worth a read:

http://circuitslab.case.edu/manuals/Probe_Fundamentals-_Tektronix.pdf

[Fungus]

I was just connecting the oscilloscope ground lead to the PCB ground.... This ground is abt 50mm away from the probing resistor....
By probe point is on the 33ohms resistor.....

Well.... there's your problem.

Almost all observed ringing/overshoot on digital signals is a measurement error due to inductance in the scope's probe. Pull all the leads/caps off your probe and use the little spring thing instead, like this:



Tell us what happens.