Digital signal overshoot and ringing - Page 1
Forum section: electronics / projects

author: rakeshm55
author_rank: Regular Contributor
timestamp: February 10, 2016, 04:38:53 am
content: 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...

author: rx8pilot
author_rank: Super Contributor
timestamp: February 10, 2016, 05:20:30 am
content: 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.

author: rakeshm55
author_rank: Regular Contributor
timestamp: February 10, 2016, 05:49:41 am
content: 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.....

author: FrankBuss
author_rank: Unknown
timestamp: February 10, 2016, 06:21:29 am
content: Quote from: rakeshm55 on February 10, 2016, 05:49:41 amThe 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-ws2812Most probes comes with something like this.

author: Jeroen3
author_rank: Super Contributor
timestamp: February 10, 2016, 06:42:06 am
content: 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.

author: tggzzz
author_rank: Super Contributor
timestamp: February 10, 2016, 10:02:09 am
content: Quote from: rakeshm55 on February 10, 2016, 04:38:53 amIt would help if you would quote your scope's bandwidth, the probe tip capacitance, and the approximate ringing frequency.For practical techniques, seehttps://entertaininghacks.wordpress.com/2015/04/23/scope-probe-accessory-improves-signal-fidelity/For theoretical reasons and better probing techniques, seehttps://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.

author: timb
author_rank: Super Contributor
timestamp: February 10, 2016, 11:45:37 am
content: Have at look at the first two sections of Linear AN47:http://cds.linear.com/docs/en/application-note/an47fa.pdfAlong 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.)

author: T3sl4co1l
author_rank: Super Contributor
timestamp: February 10, 2016, 07:17:14 pm
content: Layout? Probe type and technique? Instrument specs?Tim

author: Neilm
author_rank: Super Contributor
timestamp: February 10, 2016, 07:19:43 pm
content: I think that Dave has done a video on this, but I can't find it ATM.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 10, 2016, 07:32:58 pm
content: Quote from: Jeroen3 on February 10, 2016, 06:42:06 amProbe 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.

author: AndyC_772
author_rank: Super Contributor
timestamp: February 10, 2016, 07:37:13 pm
content: Quote from: rakeshm55 on February 10, 2016, 04:38:53 amClock 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

author: Fungus
author_rank: Super Contributor
timestamp: February 10, 2016, 07:37:21 pm
content: Quote from: rakeshm55 on February 10, 2016, 05:49:41 amWell.... 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.