PCB ground plane problems - Page 1
Forum section: electronics / projects

author: 5321488
author_rank: Contributor
timestamp: September 08, 2017, 07:26:29 am
content: Hi,I am having heaps of problems getting a ground fill on a PCB that I am designing. It just won't work. The circuit is basically 8 CMOS ICs, and then associated resistors, transistors, other passives, etc. My question is whether a ground plane will be essential or whether I could get away without it? Are the CMOS ICs inductance/capacitance sensitive or not really?btw, if anyone has any genius ideas on getting the ground plane that would be great too. I'm using the EasyEDA software.

author: ebclr
author_rank: Super Contributor
timestamp: September 08, 2017, 08:13:11 am
content: The ground plane is not essential in all circuits but is a very good practice to have because ground planes will help reduce a lot of problems and noise and even EMI. For example, to flash a LED with a 74HC04 oscillator, the ground plane will be useless, On the other hand, working with fast signals, and high gains, will definitely need a well-made ground plan. Actually, near all PCB software helps on do this.

author: tggzzz
author_rank: Super Contributor
timestamp: September 08, 2017, 08:22:18 am
content: It very much depends onwhichCMOS ICs you are using. Modern CMOS (e.g. 74LVC*) has sub-nanosecond transition times, older stuff (e.g. 4000 series) hasmuchlonger transition times.Seehttp://www.edn.com/collections/4435129/Bogatin-s-Rules-of-Thumbm particularly the first 10.

author: 5321488
author_rank: Contributor
timestamp: September 08, 2017, 09:04:01 am
content: Quote from: ebclr on September 08, 2017, 08:13:11 amThanks for your response, I'm pretty much not dealing with anything faster than 1 Hz but the circuit is moderately complex, so I guess it is somewhat warranted.Quote from: tggzzz on September 08, 2017, 08:22:18 amThanks for your response.Ok, I'm using 4000 series so would that slower transition along with the circuit's lack of high speed signals mean that it probably isn't needed?

author: tggzzz
author_rank: Super Contributor
timestamp: September 08, 2017, 09:24:28 am
content: Quote from: 5321488 on September 08, 2017, 09:04:01 amIt reduces the chance of it being necessary, but the devil is in the details.Your "lack of high speed signals" is an orange flag: the period of a signal is almost irrelevant. What matters is the transition time and whether timing constraints are violated. For examples, violating a (say) 1ns hold time is equally problematic in a 1Hz signal as in a 1MHz signal.

author: 5321488
author_rank: Contributor
timestamp: September 08, 2017, 09:54:07 am
content: So in that case roughly what transition time would warrant control measures? Would the minimum ~30 ns times of 40xx CMOS be ok? Sorry to persist and thank you for your help

author: Siwastaja
author_rank: Super Contributor
timestamp: September 08, 2017, 10:01:48 am
content: Quote from: 5321488 on September 08, 2017, 09:04:01 amProbably not true at all - as you are dealing with square waves, you are most likely dealing with harmonic content around tens of MHz at least, up to several GHz, indeed depending on logic family. Even one pulse per year is enough to cause problems, if the edge rate is fast.The question itself is tricky, and analyzing whether the plane is strictlynecessaryrequires a lot of information and calculation.You need the ground plane for most things, so the highest priority would be to learn your PCB package to use such a basic function.Taking care of proper ceramic bypass caps near all IC power rails, and series terminating fast signals with, for example, 220 ohm resistors to reduce the edge rate, will make a design perform better even if it lacks the ground plane.

author: Rerouter
author_rank: Super Contributor
timestamp: September 08, 2017, 11:01:42 am
content: A ground plane is not necessary when any fast signal has a clear return path to follow with a small loop area,The bigger the gap between the fast signal and a ground or supply return, the more noise it generates and the more sensitive to noise it is.

author: tggzzz
author_rank: Super Contributor
timestamp: September 08, 2017, 12:16:17 pm
content: Quote from: 5321488 on September 08, 2017, 09:54:07 amProbably, but it is impossible to say without knowing the wirelengths and loads - and even then your circuit might not work because of other problems such as ground bounce, marginal timing, noise from high current peripherals etc.An old technique is to simulate a ground plane:layout the ics in one or more linesside 1 of PCB: run the two PSU (Vcc, 0V) wires up the middle of each line of icsattach directly to the ic power pinsadd decoupling capacitor for each icside 2 of the PCB: at the ends of the lines, join those PSU wiresfinally insert the signal wires