Trying to determine if and where I need ground in a circuit. - Page 1
Forum section: electronics / beginners

author: bivhitscar
author_rank: Newbie
timestamp: April 09, 2017, 11:34:29 am
content: I've just started studying electronics engineering, so my theoretical knowledge is still limited.We've been given a task to (re)create a schematic and ultimately a board layout for a circuit fromfalstad(online circuit simulator). Specifically the triangle to sine converter (under the diodes option on the falstad circuit menu, if you're interested in playing along).I asked the teacher about this and got a general answer along the lines of "Well yeah you need to have ground here" and pointed in the general direction of the signal input and output.My actual questions:Does the input and/or the output need a ground?Does the DC-in also need ground?If the answer is yes to one or both of the above, then can someone explain why the simulator shows a working circuit without ground?Just to re-iterate, this is a PCB design class and ultimately an error free ERC, DRC and suitable layout are the only real requirements. I just want to learn the theory at the same time. I've assumed ground for the signal input and output and this is my schematic so far:

author: ebastler
author_rank: Super Contributor
timestamp: April 09, 2017, 12:33:12 pm
content: Quote from: bivhitscar on April 09, 2017, 11:34:29 amThe answer is "yes" to all. The input, output, and the +- 2.4V supplies need to be ground referenced. So you probably want to provide a three-terminal connector for a convenient GND connection from your dual-voltage power supply.Any voltage does not exist as an absolute quantity, but describes a potential difference, relative to some other reference potential. A common choice for the reference potential is GND. (It does not have to be "grounded" in the literal sense, of course. For a battery-operated device, the negative terminal of the battery would typically be connected to the circuit's GND potential. No connection to earth ground anywhere, but still commonly referred to as GND.)It seems that Falstad implicitly refers all voltages to some 0V or GND potential. Not good practice, I would say, and certainly not helpful for students...

author: bivhitscar
author_rank: Newbie
timestamp: April 09, 2017, 12:48:30 pm
content: I had a feeling this was the case, thanks for the clarification!

author: james_s
author_rank: Super Contributor
timestamp: April 10, 2017, 07:06:22 pm
content: The Falstad simulator has options for single and 2 wired signal sources, the single wire sources are a convenience thing, they have an implied ground reference and save you the trouble of drawing a ground symbol and connecting it. I use them often but it's important to understand what is going on, and if in doubt use the two wire sources.

author: Rerouter
author_rank: Super Contributor
timestamp: April 13, 2017, 09:31:40 am
content: equally the analog output is referenced to ground, while the scope probe output has a second pin for its reference,Falstads hides some stuff, but its not hard to figure out what as it generally has the 2 wire option directly beneath it,Equally in such a circuit, you will want to buffer your output, any significant load on the output will distort the effect.

author: Luminax
author_rank: Regular Contributor
timestamp: April 13, 2017, 01:09:47 pm
content: If this were OrCAD PSPICE, the program will stop you saying it doesn't know where to reference your voltage.Basically as ebastler mentioned, a voltage is a 'difference' of potential between two points.A ground is just a point(or points) in your circuit where you 'determine' that it's a 0 reference, hence if point A is then defined as 5V, that means the difference of potential between point A and that point you determined as ground is 5 - 0 = 5.There's a whole can of worms with voltage potential, current and etc that you need to look up if you want to go down this road really prepared.