EEVblog Electronics Community Forum
General => General Technical Chat => Topic started by: gmc on July 07, 2019, 12:30:51 pm
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I've been spending a bit of time converting schematics for old retro stuff (Atari, Commodore etc) to KiCAD and seeing a common thing where the designers used different ground symbols. At first I thought the grounds were separate but after looking though the PCB layouts the grounds are all common.
I'm curious as to why this was done, maybe worked on by different teams and there was no standard. Not sure about this though, take this component from the Commodore, pins 1 and 4 are connected together, same with the grounds on the caps above. I would expect that maybe if there were different sections of the board, but on the same component?!
(https://i.postimg.cc/HssGz2jh/Screenshot-from-2019-07-07-13-20-09.png)
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Are pins 1 and 4 actually tied together right at the LM311 comparator footprint?
It is rather common to see "analog ground" and "digital ground" as separate nodes. Even if they are tied together at some designated point. There are even other grounds as well. In your example, perhaps the power bypass capacitors go to "power ground". Note also that often there are notes in the periphery (or somewhere else in the documentation) identifying which symbols are used for which grounds.
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The different ground symbols are used to indicate different "grounds". They can merely be used to enforce star grounding (connecting only at a designated point) or be completely separate grounds as in opposite sides of an isolating device.
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The ground symbols with the bars are grounded to the chassis or to an earth ground such as a cold water pipe. The ground symbols with inverted triangle are the return path for power. Frequently the earth ground and power return (aka ground) are tied together but usually only at the power source (the input or at the power supply output). That reduces the electrical noise.
Schematics frequently just show multiple grounds instead of drawing numerous lines back to the power supply or the chassis, etc. It makes the schematic easier to read. In actuality that grounds may be tied together at a single point (aka star ground) or they may just be connected to the nearest other grounded item. Old tube type TVs used a metal chassis and the tubes and other devices were frequently grounded by soldered a short piece of wire over to the nearest piece of the chassis. That usually works ok for low frequency analog devices such as AM or FM broadcast band radio or TV but it won't work for very high frequency equipment or for very sensitive equipment or for equipment that has both analog and digital circuits in it.
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Thanks for the replies. I was just confused as to why they used different grounds if they are all common anyway. Makes sense then to use chassis ground and digital ground, even if they are common.
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Thanks for the replies. I was just confused as to why they used different grounds if they are all common anyway. Makes sense then to use chassis ground and digital ground, even if they are common.
In many cases, noise.
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Thanks for the replies. I was just confused as to why they used different grounds if they are all common anyway.
That's the hurdle that you have to get over, all "grounds" are not exactly common except in theory. There is always some resistance (or impedance) in every conductor so there is always a slight voltage difference between the various points that are grounded. Sometimes it's negligible and can be ignored but in very sensitive equipment it causes problems. Especially if the various grounding points are connected in series. In that case an increase in the current at one point causes an increase in voltage at that point and it shifts the voltage at all of the following grounding points. A star ground avoids that problem because each grounding point is tied back to the main ground via it's on conductor so that the changing load at one grounding point doesn't affect the others.
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That’s an excerpt from the 1541 schematic?
The triangular ground symbols are for the 12V rail and the striped earth symbol is for the 5v rail. They are commoned at the PSU itself but kept segregated throughout the rest of the design.
This means that noise and other things on one ground rail won’t affect the other.
Neither of them are connected to chassis ground.
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What is shown in the circuit is "power supply ground" and "signal ground". It is very common to separate the two in analog schematics, as they fulfill different demands. They are often (but not always) routed separately on the PCB.
It has absolutely nothing to do with water pipes or chassis ground, aka "protective earth". This is symbolized with the striped earth symbol inside a circle.
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A student of the art of electronics was building a circuit including some digital sections, I/O wires to external devices, a mains phase-control driver, a HF PLL oscillator locked to a mV-level reference, and a high gain amplifier. Nothing seemed to work; the digital circuit kept getting into impossible states, the I/O buffers often died for no apparent reason, mains power regulation was unstable, the PLL would barely ever remain in lock, and the amplifier output was noisy garbage.
The student had become very frustrated. Each part would work on it's own in breadboard, all his instruments showed nothing wrong, yet as a whole the thing refused to work at all.
He finally admitted defeat and showed the system to his tutor.
The master laughed, took away the student's multimeter then redrew the schematic. Exactly the same but with five different ground symbols (plus a few optos and small inductors.)
Then the student achieved enlightenment.
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A recent job I did had the opposite pattern, over a half dozen grounds (and no pours) in a mere power supply module, with erratic behavior and terrible EMI as a result. The solution I employed, marry all grounds if at all possible, on a common ground plane; between the remaining ground domains (with their own planes), use common mode chokes between galvanically connected domains, and Y caps (and more CMCs if necessary) between isolated domains. Test permutations of Y caps as needed. End result: 21dB or better improvement in emissions, elimination of erratic behavior.
True enlightenment comes with understanding that ground is just another signal, with impedance between points.
Tim
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@stevelup, well spotted. It's for the 1541.