You can draw these schematic symbols well without drawing skills.

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/ 2019-04-11 / News

There is a lot of discussion about drawing schematic symbols. It's important to make your schematic symbols understandable. Sometimes pre-made symbols in a computer-aided design (CAD) package are fine, but most symbols are not ideal. Make sure your package can easily create symbols because you may have to redraw each individual component and create a new one. The tens of thousands of symbols included in CAD software are just the basis for you to redraw them.

A good schematic should have a predictable signal flow. This flow direction requires the input section to be on the left and top, and the output section to be on the right and bottom. Of course, this is not a piece of iron, but if you want other engineers to understand your schematic at a glance, it is very important to follow this rule. If I yell at you loudly, "What is the difference between doing this?" This grammatical structure is obviously confusing, but if I say it from right to left, "What is the difference?" Can understand. While many semiconductor companies make a lot of money and provide a lot of support, many times they focus on the inside of the chip and can't do the right schematic flow (Figure 1).

Figure 1: Many companies draw The schematic symbol mimics the pin map of the component, not the signal flow.

Figure 1: Many companies draw The schematic symbol mimics the pin map of the component, not the signal flow.

The six inverter U1 in Figure 1 is not very practical. It combines six inverters into one symbol, and has input and output on the left and right sides. The pin length does not need to be that long. The U2 symbol is slightly better, the input is on the left and the output is on the right. A person like me doesn't like a colorful background, because after six black and white copies, the yellow color turns black, so you can't see anything. The U3 I created consists of different components (heterogeneous components), including six identical components and a seventh component representing power and ground. Exclusion of RP1 is a very stupid drawing,It's easy to make a mess of the schematic when these resistors should be in different positions on the schematic. RP2 shows the role of heterogeneous components at this time.

Some semiconductor companies use ANSI symbolic logic devices, which are apparently invented by people who lack linear thinking in analysis, rather than graphical thinking in the eyes of analog engineers (Figure 2).



Figure 2: Many engineers don't like ANSI/IEEE logical symbology. These symbols are simply unhelpful and harmful. The actual logical symbols are slightly better. The components included in the CAD software package are basically useless. It is better to split the components into two. It is better to separate the power supply so that the signal flow is not disturbed. The simulation engineer wants the most. It's a little drawing inside the component that shows its function.

For multi-component packages (such as many logic gates), the schematic symbols need to be broken down because you rarely do. All of these components are used in the same place in the figure. This principle also applies to two-way or four-way op amps. The symbol of the component can be de Morgan equivalent (Figure 3).I really admire engineers who can understand the circuit work through Boolean expressions, but I still like the graphical representation - the bits in the D latch can be imagined through the graphics, or the multiplexer is given Input pin.

Figure 3: As early as 1995, OrCAD 9 allowed the use of De Morgan equivalent symbols to represent NAND gates.

Altium/CircuitStudio allows users to assign different "patterns" to components to accomplish the same task. It is very convenient if you want to draw an op amp symbol with a "pin negative" mode. If there is no equivalent symbol, if you want to flip a component vertically, you will also put the positive power supply on the bottom and put the ground on the top. By calling the de Morgan equivalent symbol, you can swap the input pins.Keep the position of the power supply and ground unchanged. Another way to solve this problem is to make a heterogeneous component (U6) with an independent power supply. Now you can flip the op amp vertically and place the negative pin on it.

The schematic program of a certain era appeared in such a period: there are about 40 14-pin logic chips on the PCB, each chip is equipped with a decoupling capacitor, plus a card edge connector. In 1985, DOS OrCAD could not even draw triangles. This was the limitation of that era and the thing that needed to be worried in that era. At the time, many companies felt that there was only one power supply on the PCB, VCC (two "C" stands for "common collector" because all of these logic gates feed power to the collectors of many transistors). Therefore PCB only needs VCC and ground. The CAD company's programmers even thought that there was no need to display the power pins on the chip. They just invented the "zero length" pin, and then the layout designer would connect all the pins of the same name together. The programmer thinks that it is simply stupid for the engineer to use the schematic diagram of the last generated netlist.

Speaking to the ground,"Common" or "return" is more appropriate unless your circuit is connected to the ground pin of a wall outlet (Figure 4). I admit that this is just a personal preference, but I like the American-style power and resistance symbols, there is a circle on the transistor and MOSFET, and the MOSFET clearly indicates the N-channel or P-channel type.


Figure 4 : Various component symbols such as ground, power, resistor, transistor and MOSFET.


I have met a professor who will give you a failing judgment if he sees that you have a geodetic symbol on the car radio schematic. A car chassis is a different symbol, whether Altium calls it the earth or the triangle symbol you use on most PCBs, it means the common or the return. My personal preference is to use arrows to represent the power supply.I have never encountered an engineer who likes the resistance of European paintings like R1 and R2. Even the variable resistance symbol R3 in Altium has no meaning unless it has three feet or shorts two feet on the package. Together. I also like the circles on the transistors, the short pins, the letters N or P to clearly show the type of MOSFET, and the gate pins that help to show the type of tube, the type of P-channel that can be flipped so that the source is on top, Because more positive power is also on. I really appreciate the Altium/CircuitStudio display body diode.

In modern design, the problem with invisible power and ground pins is that the circuit often burns out when the power supply of the layout package is incorrect. I often burn. This is a very serious problem, because you may have multiple layers with power, and it is very difficult to re-do the PCB or even rebuild the prototype. For this reason, many of us will explicitly draw the power pins. There are three ways to implement a multi-element package like a four op amp (Figure 5). The first method is that you can draw power pins on each component. The second method is to draw only the power supply pin on one of the components.Be sure to put all unused components on the schematic at this time. The third method is to design the quad op amp into a heterogeneous package of five components, including four independent op amps and a separate power and ground pin component. The advantage of this method is that you can put the power supply and ground components and all decoupling capacitors together. The downside is that you may have forgotten to put power and ground components, and the resulting disaster is that the device is not powered, not the wrong power supply. One trick is to use the power pin as the first component in the package so that when you place the component, the first one is the power supply. In any case, you should put all the components in the schematic to properly bias the unused components to prevent them from oscillating.




Figure 5: Do not use zero-length pins for power and ground.


Instead,It is best to draw a power pin on each component of U1. You can also draw power pins on only one component of the package, but make sure all components are placed so you don't forget to connect the power supply (U2). The U3 package uses a separate "component" to draw power and ground. The advantage of this is that you can flip the op amp and flexibly place the negative pin above or below the positive pin, depending on your circuit needs.
After more than a decade ago, Cadence's OrCAD had these heterogeneous components. This method also broke the connector into pieces. This is also done to maintain the signal flow of the schematic, ensuring that each wire is connected to the correct connector (Figure 6). Now you can make sure that your schematic flow is from left to right, making it easier for other engineers to understand, and it will make it easier to understand when you look at it in 5 years.

heterogeneous component function in OrCAD, or the component "mode" in Altium/CircuitStudio.So that the flow of the schematic is clearer and easier to understand (b).

Another consideration is how to draw complex components such as switching power chips. Even if you move the input to the left and the output to the right, it is still difficult to understand how this component works. In this case, you can draw a simple diagram in the symbol box to indicate the function of this component. Not necessarily the block diagram in the data sheet, just a simple statement to remind you and others what this component does.

There are other conventions for schematic symbols, which are more of a preference than a good design principle. I really like to surround the transistor with a circle. It needs to be reiterated that the transistors painted by semiconductor engineers have no circles. I think the circle is very useful. Again, I really like to make a small jump when the line crosses. This leads to another important rule: there are no 4-way nodes. I have seen a schematic that was faxed, and I can't see if the wires are just crossed rather than connected. As a result, I guessed it wrong, which was a waste of my day. If all schematics are jumpered, the "no 4-way nodes" rule is less important. What makes me happy is thatThe latest version of Altium/CircuitStudio can display jumpers and automatically prevent the generation of 4-way nodes.


node is a contraindication in the schematic. Altium/CircuitStudio has the option to generate jumpers, as well as the ability to eliminate cross-junctions by setting trace offsets, as shown in the GND connection of this chip. Note that the left side of the library component is the output, and the right side is the input, as opposed to what you think.

My approach is to redraw the symbol of the component using the rules entered on the left (Figure 8). I also used separate power and ground symbols to reduce clutter. After all, we are concerned with signal flow. Most engineers understand the internal functions of the 555 timing chip. But if you don't know, or if you think the person reading the schematic doesn't know, then you can draw some or all of the block diagram inside the component. Altium/CircuitStudio allows you to place images on schematic symbols.So I found a good 555 timer block diagram on the Internet, and after some minor adjustments I put it into the schematic symbol. I have to follow their pinout structure, so there are some jumps on the schematic.

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