This page is about measures on printed circuit boards (PCBs) to avoid disturbances in logic circuits due to noise on supply lines. and also avoid this with logic IC families with high tolerance to voltage variations.
Decoupling capacitor
In electronics, a decoupling capacitor is a capacitor used to decouple one part of a circuit from another. Noise caused by other circuit elements is shunted through the capacitor, reducing its effect on the rest of the circuit. Rigorous calculations for these capacitors is complicated, but in early days there was as a rule of thumb to place by one 10nF ceramics MULTILAYER capacitor near every IC, and one electrolytic capacitor of 10µF (preferred Tantalum one) between main traces that supply groups of 10-20 IC. The 10nF caps need to be small—preferably 0402 or at most 0603 sized—to avoid the lead inductance from the package nullifying the effect of the capacitor.
The part list with Decoupling capacitors is shown below.
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ID
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Component
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Properties
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Qty.
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Tantalum capacitor Radial lead 2.5 mm 10 µF (Ø x H) 6 mm x 10 mm AVX TAP106M035SCS
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35 Vdc 20 %
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Voltage variations logic families
All ICs used for my model railway have a supply voltage of 5 volts and they must have immunity to voltage variations on the supply lines between 4.5 and 5.5 volts. Logic families with this property are mentioned in the next two sections and are contenders for electronic circuits depending on their other properties. Also stated is how popular a particular logical family is so you can see if they are readily available.
Microchip ATmega
The voltage variation for the Microchip ATmega48P/88P/168P/328P/808/809/1608/1609/4808/4809 is 5V ±0.5V.
74xx families
Logic 74xx families with immune for supply voltage variations between 4.5 and 5.5 volts are:
- 74ACxx (POPULAR)
Advanced CMOS Logic (ACL for short). The AC is faster than the HC subfamilie. Not pin compatible with TTL. Improved noise immunity.
- 74ASxx (RELATIVELY POPULAR)
Advanced Schottky TTL – Improved design over 74Sxx Schottky types optimised to increase switching speed at the expense of power consumption of about 22mW
- 74ALSxx (POPULAR)
Advanced Low Power Schottky TTL – Lower power consumption of about 1mW and higher switching speed of 4ns compared to 74LSxx types
- 74HCxx (VERY POPULAR)
High Speed CMOS – CMOS technology and transistors to reduce power consumption of less than 1µA with CMOS compatible inputs
- 74HCTxx: (POPULAR IN CERTAIN CASES)
High Speed CMOS – CMOS technology and transistors to reduce power consumption of less than 1uA but has increased propagation delay of about 16nS due to the TTL compatible inputs. This is the reason for the existence of this logical family because TTL or NMOS outputs are incompatible with the inputs of the HC family unless a pull-up resistor from the TTL output to VCC is used.
