What’s with Vdd, Vss and Vwhatnot?


Josef van Niekerk" rel="author">Josef van Niekerk

Looking at datasheets from different vendors, and schematics draughted in different standards including IEEE and IEC British standard, I’ve been wondering what the differences in the different abbreviations used when referring to voltages are. Was this to do with the different standards, was it simply by choice, or does it have any significant meaning.

On datasheets, schematics and IC pinout diagrams, we often encounter voltage supplies being referred to by means of different abbreviations. These include Vdd, Vss, Vee and more, so for the aspiring beginner electronic engineer hobbyist guy like me, and I bet loads of others, these schematics can become Greek, and right down confusing if we don’t have the proper understanding of the meanings of these abbreviations.

I’ll try and keep it simple. The short story has to do with the way electronics have evolved, and in the 90’s, or perhaps even earlier, voltage supplies were indicated in association with leads on different types of transistors, and some of these stuck with us.

With older BJT (Bipolar Junction Transistors), there are three legs, a Base, Emitter and Collector. In relation to these, voltage supply was referred to as Vee, where the e points to the Emitter lead. In more modern FET (Field Effect Transistors) there are elso three legs, referred to as the Source, Gate and Drain. In the latter, voltage supply was thus referred to as Vdd, where d refers to the voltage drain.

These conventions stuck with us, and it seems that nowadays, they are interchangeably used. In short, Positive Voltage Supply can typically be indicated by Vin, Vdd, Vcc, V+, Vs+. Negative Voltage Supply can be indicated as Vee, Vss, V-, Vs- and also GNDAVdd or Vdda would commonly indicate voltage supply for analog circuits, or typically pins on MCU’s that supply voltage to internal ADC (Analog to Digital Converter) Modules.

In another post, a worthy question might by why double characters are used to denote the voltage supply instead of a single. Why Vdd and not simply Vd?