Overlapping Circles Circuit Symbol

[Adam Y.]

Im reading A Designer's Guide to Instrumentation Amplifiers and I have a question about the one circuit symbol. I know a single circle typically represents a voltage or a current source. In the third chapter of this book they have a symbol I've never seen before. This symbol is two overlapping circles. I know that they are base current compesenators because they are labeled. Is this a standard symbol and what do they typically represent?

[Richard Wallace]

Two overlapping circles with connection lines extending on the long axis is a fairly common symbol used to indicate a constant direct current source. Typically the symbol will be accompanied by an arrow off to one side, oriented parallel to the connected direction, indicating the polarity of the current.

The symbol is equivalent to a circle with an arrow inside, oriented in the direction of the connection lines. The single circle symbol is more common in US based technical literature, while the overlapping circles symbol is more common in Europe. A colleague in my office who was educated in the UK recalls that one of his instructors explained the use of overlapping circles for current sources as a redundant method of distiguishing them from voltage sources, which are represented by single circles.

I have only seen the overlapping circle symbol used on data sheets for analog devices, such as the instrumentation amplifiers you are reading about.