[sdiy] Equation question

[David G. Dixon]

> When voltage - or a percentage of voltage (i.e VCC2) - is used as part of
> an equation in a single supply circuit, then Voltage (obviously) =
> Voltage.
> 
> When using a bipolar supply (i.e. +/- 12 Volt) is total voltage (or say, a
> third of the supply voltage) calculated using 12V or 24V?

Generally, I would say the latter.

Voltage is only meaningful as a difference.  Hence, it must always be
expressed relative to some reference value.  We tend to fall into the lazy
trap of always assuming that ground is that reference value, and assigning
ground a voltage of zero, but that doesn't change the basic fact that
voltage is always a difference.  You could, for example, power a
single-supply opamp between GND and -12V in a +/-12V system, and this would
be no different than powering it between 12V and GND, except that it's
ground reference would be -12V instead of GND.

Gurus?