[sdiy] Equation question

[Scott Gravenhorst]

Justin Owen <juzowen at googlemail.com> wrote:
>> 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? 
>
>
>>If I understand the question, it depends on the reference.  
><edit>
>> - in this case it isn't, it's the negative rail.
>
>Thanks Scott and David,
>
>So - for an op amp that has V+ supplied by +12 Volts and V- 
>supplied by -12 Volts - would an equation that refers to 'a third 
>of the supply Voltage' be talking about 12 or 24 Volts?

I would assume that the reference here is ground, so it would be 12 volts
- however, I am _assuming_ and we know where that gets us.  In these
cases, I look at the schematic for the final answer.
 
>This is the first time I'll be using an op-amp under dual supply 
>- I could just plug it all in and see what happens (which is what 
>I normally do...) but I thought it might be interesting to try 
>and calculate/predict some results. 

That is the right track to be on.  It can save you money on blown ICs,
diodes, transistors - some of which might be expensive...  And there is
also a measure of pride involved in being able to calculate a safe value
for a resistor or other current or power sensitive component.

>Sorry if I'm asking how long a piece of wire is...

12.478 inches?  (c;

-- ScottG
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