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prof....@gmail.com
May 19, 2008, 5:50:16 PM5/19/08
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>> I have a question for you though. What primarily determines the accuracy of these opamps?
>> Is it the open-loop gain? I realize more open-loop gain gives better driving strength. Also, I know
>> when I upped my open-loop gain to > 12k my accuracy improved significantly; however, at the
>> same time, if I fooled around with the capacitances then my accuracy would also change. Is
>> there a standard method for determining how accurate your opamp will be (before going through
>> the whole design stage) besides trial-and-error? I know you said in class that we probably
>> needed > 12k open-loop gain. Where did you get this number from? (Although, I definitely
>> realized you were right in that number).
>> Is it the open-loop gain? I realize more open-loop gain gives better driving strength. Also, I know
>> when I upped my open-loop gain to > 12k my accuracy improved significantly; however, at the
>> same time, if I fooled around with the capacitances then my accuracy would also change. Is
>> there a standard method for determining how accurate your opamp will be (before going through
>> the whole design stage) besides trial-and-error? I know you said in class that we probably
>> needed > 12k open-loop gain. Where did you get this number from? (Although, I definitely
>> realized you were right in that number).
You know the formula for gain error as a function of amplifier gain
and feedback factor: it's approximately 1/Af . So if your feedback
factor is 1/16 (1/17 actually), and you need the error to be less than
0.2%, then you need 16/A < 0.2%, or A> 500*16.
That's a *necessary* condition for your system to work. It's not
sufficient - meaning that there are lots of other things that have to
work too.
ksjp
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