- Wed Sep 11, 2013 6:46 pm
#163417
Again look at the specs for an op-amp. You will see both specs. That's because the simple "1'st order" model of how an op-amp works isn't the whole truth. For example the simple assumptions that Ad is infinite and Acm = 0. When you get into demanding applications these assumptions will lead to a circuit that may not work as desired. Ad has some limit and that has effects; on linearity, on frequency response to name two. That Acm != 0 means there will be some output even when the + and - signals are identical but non-zero. If you're designing a differential amp using op-amps, you need to understand that and then do the math to see if the effect of Acm is important (or not) for your design.
A lot of circuitry is so complex that you, as a designer, don't have the time or the resources to understand all the inner workings. Instead, those that do understand their design and it's inner workings, will give you as set of specs that try to describe how the "black box" will work in almost all usages ... assuming you understand the specs and what they mean. That's one reason why EE's get paid the medium $$s.
A lot of circuitry is so complex that you, as a designer, don't have the time or the resources to understand all the inner workings. Instead, those that do understand their design and it's inner workings, will give you as set of specs that try to describe how the "black box" will work in almost all usages ... assuming you understand the specs and what they mean. That's one reason why EE's get paid the medium $$s.