RobG
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Sure, if you have external components big enough to keep all the noise from getting to the opamp you can use a poor opamp, but just using a low bandwidth opamp will likely cause problems. Similarly, a passive filter on the output will have series resistance so you are going to have poor DC load rejection. If you want to use an external cap for filtering you can make a case for passive filtering, well if bond wire inductance don't get ya coming back in... but it is hard to do with on-chip caps as a general rule.
So... a poor opamp with external components or a good internal opamp... you need the BW to be way lower or way higher than the signals of interest... that is kind of what it boils down to.
As I mentioned before, the transfer function from the power supply to the opamp output is non-linear so noise on the power supply will cause a DC shift in the output voltage unless there is enough gain to reject it -- even if you have a very large external cap. Be careful... it depends on the application.
rg
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