AD811 vs. LM7171

Have you checked the datasheets?

http://www.analog.com/static/imported-files/data_sheets/AD811.pdf

That's where I would start.

Crikies, I just looked at those datasheets (thanks Mikero). Reason was obvious without looking at the specs...SIZE.

The AD811 has 20 pins. 15 of those pins are NC, a waste of space.

The LM7171 has 8 pins. 3 of those pins are NC, much better

...I would also hazard a guess that after I briefly looked at some specs, there are issues with heat dissipation, explaining the unusual size of the AD811 with respect to its internal topology. The LM7171 runs relatively cooler than the AD811, allowing for a smaller physical size.

Economics is a funny thing, I would have designed a small 8 pin AD811 with a vertical heatsink or change the package to a ZIP, PENTAWATT or T7-TO220.

These are the packages for AD811. I don't think that AD designed the AD811 having in mind comparing with LM7171. I think it was the opposite way.

Anyways, when I need a professional component I go to AD. If I design a commercial device, I might consider National (or whoever makes the LM7171), although the prices that AD has are very competitive.

You, also, can see, from the table, a reason to use the 20 pin package.

but im using an 8 pin AD811.

lol...well that changes things.

The AD 811 is a current feedback Op-Amp and the LM 7171 is a voltage feedback Op-Amp. Have a look at page 16 of the PDF document provided by Mikerho for the LM7171. Has the differences there.