Geoff,

As you have discovered, they are quite similar.  In general the OP177 
has slightly "better" specs, though the noise specs for the OP27 are 
maybe 3x lower.  I think what you will see is that they are both good 
enough that overall performance will probably not be improved with a 
better op-amp.  Something else in your setup will likely be limiting 
the performance.

In some of his 
designs,   http://bnordgren.org/seismo/gif_images.htm   Dave has 
found that the AD706 is "best" by a small margin.  Several other 
parts work nearly as well and their performance differences can only 
be seen by careful measurements of the instrument noise spectrum.

There are other factors that might suggest something different, such 
as the need for very low power consumption, small size, the need for 
a larger output voltage or current range, or the need to work with 
unusually high resistance (impedance) circuits, but in typical 
seismometer circuits, the AD706 seems to work rather well.

Regards,
Brett

At 12:23 PM 3/14/2011, you wrote:
>I just purchased an OP027 to replace the OP177
>which I now use but saw no difference in the results.
>The baseline shifted 200mV but that compensated
>nicely using the zero feature at pins one and eight.
>
>It seems to me these two chips are about the same ??
>Pinout is identical.
>
>There seems to be a military spec chip
>for $20+USD but I got the $2USD one instead.
>
>Can anyone say there's a better Op Amp than these
>to use as a Preamp ?
>
>geoff
>__________________________________________________________


__________________________________________________________

Public Seismic Network Mailing List (PSNLIST)

To leave this list email PSNLIST-REQUEST@.............. with 
the body of the message (first line only): unsubscribe
See http://www.seismicnet.com/maillist.html for more information.