OPA170-Q1: OPA170, OPA1678 BPF issue

Hi Tseng,

Can you please be more specific on what "abnormal" means? When I ran the circuit the gain and phase did not match what I would expect but I want to make sure I experienced the same thing you did.

I replaced the OPA170 model in your circuit with a more up to date TINA model that is available on the OPA170 product webpage and made new wire connection and it now runs much better (please see attached circuit). I believe the issue was due to a wiring connection issue. However, there is a difference in gain between the circuit using the OPA1678 and the OPA170. This may be because the phase margin of the OPA170 is below 45 degrees indicating a stability issue. Have you checked the phase margin of the device? I recommend watching our TI Precision Lab Videos on Stability for more information.

8304.OPA170.TSC

Thank you,

Tim Claycomb

Hey Tseng, that circuit looked pretty familiar, yes we covered this back in April in this thread, 

https://e2e.ti.com/support/amplifiers/f/14/p/792864/2937336?tisearch=e2e-quicksearch&keymatch=opa2189#2937336

You have changed the design to a gain of 10V/V design? Yes, the OPA170 is likely too slow

Hello Tseng, 

I guess I am curious why you are moving to the OPA2172 vs the OPA1678 which is quite a lot less expensive? I thought Tom's suggestion of the OPA1678 was perfect for this BP filter application where you do not care about DC precision since the signal path is basically AC coupled.