**Part Number:**OPA348

**Other Parts Discussed in Thread:**OPA378

Hello

I am struggling with a few things designing a double stage Sallen-Key LPF and between PSpice and real circuit there are up to +/- 1 dB of variation at 10 kHz.

The circuit is:

I have downloaded the PSpice library for OPA348 from the TI website and applied, against the IDEAL OPAMP there are some differences as should be, first of all the gain and the unit GBW, the input and output impedances that are important in the Sallen-Key arrangement.

The circuit has an input buffer, still OPA348, unit gain with an RC at + input: 3k32 and 4n7.

The response curve is a modified Bessel to recover the non-linear response of the input transducer at frequencies higher than 2 kHz (i.e. the transducer has a +3 dB @ 11 kHz).

What's is weird in example that if R48 is 1k91 ohm, the simulation cannot converge, with 1k911 works. I have 3 boards, the prototype one is nearly like PSpice +/- 0,1 or 0.2 dB max along the whole response curve.

So, I have assembled other 2 boards but one has + 1 dB @ 10 kHz respect the PSpice and the other - 1 dB.

I have checked several times the boards and appears the same, components are in the tolerance (resistors 1%, capacitors 2%) but the final behaviors are different.

Any suggestion on what I have mistaken or missed?

Thanks a lot.

Maurizio