Why do 2 different online active filter calculators give way different GBW requirements for the same frequency range?

Question

I am confused about 2 different online active filter calculators giving different GBW requirements for the same frequency range. One calculator is giving GBW requirements in the kHz range while the other calculator is saying I need a very high GBW in the MHz range - up to 40 MHz.

I am trying to calculate an 8 pole bandpass filter in the range of 5400 Hz - 7500 Hz and this is what each says about GBW requirement for each section:

first website is https://tools.analog.com/en/filterwizard/:

GBW for each section with the above calculator: 768 kHz, 654 kHz, 741 kHz, 547 kHz

2nd website is https://webench.ti.com/filter-design-tool/filter-type:

GBW for each section with the above calculator: 3.4 MHz, 4 MHz, 27.4 MHz, 39.6 MHz

Answer 1

Without knowing details about your filters (topology, gain, approximation), the "required" GBW depends primarily on the required accuracy of the filter function. Of course, an "ideal" opamp would be best - however, it does not exist.

Therefore, we require an opamp "as good as possible" under technical, economic and application-specific considerations. In this context, it makes no sense to ask for an accuracy that is better that the unavoidable tolerances of the passive parts (R,C), which may be different for the variuos applications. More than that, some filter topologies are more sensitive to GBW limitations (multi-feedback) and some other are less sensitive (Sallen-Key, GIC-structures).

Therefore, it is common practice to consider a certain "safety factor" as far as the recommended (or required) GBW is concerned. And it is no surprise that each filter design software will contain its own "safety factor".

Comment: Instead of a cascade of 4 second-order stages, a direct realization might be better - starting from a fully designed passive RLC bandpass of eighth order. There are different strategies to convert such a passive filter into an active realization.

Answer 2

The recommended op-amp AD4096 is selected based on the low power requirement.

As you pointed out in the comments, after defining the filter specs, the component picker reveals the GBW:

1. "Pick for me"
2. Optimization "Low Power"
3. "I want to choose" shows the ADA4096 for a GBW of 786KHz, and they calculate the required GBW at 178KHz, which seems quite reasonable

However, if you choose Optimization "Low Noise", then the "I want to choose" option takes you to the LT6233, with GBW 60MHz.

I didn't check the TI settings, but it's possible that their tool defaults to low noise rather than low power.

Set optimization to low noise:

I want to choose:

See the GBW:

This was an excellent question. User beware.