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Filter wizard: noise in Sallen Key vs. Multiple Feedback

Question asked by sfx on Jun 30, 2015
Latest reply on Jul 2, 2015 by sfx



I am designing a 2-stage amplifer/filter with a high pass (>10kHz) and low pass (<100kHz) and an overall gain of 1000-2000 (gain ~30-40V/V per stage). My input impedance is approximately 5k (piezo transducer), which limits the smallest resistors a bit for noise reduction.


In the Filter Wizard, when comparing the noise simulation figures, it seems that Multiple-feedback topologies have significantly less noise (e.g. a peak of 100nV compared to >1uV for comparable Sallen-Key). By tuning resistor/capacitor values, the noise spectrum can be made quite flat and very low compared to Sallen Key.

Is this a real effect, or just an artefact of the simulation?

This video: Filtering 101: Sallen-Key vs. Multiple Feedback - YouTube  by an Analog Devices engineer says that the noise gain of Multiple Feedback is slightly higher.

If the Filter Wizard is right, could someone explain the much lower noise?

Independent of this issue, what would be your recommendation for the two stages to minimise noise if I want to limit it to 2 opamps? Would it be better to add a 3rd opamp as an input buffer (accepting additional board space) to interface with the high input impedance, and reduce resistor sizes in the following stages?

What is the impact of a virtual ground on noise in your experience? I built the above design with the eval kit using virtual ground, and noticed slightly higher noise when the transducer is referenced to GND (9mV RMS) than referenced to VGND (4.7mV RMS).