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low voltage amp for sallen-key filter

Question asked by bdolf on Apr 10, 2017
Latest reply on Jul 12, 2017 by ARoxas

I am looking for a low-voltage amp for a Sallen-Key low-pass filter with ~30kHz cutoff and gain=1. The Op-Amp must operate from a 5V single-supply and must provide rail-to-rail input and output. In simulation I noticed that some devices seem to suffer from common-mode distortion. With an AD8646 for example, THD can be reduced significantly by placing a resistor in the OUT -> IN- feedback path (Rfb ~ R1+R2, the resistors in front of the IN+ input). This resistor however creates an additional pole and attenuation at frequencies above 1MHz is sacrificed. This can be compensated by a small capacitor in parallel to Rfb, but this all ends up in a significant increase in component count. Since space and costs are important in this application: which low-cost, low-voltage amp do you recommend for this circuit, which provides good distortion figures (<0.005% at 1Vrms) without the need to externally compensate CM-distortion.

Many thanks,

Adolf Klemenz

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