8.2.1.1 Design Requirements

For both designs, target the following filter shape characteristic:

• Gain of 1 V/V
• 100-kHz Butterworth response
• Q = 0.707 gives a flat Butterworth design

Scale the resistors down to reduce their noise contribution. In the MFB design, the input resistor is the in-band load to the prior stage. Use values slightly below the gain of –1 V/V in Table 3. The Sallen-Key filter shows a high impedance input in-band, so scale those resistors down further to improve noise.

The output DC error and drift can be improved by adding bias current cancellation resistors. For the MFB filter that is a resistor (and a noise filter capacitor) on the noninverting input to ground equal to the resistor inside the loop times the noise gain. For the Sallen-Key design, add a feedback resistor equal to the sum of the two input resistors.