Patent ID: 8436679

Claim:
A low frequency notch filter circuit, comprising: first, second and third operational amplifiers connected together in a modified Tow-Thomas biquadratic filter circuit, the first operational amplifier being configured as a lossy integrator, the second operational amplifier being configured as an inverting integrator, and the third operational amplifier being configured as an inverting amplifier, each of the operational amplifiers having an inverting input, a noninverting input, and an output, the noninverting input of each of the operational amplifiers being connected to ground; means for accepting an input signal voltage into the first operational amplifier; a first R-2R ladder circuit connected in parallel with a first capacitor connected between the output and the inverting input of the first operational amplifier, the first R-2R ladder circuit having a first current output connected to the output of the first operational amplifier, the first R-2R ladder circuit having a first voltage input connected to the inverting input of the first operational amplifier, the output of the first operational amplifier providing a notch-filtered version of the input signal voltage forming the output of the low frequency notch filter circuit; a second R-2R ladder circuit having a second current output portion connected to the output of the first operational amplifier, and a second voltage input portion connected to the inverting input of the second operational amplifier; a third R-2R ladder circuit having a third voltage input portion and a third current output portion; means for feeding the input signal voltage into the third current output portion of the third R-2R ladder circuit, the third voltage input portion of the third R-2R ladder circuit being connected to the inverting input of the second operational amplifier; and a fourth R-2R ladder circuit having a fourth current output portion connected to the output of the third operational amplifier and a fourth voltage input portion connected to the inverting input of the first operational amplifier.