Adjustable high-frequency notch filter

Description: Although you can obtain universal, resistor-programmable switched-capacitor filters that are configurable as notch filters, most cannot operate at bandwidths higher than 100 kHz. Further, the typically 16- to 20-pin packages do not include a continuous-time, antialiasing filter to prevent spurious signals from appearing at the output. By using an eight-pin, dual operational amplifier and an eight-pin, switched-capacitor bandpass filter, you can construct a notch filter. IC2, a TLC082 is a dual BiCMOS op amp, replacing the older JFET-input stage with lower noise CMOS but retaining the bipolar output for high drive capability.

The described circuit utilizes a dual operational amplifier (op-amp) and a switched-capacitor bandpass filter to create a notch filter suitable for applications requiring precise frequency selection. The operational amplifier, specifically the TLC082, is notable for its BiCMOS technology, which combines the advantages of both bipolar and CMOS processes. This results in lower noise levels and improved performance characteristics, making it ideal for filtering applications.

In constructing the notch filter, the configuration typically involves connecting the output of the bandpass filter to the inverting input of the op-amp. The non-inverting input is usually grounded or connected to a reference voltage, depending on the desired configuration. Feedback from the output to the inverting input is established through resistive and capacitive components, which determine the filter's characteristics, such as the notch frequency and bandwidth.

The switched-capacitor filter operates by periodically sampling the input signal and transferring charge through capacitors, effectively simulating resistive elements. This allows for programmable filter characteristics without the need for large passive components, making the design compact and efficient. The choice of capacitors and the timing control of the switching determine the frequency response and quality factor of the notch filter.

To further enhance the performance of the notch filter, it is essential to consider the layout and grounding techniques to minimize noise and interference. Proper decoupling of the power supply and careful routing of signal paths can significantly improve the overall performance of the circuit, particularly in high-frequency applications exceeding 100 kHz.

In summary, the combination of a dual op-amp like the TLC082 and a switched-capacitor bandpass filter provides a robust solution for constructing notch filters with high precision and low noise, suitable for various electronic applications.Although you can obtain universal, resistor-programmable switched -capacitor filters that are configurable as notch filters, most cannot operate at bandwidths higher than 100 kHz. Further, the typically 16- to 20-pin packages do not include a continuous -time, antialiasing filter to prevent spurious signals from appearing at the output.

By using an eight-pin, dual operational amplifier and an eight-pin, switched-capacitor bandpass filter, you can construct a notch filter (Figure 1). IC2, a TLC082 is a dual BiCMOS op amp, replacing the older JFET-input stage with lower noise CMOS but retaining the bipolar output for high drive capability.