Low-pass filter model

This is a behavioral low-pass filter supporting both first-order (−20 dB/dec) and second-order (−40 dB/dec) responses, implemented in the s-domain via Verilog-A's laplace_nd — meaning it behaves correctly in both AC and transient simulations without requiring explicit R and C components. Cutoff_frequency sets the −3 dB corner and Filter_Order selects the roll-off slope (1 or 2). Use first-order for simple pole models (e.g. an LDO output filter or amplifier bandwidth limit) and second-order when you need a sharper cut, such as an anti-aliasing filter in a sigma-delta modulator or a loop filter in a PLL behavioral model. This model is most valuable early in a design when you want to validate system-level frequency-domain behaviour before committing to component values.

This article contains Verilog-A model for a low-pass filter. This model supports two types of filters - 1st-order and 2nd-order.

Usage:

1. Create a new cell in Library Manager named LPF and select cell type Verilog A;
2. Copy and paste the code provided;
3. Specify Cutoff_frequency variable to be -3dB frequency;
4. Specify Filter_Order variable to be 1 if you want -20dB/dec or 2 if you want -40dB/dec ;
5. Perform Check and Save;
6. A cell symbol will be created;
7. Instantiate LPF cell into your design;
8. Perform Check and Save and run the simulation.

LPF model testbench

LPF model simulation result (Filter_Order = 1 - purple, Filter_Order = 2 - green)

Cell name: LPF

Model type: Verilog-A