✍️05-Engineering Signals and Analog Filters

Tools used in this book

Though we are starting from fundamental principles and emphasizing manual calculation, it is also beneficial to use tools that can help streamline the design process, and these tools offer powerful, free, and fast solutions.

LT SPICE

A powerful, fast, and free SPICE software to simulate analog circuits

WolframAlpha

An online based knowledge engine for fast math & science related computations

Analog Filter Wizard

An online based design tool for active filters by Analog Devices

Marki Microwave

An online based design tool for passive filters by Marki Microwave

Some Filters Introduced in the Book

The book introduces several types of filters and explains the concept of transfer functions and their derivation. This process involves analyzing circuit components and applying mathematical techniques like Laplace or Fourier transforms. Understanding transfer functions helps readers gain insight into a filter’s frequency response, which is key to designing and optimizing filters in practical applications.

SPICE Simulations

Contains the LT SPICE simulation files used in the book (from Chapter 3 to Chapter 5)

Feel free to download and use them directly to save you some time. While I do encourage you to create these files by yourself if you are new to this powerful simulation tool.

Audio File Used in Section 6.2

This audio file is used in Section 6.2 of the book as the soundtrack sample to be fed into the filter. You may also use any music of your choice, as long as it is in .WAV format. The recommended length is within 30 seconds, as longer files may cause LT SPICE to take more time for processing.

Schematics for the ECG Circuit in Section 6.3

For educational purposes, the schematics of the four PCB modules that make up the complete ECG circuitry are provided below. Note that certain sections of the circuit, such as the input protection circuit, are not shown here but are part of the overall design. The focus is on the core components.

Stage1

Stage2

Stage3

Stage4

Instrumentation Amplifier

This instrumentation amplifier with a gain of apprximately 100 is used to amplifier the differential signal between the two electrodes placed across the heart of the patient; IN+ and IN- are attached to the measurment probes and the third probe (common) is connected to the ground

High Pass Filter

This high pass filter is used to suppress the low frequency noise due to DC drifting, breathing and muscle contractions of the patient, while providing an additional gain of ~10 to boost the differential signal. The signal measured at the output is expected to be more "zero centered"

Notch Filter

The notch filter implemented using Multiple Feedback topology removes the 50/60Hz power line noise coupled from surrounding enviroment when the patent's body serves as a big capacitance that captures the power line hum into the circuit

Low Pass Filter

The 3rd order Butterworth low pass filter cleans out higher frequency noises (usually above 150Hz) from the harmonics, EMI and noise on the ground, while maintaining the lower band frequency maximally flat. The output is low impedance output which can be directly connected to next stage circuit for measurement and processing.