The difference is in the electronic methods employed to filter the signal. One uses analogue electronics, whilst the other digital electronics.
The main difference between the two methods is that a digital filter circuit has to sample the analogue signal and convert it into a set of binary numbers. These numbers are stored as digital data in a hard disk drive, processed, and manipulated digitally.
In contrast, analogue filters do not have to do this type of conversion and the signal remains in its pure analogue form throughout the filtering process.
A digital filter will require an Analogue to Digital Converter (DAC). This converts the analogue voltage levels in the waveform into binary numbers.
A fast microprocessor (µP) processes the binary numbers. The processor typically follows an algorithm written by a programmer. This algorithm determines the filtering characteristics. This may involve an arithmetic operation on the sampled numbers.
Once those binary numbers are processed - or mathematically manipulated - they feed to another circuit called a DAC, which is a Digital to Analogue Converter and as its name suggests converts the binary values into voltage levels thereby reconstructing the signal back into an analogue form.
Analogue filters are much simpler than digital filters because they do not have to convert the signal into a digital form. Hence, there is no requirement for an ADC or DAC. In this type of filter, the signal remains in its true analogue form throughout the process.
RC networks, which are made of simple resistor and capacitor components, do the filtering. There is also often an op-amp involved to amplify the signal to the required level.
Advantages of Analogue Filters
- An analogue filter is simple to implement, as there is no need for a computer processor.
- There is no requirement for a software programmer to write the algorithm.
- Simple RC filters require minimal components.
Advantages of Digital Filters
- In modern gadgets that already utilise a processor, a digital filter is essentially an algorithm, which will require very few additional components.
- Filters can be standardised, as they are simply software modules operating on a number of different computer platforms.
- Digital filters can filter very low frequencies.
- Digital filters can also be adaptive to change their filter characteristics based on the input signal parameters.