# Difference Between Analog and Digital Filters

## Main Difference – Analog vs. Digital Filters

In electronics, a signal can be made of a combination of component signals at different frequencies. Filters are components used in electric circuits to remove unwanted frequency components in a signal. The main difference between analog and digital filters is that analog filters process analog signals directly, whereas digital filters need to first convert analog signals to digital signals, before processing. After processing, the signal needs to be converted again from digital to analog signals.

## What are Analog Filters

Analog filters make use of resonance in electrical circuits. Combinations of resistors and inductors provide different levels of impedance to currents with different frequencies. Therefore, they can be used in circuits to suppress unwanted frequency components in a signal.

For instance, a band-pass filter suppresses frequencies which are outside a given range of frequencies. The diagram below shows how such a filter may be constructed using capacitors (C) and inductors (L):

A Band-pass filter which suppresses frequencies which are not within its range.

And, in the output signal, all frequencies which are not in the bandwidth range (B) are suppressed:

A band-pass filter suppresses frequencies which are not in a region called the bandwidth.

Analog filters can filter signals continuously. In some cases, this gives a slight advantage to analog filters, where all unwanted frequencies need to be removed. The ability of a digital filter to do this depends on its sampling rate (see below).

## What are Digital Filters

Digital filters can work with digital (i.e. non-continuous) signals. A digital filter, therefore, consists of an Analog to Digital Converter (ADC), which first converts any analog signal to a digital signal. The ADC does this by sampling the signal at regular intervals. After the conversion, the digitized signal is processed and unnecessary frequency components are removed. Then, the resulting digital signal needs to be converted back into an analog signal using a Digital to Analog Converter (DAC).

Digital filters have several advantages over analog filters. Once they are programmed, the programs in digital filters could be easily altered by re-writing their algorithms (to reprogram an analog filter, the circuitry itself needs to change). This also allows digital filters to perform adaptive filtering: a form of filtering where filtering parameters could be altered over time, depending on how the input signal changes. Digital filters also tend to be more accurate since the accuracy of an analog filter largely depends on the capacities of its components. Digital filters are also able to handle very low frequencies that analog filters are unable to handle.

## Difference Between Analog and Digital Filters

### Type of Signal Processed

Analog filters can process analog filters directly.

Digital filters need to first convert analog signals to digital, before processing. After processing, the signal needs to be converted again from digital to analog.

### Altering the Functionality

To alter the functioning of analog filters, the components themselves need to be re-configured.

Digital filters are usually governed by algorithms, and the functionality could be easily changed by altering these algorithms.

### Rate of Processing

Analog filters filter out unwanted frequencies continuously.

The rate at which digital filters can work depends on their sampling frequency.

Image Courtesy

“A schematic diagram of an example of a band-pass filter, in T-section Cauer topology…” by Inductiveload (Own work) [Public Domain], via Wikimedia Commons

“ A diagram showing the definition of bandwidth (B) for a bandpass filter…” by Inductiveload (Own work) [Public Domain], via Wikimedia Commons