Analog Filters for Data Conversion

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Analog Filters for Data Conversion - PowerPoint PPT Presentation

Analog Filters for Data Conversion. Types of analog filters used as anti aliasing filter. Three types of analog filters are commonly used: Chebyshev , Butterworth ,and Bessel Each of these is designed to optimize a different performance parameter.

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Presentation Transcript
Types of analog filters used as anti aliasing filter

Three types of analog filters are commonly used: Chebyshev, Butterworth,and Bessel

Each of these is designed to optimize a different performance parameter.

The complexity of each filter can be adjusted by selecting the number of poles and zeros, mathematical terms that will be discussed in later chapters.

The more poles in a filter,the more electronics it requires, and the better it performs.

Four, six, and eight pole filters are formed by cascading 2,3, and 4 of these circuits, respectively.

If you need a high-pass filter,

Simply swap the R and C components in the circuits (leaving Rf and R1 alone).

switched capacitor filterare easier to manufacture on IC’s

Note: C and C/100

Switched capacitor filters use switches and capacitors to mimic resistors.

It shows the equivalent step responses, two capacitors and one switch can perform the same function as a resistor-capacitor network.

the cutoff frequency of the filter is proportional to the clock frequency used to drive the switches.

Some definitions
• For audio CDs have a sampling frequency =44100 Hz. The Nyquist frequency = 22050 Hz
• Cutoff frequency or corner frequency is the frequency either above which or below which the power output of a circuit, such as a line, amplifier, or electronic filter is the power of the passband.
• Because power is proportional to the square of voltage, the voltage signal is of the passband voltage at the corner frequency. Hence, the corner frequency is also known as the −3 dB point because is close to −3 decibels
Step response of a filter

Step response of the three filters.. The Bessel is the optimum filter when overshoot and ringing must be minimized.

- 3 characteristics of a filter

Roll-off

Cut-off frequency

Step response

Pulse response of the Bessel and Chebyshev filters

a

c

b

A key property of the Bessel filter is that the rising and falling edges in the filter's output looking similar. This is called linear phase. Figure (b) shows the result of passing the pulse waveform in (a) through a 4 pole Bessel filter. Both edges are smoothed in a similar manner. Figure (c) shows the result of passing (a) through a 4 pole Chebyshev filter.

The left edge overshoots on the top, while the right edge overshoots on the bottom. Manyapplications cannot tolerate this distortion.

The goal is to eliminate high frequencies (that will alias during sampling), while simultaneously retaining edge sharpness (that carries information).

Figure (a) shows an example analog signal containing both sharp edges and a high frequency noise burst. Figure (b) shows the digitized signal using a Chebyshev filter. While the high frequencies have been effectively removed, the edges have been grossly distorted. This is usually a terrible solution. The Bessel filter, shown in (c), provides a gentle edge smoothing while removing the high frequencies. Figure (d) shows the digitized signal using no antialias filter. In this case, the edges have retained perfect sharpness; however, the high frequency burst has aliased into several meaningless samples.