Low pass analog filter design

Active analog filters can be seen in almost all electronic circuits. The audio system uses filters for band limiting and balancing. Communication system designers use filters to tune specific frequencies and eliminate other frequencies. In order to attenuate high frequency signals, all data acquisition systems have an anti-aliasing (low-pass) filter in front of the analog-to-digital converter (ADC) or an anti-mirror (low-pass) behind the digital-to-analog converter (DAC). filter. This analog filtering also removes high frequency noise superimposed on the signal before it reaches the ADC or after leaving the DAC. If the input signal of the ADC exceeds half of the converter's sampling frequency, the size of the signal is reliably converted; however, as it changes back to the digital output, the frequency also changes.

Low-pass, high-pass, bandpass, or band-stop filters can be efficiently designed using TI's WEBENCH filter designer software. This application replaces TI's FiLTErPro and previous National Semiconductor's WEBENCH active filter designer software. It uses these programs and formulas when generating active filters. However, it allows depth adjustment of various filter variables, optimization of the filter, finding the right TI op amp for the filter circuit, and SPICE simulation, which is more powerful than the two programs above.

Important design parameters for low pass analog filters

The frequency domain specification of the low pass analog filter includes four basic parameters:

Fc, the -3-dB cutoff frequency of the filter

Ao, the gain of the filter

Asb, the stopband attenuation

Fs, the interruption frequency of the stop band attenuation

The filter type window of the WEBENCH filter designer shown in Figure 1 lists these parameters. The frequency range of the DC to the cutoff frequency (fc) is the band pass area. In Figure 1, Ao is the bandpass response level. When using a Butterworth or Bessel filter, the bandpass response can be flat and ripple free. Conversely, Chebyshev filters have ripples up to the cutoff frequency. The ripple error of the Chebyshev filter is 2 ΔAMAX.

Figure 1 WEBENCH filter designer important analog filter parameters

When the filter response exceeds fc, it will fall through the transition band to the stop band region. The filter approximations (Bartworth, Bessel, Chebyshev, etc.) determine the bandwidth of the transition band and the order of the filter (M). The number of poles of the transfer function determines the order of the filter. For example, if a filter has 3 poles in its transfer function, it is a third-order filter.

In general, the transition band becomes smaller as more poles are used to implement the filter design, as shown in the Butterworth low-pass filter in Figure 2. Ideally, low-pass, anti-aliasing filters should have a "brick wall" response with minimal transitions. In fact, this is not the best anti-aliasing method. An active op amp is required for every two poles when designing an active filter. For example, a 32-order filter requires 16 operational amplifiers, 32 capacitors, and up to 48 resistors.

Figure 2 Butterworth filter pole points increase to bring a more significant drop

Analog filter approximate calculation type

Figure 3 shows some of the available low pass filter types in the solution window of the WEBENCH Filter Designer Watch screen. This screen appears after the user clicks the "Start Filter Design" button (Fig. 1).

Figure 3 Low-pass filter type of the WEBENCH filter designer

Butterworth, Bessel and Chebyshev are some of the more popular types of filter approximations. After looking at the magnitude and frequency domain comparisons as well as the magnitude and time domain comparisons, you can know the filter type.

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