Active Filter Design Made Easy With WEBENCH® Active Filter Designer

Active Filter Design Made Easy With WEBENCH® Active Filter Designer
Custom Active Filter Designs Including Spice Simulation Filters are vital in modern electronics. Every data acquisition system has an anti-aliasing filter (lowpass filter) to attenuate high frequency signals before the analog-to-digital converter or an anti-imaging filter (again a lowpass filter) to attenuate high frequency signals after digital-to-analog converter. Instrumentation relies on filters for accurate signal measurements. Active filters can attenuate frequencies that range from sub-1Hz to 10MHz, where passive filter designs require prohibitively large component values and sizes.

WEBENCH® Active Filter Designer: Active Filter Designs Within Minutes!
Choose a Sensor & Signal Bandwidth Select a Filter Type 2. Design Frequency response 3. Analyze with SPICE The WEBENCH Filter Designer steps are SELECT, DESIGN, and ANALYZE. This web software lets you design, optimize, and simulate complete multi-stage active filter solutions within minutes. You can create optimized filter designs with a selection of TI operational amplifiers and passive components from TI's vendor partners. SELECT filters from low-pass, high-pass, band-pass, and band-stop types. Then, as an option, specify performance constraints for attenuation, group delay, and step response. You can then choose from a variety of filter responses such as Chebyshev, Butterworth, Bessel, transitional Gaussian to 6dB, transitional Gaussian to 12dB, linear phase 0.05°, and linear phase 0.005°. From this list, you determine the filter response best suited for your design by optimizing for pulse response, settling time, lowest cost, pass-band ripple, and stop-band attenuation. DESIGN your filter using Sallen-Key or Multiple Feedback topologies. You then select the best operational amplifiers for your design by evaluating gain bandwidth vs. current vs. cost and other parameters. Specify your resistor/capacitor tolerances between ideal, 0.5%, 1%, 2%, 5%, 10% and 20% values. You can experiment with user defined capacitor seed values, which adjusts the resistor value range in your filter design. You can also optimize your filter topologies for sensitivity, lowest cost, and smallest footprint. ANALYZE your design by running SPICE electrical simulation with closed-loop frequency response, step response, and sine-wave response options. Use these options and adjust your input conditions in order to evaluate different output results.

Accessing Filter Designer
ti.com/webenchfilters Select your filter type Start Design To access the WEBENCH® Filter Designer application, use ti.com/webenchfilters as your URL. Once you arrive on this page, select the filter type that you are interested in designing and then click on the “Start Design” button.

NEW Filter Designer Requirements page

Changes to specifications
Gain units  V/V Gain = 20 V/V -3 dB = 5 kHz fs = 25 kHz

NEW Visualizer page Choose 0.5 dB Chebyshev filter

Filter Design Summary page
Note Min OpAmp GBWP values Note OpAmp Bandwidth – 10MHz

Filter Design adjust gain values
Change Gain = 10 Update Note Min OpAmp GBWP = 2.104 MHz Change Gain = 2 Update Note Min OpAmp GBWP = 3.032 MHz Note OpAmp GBWP = 4.0 MHz

Electrical Simulation
Closed Loop Frequency Response, Sine Wave Response, Step Response Click to Run Sim Select Sim Type Select the simulation that you would like to run. Simulation options are Closed Loop Freq Response, Sine Wave Response, and Step Response. Click on the input or output component to modify the stimulus. Click “Start New Simulation” to run the simulation.

Everything old is new Is live today
Big changes in Filter Type (page 1) Bigger changes in Visualizer (page 2) ti.com/filterdesigner

Hands-on Exercise Design Problem: Goals:
Customer would like a bandstop filter at 1000kHz with the following constraints: Type: Bandstop Center Frequency: 1kHz Gain: 1 Passband Bandwith: 1kHz Stopband Attenuation: -45 dB Stopband Bandwidth: 100Hz Dual Supply: +/- 5V Filter transfer function: Linear phase .05deg, 6th order Generate a filter What is the output ripple of a 1V input sine wave at 1kHz? How can this be improved? 11

Hands On Problems 10kHz Low Pass Filter Optimize Low Pass Filter
Go to hands on problem set for Signal Chain Work the problems from the following: Active Filter Designer 10kHz Low Pass Filter Optimize Low Pass Filter Anti-aliasing filter

Active Filter Design Made Easy With WEBENCH® Active Filter Designer
Custom Active Filter Designs Including Spice Simulation Filters are vital in modern electronics. Every data acquisition system has an anti-aliasing filter (lowpass filter) to attenuate high frequency signals before the analog-to-digital converter or an anti-imaging filter (again a lowpass filter) to attenuate high frequency signals after digital-to-analog converter. Instrumentation relies on filters for accurate signal measurements. Active filters can attenuate frequencies that range from sub-1Hz to 10MHz, where passive filter designs require prohibitively large component values and sizes.

Common Filter Applications
Band limiting filter in a noise reduction application The range of frequencies processed by an electronic circuit may be limited to assure maximum performance or accuracy. Frequencies laying outside the desired range will add unnecessary energy to the circuit which may result in less-than-optimum performance from the circuit. Noise is truly random in frequency and amplitude, but we often apply the term to annoying interference such as 50 or 60Hz hum in an audio system. Bandwidth limiting a system with filters can result in the elimination of much of this unwanted noise resulting in higher system performance.

Common Filter Applications
Analog Input A.) RAW SIGNAL f1 f3 f4 f2 Nyquist Sampling B.) AQUIRED SIGNAL fC Anti-aliasing Filter Analog filters are often employed in digital signal processing systems. This examples shows an anti-aliasing lowpass filter placed before the analog-to-digital (A/D) converter. Its purpose is to attenuate frequencies above the half-sampling (fs/2) frequency from being converted by the A/D converter. Such frequencies will fold back into the desired frequency range producing undesirable beat frequencies (tones). In the reverse process, an anti-imaging (or reconstruction) filter is used at the output of a sampled system to eliminate spectral images of the desired signal. These are generated at integer multiples of the sample rate. The ideal filter specification is the same as the anti-aliasing filter. Any passband errors will have the same effect for both types of filter. The consequences of the rejection band deviating from the ideal are not the same and depend on the response of the following equipment to the images that may be generated. Both types of filters can be implemented in the analog or digital domains. anti-aliasing filter – A lowpass filter used at the input of an A/D converter to attenuate frequencies above the half-sampling frequency to prevent aliasing. anti-imaging filter – A lowpass filter used at the output of digital to analog converters to attenuate frequencies above the half- sampling frequency to eliminate image spectra present at multiples of the sampling frequency

Filter Types Lowpass, Highpass, Bandpass, and Bandstop Filters
A lowpass filter the bandwidth is equal to DC to fc A highpass filter has a single stop-band DC to fc, and pass-band f >fc A bandpass filter has one pass-band, between two cutoff frequencies fL and fu>fL, and two stop-bands 0<f<fL and f >fu . The bandwidth = fu-fL A bandstop (band-reject) filter is one with a stop-band fL<f<fu and two pass-bands 0<f<fL and f >fu Adopted from: Introduction to Filter Theory – by David E. Johnson A lowpass filter passes lower frequencies up to the cut-off frequency (fC) and attenuates the higher frequencies that are above the cut-off frequency. This filter gain response is relatively flat up to the cut-off frequency. Beyond that point the filter magnitude attenuates with frequency. The highpass filter passes higher frequencies above the cut-off frequency. In this manner, lower frequency signals are filtered. This filter gain response is relatively flat above the cut-off frequency. Below that point filter magnitude attenuates with decreasing frequency. The bandpass filter allows signals in a frequency window to pass. This window is programmed with the frequencies fL and fU, where fL is the lower frequency and fU is the higher frequency. All signals below fL and above fU are attenuated. The bandwidth of the bandpass filter is equal to fU minus fL. The bandstop filter is the opposite of the bandpass filter. The bandstop filter rejects the frequencies between fL and fU and passes all other frequencies below fL and above fU. Again, fL is the lower frequency and fU is the higher frequency. All signals below fL and above fU are passed. The bandwidth of the bandstop filter is equal to fU minus fL.

1-kHz Lowpass filter gain vs. frequency
Filter Types 1-kHz Lowpass filter gain vs. frequency This slide shows the frequency response of a lowpass filter. At DC the gain of this filter is zero. Passband region occurs while the filter gain remains at zero dB. At 1 kHz you can see how the signal starts to attenuate. Actually at 1 kHz a signal is down by 3 dB. Beyond 1 kHz, and in the stopband region, the signal starts to attenuate at a rate of 40 dB per decade. With a -40 dB per decade attenuation rate this is a second order lowpass filter. This differs from the ideal brickwall response. To achieve the ideal brickwall response, the filter would need nearly an infinite number of poles.

Filter Types 1-kHz highpass filter gain vs. frequency
The slide shows a frequency response of a highpass filter. In contrast to the lowpass filter, at the lower frequencies the gain of the filter is not zero. Rather it starts to move toward zero dB while arriving at the corner frequency of 1 kHz. The cutoff frequency of this filter is 1 kHz. The passband region of this filter starts at 1 kHz and goes up in frequency with the filter response a constant zero dB. The stopband region of this filter starts at 1 kHz and proceeds downwards in terms of frequency. The attenuation in the stopband region is 40 dB per decade. Again this indicates that this diagram is showing the behavior of the second order highpass filter. The green curve in this diagram differs from the ideal brick wall filter response. If this filter had brickwall response it would require an infinite number of zeros.

Filter Types 1-kHz bandpass filter gain vs. frequency
The required level of attenuation is specified at the stop-band BW The bandpass filter is shown in the slide. This type of filter literally passes of band of frequencies while attenuating the signals in the frequency below the lower cutoff frequency (fL) and above the higher cutoff frequency (fH). The level of attenuation is defined by the distance between the three dB cut-off frequency and the stopband frequency. This filter response is symmetrical around the center frequency which is called f0. The bandwidth of this filter is equal to fH minus fL.

Filter Types 1-kHz bandstop, or band-reject filter gain vs. frequency
The bandstop filter is nearly a mirror image of the bandpass filter along the y-axis. With this filter, the signals within the low cutoff frequency (fL) and the high cutoff frequency (fH) are filtered. All other high and low frequencies are passed through this filter. This filter is defined by the center frequency which is called f0, the 3 dB cut-off frequencies (fL, fH), and the stopband frequency. The bandwidth of this filter is equal to fH minus fL.

Filter Order Gain vs. frequency behavior for different lowpass filter orders Pass-band Stop-band fC (-3dB) 1kHz The filter order plays a big roll in defining a filter’s pass-band and stop-band regions. Increasing a filter’s order usually requires cascading additional first, and/or second-order filter stages. The gain roll-off in the stop-band region of the lowpass filter example is -40dB/dec, for each 2nd-order stage. The benefit of higher-order filters is a more ideal response within the pass-band and a greater attenuation rate in the stop-band. Higher order filters comes at the expense of greater circuit complexity and more stringent component tolerances.

Filter Order 2nd-order lowpass, highpass and bandpass gain vs. frequency slopes This plot provides examples of the amplitude vs. frequency responses for lowpass, highpass and bandpass filters normalized to 1kHz. The primary application for such filters is to reduce the amplitude of a particular range of frequencies. These responses are for 2nd-order Butterworth filters (more about this later). For the lowpass and highpass filters the filter cut-off frequency (fc) is at 1KHz, where the gain is down -3dB. The bandpass center frequency is 1kHz and the gain falls off below and above this frequency. An important point to note is the 2nd-order lowpass and highpass filters decrease or increase at a 40 dB/dec rate, respectively, beyond fc. Observe that although the 2nd-order bandpass response decreases at rate similar to the lowpass and highpass filters close to fc, it eventually settles at a roll-off rate of -20dB/dec; half that of the other filter types. Therefore, to twice as many filter stages are required for the bandpass filter to achieve the same eventual gain vs. frequency rates as the lowpass or highpass filters.

Why Active Filters? A comparison of a 1kHz passive and active 2nd-order, lowpass filter Inductor size, weight and cost for low frequency filters may be prohibitive Inductor magnetic coupling considerations Active filter size is small and low in cost R and C values are easily scaled in active filters Active filters have replaced conventional passive LC (inductor/capacitor) filters in many low-frequency, filter applications. The size, weight, availability, cost and magnetic characteristics of large inductors often make their use less convenient, or undesirable. LC filters are still the most viable, and sometimes the only choice, for applications involving radio frequencies and where power levels and frequencies exceed an active filter’s capabilities. Dynamic range limitations must be considered for all applications and that too may dictate the use of a passive filter, over an active filter. The circuit shown in this slide makes a comparison between a 1kHz passive lowpass filter and a 1kHz active lowpass filter. The LC filter requires specific source and load impedances to provide a prescribed response. In this slide, the lossless case is shown. Losses associated with the inductor(s) will cause the gain and phase responses to deviate from the ideal. The active-filter must be driven by a very low impedance source, or the source impedance must be included in the filter design to keep from corrupting the ideal response. When these two filters are driven by an equivalent source voltage it can be seen that both the gain and phase response are identical. A different active filter topology may invert the output to input signal relationship and that needs to be considered in an application.

Popular Active Filter Topologies
2nd-order Active filter topologies used by WEBENCH Active Filter Designer Component type for each filter topology Pass Z1 Z2 Z3 Z4 Z5 Low R1 R2 C3 C4 na High C1 C2 R3 R4 Pass Z1 Z2 Z3 Z4 Z5 Low R1 C2 R3 R4 C5 High C1 R2 C3 C4 R5 Band The active filter stage is a filter comprised of one or more operational-amplifiers and associated resistors (R) and capacitors (C). The operational-amplifier’s output is fed back to one, or both inputs, via the paths containing the resistors and capacitors. Imaginary poles are intentionally developed at specific frequencies to modify the amplitude vs. frequency response of the amplifier stage. Without the use of the operational-amplifiers and feedback these poles could only be developed with the use of inductors and capacitors. The two most common active filter topologies are the Sallen-Key, which is a non-inverting, voltage-controlled, voltage-source (VCVS) topology and the infinite-gain, multiple-feedback topology (MFB). The latter is an inverting topology, where the output polarity is inverted from the input at DC.

Filter Responses Response Considerations Amplitude vs. frequency
Phase vs. frequency Step and impulse response characteristics Filters are employed in circuits to act upon an applied input and provide a specific response characteristic. Most often this is to modify the pass-band characteristics of the applied signal, but it may involve something other such as modifying the timing or impulse behavior in a very specific, controlled manner.

Filter Reponses Common active lowpass filters - amplitude vs. frequency Examples of the amplitude vs. frequency responses for several different, but common, 1kHz lowpass filters are shown. All of these filters are of the same 4th-order, but as can be seen they exhibit different responses over frequency. Different applications may call for one response, or another. Here’s some details regarding each of these responses: Butterworth – This is referred to as a maximally flat response. The roll-off in the pass and stop-bands is very predictable and a function of the filter order. The amplitude vs. frequency response is as mathematically flat as possible within the pass-band. It is the most often applied filter response in general filter and signal-processing applications. Chebyshev – Features earlier roll-off as compared to most other filter response types, but at the expense of pass-band ripple. They minimize the error between the idealized and the actual filter characteristic over the range of the filter. A faster initial roll-off rate is associated with higher pass-band ripple. Linear phase – The phase response is a linear function of the frequency. All frequency components have equal delay time. Therefore, the filter has a constant delay or group delay. Bessel – Has a maximally flat group delay (maximally linear phase response). They are often used in audio crossover systems. Analog Bessel filters are characterized by almost constant group delay across the entire pass-band, thus preserving the wave shape of filtered signals in the pass-band. Gaussian – A filter whose impulse response is a Gaussian function. Gaussian filters are designed to give no overshoot to a step function input while minimizing the rise and fall time. This behavior is closely connected to the fact that the Gaussian filter has the minimum possible group delay.

Filter Reponses Common active lowpass filters – other responses
Phase vs. frequency Impulse response Group Delay This graph shows examples of phase versus frequency, output versus time is the impulse response, and a group delay versus frequency. All of these filters are fourth order, but you can see that they exhibit different responses over frequency. Referring to the phase versus frequency plot, the Bessel filter has the slowest change and phase versus frequency as compared to the other filters. On the other hand the Chebyshev filter moves quickly through a phase of 0° down to -360° rather quickly in terms of frequency. The impulse response also tells a story about these filters. Of all the filters, the Chebyshev exhibits a great deal of ringing over time and it takes a long time to settle. However, the overshoot is not as high as the Gaussian, Bessel, or the Linear Phase filters. These three filters settle relatively quickly. The Butterworth is a middle-of-the-road filter with a fairly low overshoot. However it does take a little more time to settle, but not as long as a Chebyshev. The group delay versus frequency plot provides a insight into the impulse response plot above. A filter that displays high group delay will also have a great deal of overshoot and ringing. You'll notice that the Chebyshev filter has the highest group delay.

Specify Filter Requirements
Select Filter Program Frequencies Click to Continue You can choose between a Lowpass, Highpass, Bandpass, or Bandstop filter. Additionally in the view you can specify your Attenuation and Flatness constraints for the filter, as well as your power supply preference. After completion of all the entries on the page click “Start Filter Design” to proceed to the FILTER DESIGNER VISUALIZER view.

View / Select Filter Response
Performance Graphs Select Filter Approximation In this view, the Optimizer Knob allows you to optimize the solutions for pulse response, settling time, cost, passband ripple, and stopband attenuation. Modify your constraints and see the effects of the modifications immediately. Refine the results to narrow down on your desired filter response.

Design Summary: Modify your Design
Optimizer Dial Topology and Component Specifications Tweak Design Current Design and Design Notes Share or Copy Design Also included in the Design Summary are areas where can modify the filter system. These options in the left column include an Optimizer Knob, Topology Specifications, Tweak Design, Design Summary, and Share or Copy Complete Design.

Hands-on Exercise Design Problem: Goals:
Design a low pass filter with fast falling after the cut-off frequency. Filter Low pass Gain = 20 V/V -3db = 5000 Hz Stop band frequency = Hz Stop band attenuation = - 45 dB Chebyshev – allowable ripple <= 0.5 dB Optimize the amplifier bandwidths to be as low as possible. 32

Filter Designer Landing Page (http://ti.com/filterdesigner )
Start Design

Optimizer Knob Click on the Optimizer Knob and select the preferred optimization for your Filter Response solutions. As you make your selection, you will notice that the Solutions table re-sorts placing your new preference at the top.

Modify Constraints Change inputs and click “Recalculate”
With in the second button in this view, you can modify your original constraints that you programmed on the first view (Filter Designer Requirements). If you make modifications in the view, make sure that you click” Recalculate” to immediately see the results of those modifications.

Refine Results The third button along the top allows you to refine your results. Click and move the sliders to narrow down on the filter response choices that are displayed. Again, you will see your changes reflected in the Solutions table.

View/Optimize Filter Response Solutions
Optimization Chart Filter Response Solutions Performance Graphs You will be presented with a page displaying the Filter Response solutions that meet your requirements. The Optimization Chart allows you to compare the performance parameters of each solution in a three dimensional format. The Performance Graphs allow you to compare the gain, phase, group delay, and step response performance for each of the solutions.

Optimization Graph Modify Axis Parameters
The optimization graph allows you to sort your solutions using the categories of Group Delay, Attenuation, Filter Order, Passband Ripple, and Maximum Q. By default, the Optimization Graph shows Group Delay on the X axis, Attenuation on the Y axis, and Filter Order as the bubble size. Click on the axis configuration drop down to select an alternate parameter to display on the axis.

Charts The Charts or Performance Graphs allow you to compare the gain, phase, group delay, and step response performance for each of the solutions. In this view you can click of the graph that you interested in. Once in this detail view you can window into the areas of interest.

Select a Filter Response
In this table you are given the possible approximation options for your filter design. This table provides columns with a variety of specifications. If you click on the column head, that column will sort. When you are ready, click on the “Select” button of your desired Filter Response to continue.

Design Summary: Filter Topology Configuration
Update per stage Filter Stage Schematic BOM Now that you have selected your Filter Response, you can now configure your Filter Topologies. In this page, you will initially notice the filter circuit diagrams with their Bill-of-materials (BOM) listing below. For this presentation, we have selected a 4th order Chebyshev filter which is represented by two 2nd order, Sallen-Key stages. Each filter stage shows a schematic and a BOM as well as summary information above the filter stage.

Update Gain/Topology per stage
Click to update In the summary section over the top of each filter stage, you will find a description of the filter topology, gain, cutoff frequency and the minimum op amp gain bandwidth product (GBWP). The options for second order topologies are Sallen-Key and MFB (multiple-feedback). The gain reflects your initial programmed value in prior views. If you choose a gain greater than one, that gain value will be divided equally between the stages on this page. You can once again modify your gain and also control the individual gain of each stage. The Minimum OpAmp GBWP (Gain Bandwidth Product) is a recommended guidance as you choose your amplifier for your filter. Filter Designer chooses an appropriate amplifier for you in the BOM listing below the circuit diagram. If you choose to change your amplifier, Filter Designer only presents amplifiers with GBWPs higher that this value. The circuit topology and gain can be modified at this level. If you choose to do any modifications, click the “Update” button to implement your changes.

Filter Stage Schematic: View Component Values
Looking at the filter topology schematic in the middle of this view, you can hover over any of the passive component to view the details for that component.

Filter Stage: Bill of Materials
Select Alternate Part Each Filter Topology has a Bill of Materials associated with it. View the details of the components associated with the Filter Topology. Click Select Alternate Part to select an alternate component.

Select Alternate Part Select Alternate Part
View the alternate passive or active components select an alternate.

Design Summary: Modify your Design
Optimizer Dial Topology and Component Specifications Tweak Design Current Design and Design Notes Share or Copy Design Also included in the Design Summary are areas where can modify the filter system. These options in the left column include an Optimizer Knob, Topology Specifications, Tweak Design, Design Summary, and Share or Copy Complete Design.

Optimization Knob Click on the Optimization Knob to optimize your Filter Topologies for smallest footprint, lowest BOM cost, or sensitivity.

Filter Topology Specification
Select Topology for all stages Cap seed and tolerances Select Alternative Amplifier You can select an alternate OpAmp and Filter Topology for your entire design. In the Filter topology panel there are four options to work with: Topology, Cap Seed Value, Res Tolerance, and Cap Tolerance. The available topologies in Filter Designer are the Sallen-Key and MFB options. If you change the topology, you will impact the complete circuit diagram. The CapSeedValue will change one of the capacitors in each stage to match your request. Any other capacitors, along with resistors, will adjust accordingly to maintain your filter response. The tolerance of your resistors and capacitors can also be adjusted. When you complete changes in this view make sure that you click on “Update” to ripple your changes through the filter system.

Tweak Design At this level you have an opportunity to change the filter approximation type (Bessel, Butterworth, Chebyshev, etc.) and/or the filter order. When you complete changes in this view make sure that you click on “Update” to ripple you changes through the filter system.

Notate and Share Design ID Share Design
At this level, you can rename your design, add notes, and view the product folder for your selected op amp. Click “Share Design” to share a copy your design with a colleague.

Click to Simulate You now have your complete filter for design. Produce a design report and proceed to build your filter or proceed onto the analysis section in this software. On the main navigation panel in the Filter Designer Design Summary view, click on the “Sim” button to enter the Electrical Simulation view.

Simulation Results Waveform List View the simulation results on the right. The list of simulations you have performed are below the schematic. Click on the entry in the list to view the results for that simulation.

Review Past Simulations

Export to External Simulator
3) Schematic opens in Tina or Altium (Altium requires v14 and TI plug in) 1) Click export 2) Choose sim type and export format

View and Print PDF Report
Design Report View and Print PDF Report Finally, you will be able to print a Design Report by clicking the “Print” button. With this action you will be able to view and print a PDF summary of your complete design.

Active Filter Design Made Easy With WEBENCH® Active Filter Designer

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Active Filter Design Made Easy With WEBENCH® Active Filter Designer

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