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Analog Active Filter Design with Free Software

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Analog Active Filter Design with Free Software

Introduction


TI is well-known for its digital chips including DSP and application processors. Additionally, it also offers wide range of analog products. For detail information, please visit its analog eLab design center. You can learn the courses from the video cast and presentation, select the proper products to setup the prototype design and simulate your design with some useful free software tools for DC/DC (SwitcherPro), ADC (ADCPro and MADCBufferPro), programmable clock (ClockPro) and active filter (FilterPro). These tools are specially designed for specific TI components.


Today most of the engineers are quite familiar with the development for the digital parts, from VHDL to firmware development. The design for analog part also requires experience. The free tools can help us for the analog designs. Let us illustrate how to design a filter with available free software (FilterPro and TINA-TI).


The FilterPro is a simple tool to help you design an active filter by selecting the passband, circuit type, filter type, pole numbers and cutoff frequency and fine-tune the circuit performance.


TI also teams up with its partner DesignSoft to present a great simulation solution for its customers. TINA from DesignSoft is a powerful Spice based simulation tool for both analog and digital circuits. In the recent release, besides its existing schematics capture, simulation, PCB layout and virtual instruments, TINA even features VHDL simulation and MCU co-simulation. Although it is commercial software, DesignSoft offers a complementary version, TINA-TI, with intensive support to all of the analog product lines from TI. You can check out the EXAMPLE folders after you install the software. There are many sample circuits in the sub-folders for audio, comparators, Difference amps, FilterPro, power amps, SMPS and much more. All of the samples in the TINA-TI use TI analog product as much as possible.

Filter Design Background and FilterPro Capabilities


An ideal low-pass filter would completely eliminate signals above the cutoff frequency, and perfectly pass signals below cutoff (in the pass-band). In real filters, various tradeoffs are made attempting to approximate the ideal. Some filter types (like Bessel or Thomson) are optimized for gain flatness in the passband, some (like Chebyshev) trade off gain variation (ripple) in the pass-band for steeper roll-off, still others (like Butterworth) trade off both flatness and rate of roll off in favor of pulse-response fidelity. FilterPro supports the three most commonly-used all-pole filter types: Butterworth, Chebyshev, and Bessel. If filter type is toggled between these options, you will find the circuit is the same, while the component values, curves and Fn/Q are changed. The filter type changes the AC transfer characteristics of the filter.


The passband types supported by FilterPro are low-pass, high-pass, band-pass, notch, wide band-pass and band-reject. If you toggle the passband type between low-pass and high-pass, the capacitors and resistors are swapped. The other passband are the combinations of different types.


The software aids designer to design the filter in three kinds of circuit topologies: MFB (Multiple Feedback, it is also named as infinite Gain or Rauch), Sallen-Key (which has excellent gain accuracy, is suitable for high Q high frequency filter) and MFB full differential. You can see the different circuit topologies between them. They are quite different in circuit component BOM count and performance.


The order of an active filter is determined by how many capacitors are used. You can check it out by changing the pole numbers. FilterPro designs all pole filters and each of these poles represents an order. For example, in FilterPro a third order filter would have 3 poles and would contain 3 capacitors. The FilterPro program automatically places lower Q stages ahead of higher Q stages to prevent op amp output saturation due to gain peaking. Besides, even-order filters designed with this program consist of cascaded sections of complex pole pairs. Odd-order filters contain an additional real-pole section shown at the input on the filter, depends on configurations, some types are better with the real-pole section following. FilterPro supports filters from 2 to 10 poles for all supported filter types except Band-Pass and Notch. A single pole filter is available in the High-Pass and Low-Pass Bessel filter type.


Compared to resistors, capacitors with tight tolerances are more difficult to obtain and can be much more expensive. The Capacitor fields allow you to enter actual measured capacitor values. In this way, an accurate filter response can be achieved with relatively inexpensive components. That is an important feature of FilterPro.

Prototype with FilterPro


Let us design a simple Sallen-Key Butterworth low-pass filter for audio applications (less than 20 KHz). (Caution: FilterPro has bugs in refreshing the circuit schematics diagram sometimes. You can use File | New command to refresh the screen. Alternatively, close and run it again.)


Let us try the hardware part (circuit) first. The default configuration is Low-Pass, 3 poles, MFB Single-Ended, Butterworth, with Cutoff frequency at 1 kHz and Cursor Frequency at 10 KHz. The diagram displayed in the middle of the window is mixed up with Bode diagram (Amp vs. Freq and Phase vs. Freq) and Group delay diagram. The Gain trace is in green, the Phase trace is in red, and the Group delay is in black. Of course, you can change the color setup if necessary. The Phase trace is not corrected 180 for inverting stages, as marked on the right bottom corner of the window. Later you can compare the simulation diagram of phase response with FilterPro to understand what it means. The cursor frequency can be changed either by entering a new frequency in cursor frequency field or by clicking the desired frequency in the diagram. Then you can check the table for passband gain, Fn, Q and Gain/Phase at cursor frequency. 


Run the FilterPro, and then change Circuit Type to "Sallen-Key". You will see the circuit configuration is changed, so an inexperienced designer will know the hardware configuration for different filter type. Leave Passband (Low-Pass) and Poles (3) unchanged. Then enter 20k in the cutoff frequency filed,  all the RC values are changed to meet the desired performance. The Bode diagram and Group Delay diagram change according to your circuit configuration. I recommend you toggle the filter type between Bessel and Butterworth to check out the difference of group delay, and verify them in the TINA simulation software.


Pretty easy for FilterPro, hum? Now let us try TINA.


Simulate with TINA


You can export (draw the schematics manually) the reference design in Filter Pro to TINA-TI to analysis and fine tune the overall performance. According to our design goals, open TINA 7 -TI\EXAMPLES\Filters_FilterPro\Sallen-Key 3rd Order Butterworth LPF.sch schematic file in TINA-TI. You will see a Sallen-Key filter with attached diagrams in the window. Of course, this circuit was designed for 1 KHz LPF, so you have to change the values of the components according to the values in FilterPro, and then start to simulate. To change the value of the components, please double click the selected component , change the resistance (or capacity) field, and then click OK.




LPF circuit

To simulate, use "Analysis | DC Analysis | DC Transfer Characteristics..." command to get the DC transfer characteristics. Of course, AC analysis is more important for a filter. You can use "Analysis | AC Analysis | AC Transfer Characteristics..." command. In the dialog window, you can setup start (10Hz) and end (100 KHz) frequency, sweep type and diagram types. Here I enable "Amplitude & Phase" and "Group Delay" check boxes. Click OK, you can see a diagram window, which contains multiple diagram tabs for Bode diagram and Group Delay diagram.


Bode Diagram

There are some other features in the diagram window. For example, you can find two cursors A and B, you can click these cursors and put them on the diagram, so you can read/measure the curves by dragging the cursors. Of course, you can use the probe tool to read the curves on the bottom status bar. You can also assign labels, draw lines or put text to the selected curves.


LPF Group Delay

In general, TINA and FilterPro should have identical result (curves). If the TINA and FilterPro give different diagrams, you may have some mistakes in your schematics, you have to find out by yourself.


TINA-TI has offered all necessary features for filter design, very helpful and informative. However, you have to buy a full functional TINA from DesignSoft, if you want more features from the simulation. Please visit www.tina.com for detail information.

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Possible Improvement


Filter Pro and TINA-TI is a perfect combination to speed up the filter design. The Filter Pro has prepared the sample TINA circuits under the folder of FilterPro\TinaFiles. On the other hand, TINA also includes a dedicate folder for FilterPro under TINA 7 -TI\EXAMPLES\Filters_FilterPro\. The designer can reuse these schematics files in TINA-TI and fine tune them according to the application requirement. However, it is better to generate a schematics file which can be exported into TINA directly.


Because FilterPro has no update since 2006, so far, you have to rely on yourself to edit the existing sample schematics files. Fortunately, drawing/editing the schematics in TINA is quite easy.


Finally, if possible, I hope TINA-TI can support MCU simulation for MSP430. Although even TINA Pro has not support this micro yet.

Other Software for Filter Design


Filter Solutions, 20 day evaluation, Nuhertz,

http://www.filter-solutions.com/


Filter Wiz Pro, perfect for filter with discrete components, $199, Schematica

http://www.schematica.com/filter_wiz_files/FWPRO.htm


FilterCAD, Linear, free,

http://www.linear.com/software/


FilterLab, Microchip, free,

http://www.microchip.com/stellent/idcplg?IdcService=SS_GET_PAGE&nodeId=1...


TINA Pro, DesignSoft

http://www.tina.com/

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