Design Guide
August 2003
 

Available Technology

Electronic Filter Design Guide

SELECTING A FILTER TECHNOLOGY

In addition to specifying transfer functions, designers who need signal filtering must choose among passive, linear-active, switched-capacitor, and digital-signal-processing (DSP) filter technologies.

Passive Filters
Passive filters contain resistors, inductors and capacitors that provide polynomial approximations of ideal filters. They often come packaged in metal cans to reduce inductor magnetic pickup. Corner frequencies generally range from hundreds of Hertz to many mega-Hertz. Passive filters require no power (and therefore no power supply) and generate no DC offset.

Low-frequency passive filters are large and heavy, and manufacturing them is expensive. Input signals also undergo "insertion loss" (attenuation) in the pass-band. The non-linearity of the magnetic materials in the inductors makes building low-distortion filters of this type difficult. An engineer who wants to design a custom filter may have trouble obtaining precision inductive components and tuning the filter to a specific corner frequency requires considerable expertise. Passive filter circuits are not easily programmable.

Linear Active Filters
Linear active filters contain resistors, capacitors, and linear operational amplifiers. Corner frequencies range from 0.001 Hz to 30 MHz. Unlike passive filters, linear-active filters require external power. Since target systems also require power, this does not generally present many impediments to designs, however, corner frequencies above 100 kHz call for wide-band amplifiers that demand significant currents.

Some semiconductor manufacturers have created monolithic-silicon linear-active filter designs. This approach diffuses or layers internal capacitors and resistors onto the same silicon substrate as the semiconductor amplifiers. Attainable capacitor values and stability of the diffused capacitors and resistors limit this technique's applicability to higher frequencies, especially for high-order filter functions.

Switched Capacitor Filters
In switched-capacitor filters, a switched capacitor simulates a resistor at an amplifier input, thereby creating an integrator as shown in Figure 18.

Figure 18

The circuit momentarily connects to "A", charging capacitor "Cs" to the input voltage that is present at that moment. It then switches to "B", dumping the charge onto the amplifier's negative input. The amplifier then transfers the charge to the integrating capacitor "Ci", where it remains until the next cycle either adds or subtracts charge. The higher the switch frequency, the more often "Ci" receives charge, which changes the integrator's time constant and therefore the resulting filter's corner frequency. Varying clock frequency permits programming filters "on-the-fly".

Cascading sections permits constructing multi-pole filters. In some universal designs, a filter-section's corner frequency is not an exact sub-multiple of the clock. Cascaded multi-pole versions of such designs require care to ensure that pole frequencies are correct. By switching the capacitor at 50 to 100 times the corner frequency, these filters can attain a good approximation of theoretical performance.

Since a switched-capacitor filter is a sampling device, it experiences aliasing errors, frequency components near the sampling frequency that must be eliminated to ensure accuracy. Also, this technology produces clock feed-through. Clock feed-through is an extraneous signal that switched-technology filters create. Although feed-through resides at 50 to 100 times the filter's corner frequency, its amplitude can exceed the resolution or noise floor requirements of the application and can cause additional aliasing problems. Manufacturers often do not include this factor in their noise specifications, yet users must make accommodations for clock feed-through in their system design. Fortunately, its high frequency makes removal fairly easy with simple second or third-order linear-active filters.

Switched-capacitor designs are available as complete filters or as universal building blocks requiring external resistors to function. Driving clocks may be internal or external to the filter itself. These filters can be small (DIPs and SOICs ) and inexpensive because they are manufactured as silicon chips.

Digital Signal Processing (DSP) Filters

Due to the unique design considerations and requirements associated with digital filters, along with the ever-changing data conversion (A/D, DSP, FPGA...) technology, a separate section of Frequency Devices Filter Design Guide has been designated for Digital Filters.


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