Page 340 - Analog and Digital Filter Design

Filter Integrated Circuits 33

parallel, to increase the range of cutoff frequencies available. The internal capac-
itors are laser-trimmed to a tolerance of 0.5%.

A computer program called FILTER42 is available from Texas Instruments and
is described in Application Bulletin AB-035 (downloadable from the TI Inter-
net site, www.ti.com as file sbfa002.pdf). FILTER42 calculates the component
values for Bessel, Butterworth, Chebyshev, and Inverse Chebyshev (but not
Cauer) responses. The Inverse Chebyshev cutoff frequency is considered by this
program as being at the beginning of the stopband, rather than the -3dB point.
The Chebyshev cutoff frequency is considered by this program to be where the
amplitude response falls below the passband ripple limit.

Alternatively, design equations are given in the UAF42 data sheet. Unfortu-
nately, these equations give on, Q, and gain in terms of resistor and capacitor
values. Usually w,,, Q, and gain are known, so the equations may need trans-
posing to find, say, RF, in terms of the known values.
The FILTER42 program is in the FilterPro’” series; other programs are
FILTER1 and FILTERZ. Both FILTER1 and FILTER2 can be used to help
design lowpass filters; they cannot help in the design of highpass, bandpass, or
bandstop filters. FILTER1 produces component values for Sallen and Key
lowpass active filters. FILTER2 is described in Applications Bulletin AB-034B,
and this program produces component values for both Sallen and Key, and
Multiple Feedback (MFB) filters. The MFB topology is sometimes called
Raunch, or Infinite Gain, and is less susceptible to component variations.

Texas Instruments also have the FilterPro MFB lowpass filter design program
available for download from their Internet site. This program finds component
values for Bessel, Butterworth, and Chebyshev filters constructed from op-amps
and discrete components. Filter designs up to the tenth order are possible using
this program.

lnfegrated Circuit Filter MAX274
The MAX274 can be used to produce all-pole lowpass or bandpass active filters
with cutoff frequencies of up to 150kHz. A notch can be created by the addi-
tion of an external up-amp stage. The device contains four second-order sec-
tions. Each section is made up using four op-amps. A circuit diagram of one
second-order section is given in Figure 14.2.