BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 61

we recommend that you try the two and three op-amp topologies that will allow you more
ability to “tweak” the end result. In addition, a good way of designing well-behaved fi filters is

to base them on one of the various active fi filter building blocks of ff ered by analog IC vendors.

For example, Burr-Brown (now part of Texas Instruments) of ff ers the UAF42, a universal

active fi filter that can be con fi gured for a wide range of low-pass, high-pass, and bandpass

fi filters. It implements fi filter functions through a state-variable topology with an inverting

ampli er and two integrators. The integrators include on-chip 1000-pF capacitors trimmed

to 0.5%. This solves the di

fi

cult problems of obtaining tight-tolerance low-loss capacitors.
The UAF42 is available in 14-pin DIP and SOL-16 surface-mounted packages.
Burr-Brown's free DOS-compatible FilterPro program lets you design Butterworth,
Chebyshev, and Bessel filters, enter the desired performance, and then obtain the passive

values required. You can force the program to use the nearest 1% resistors, set some resistor
values, enter realistic or measured capacitor values, and then plot the actual gain/phase ver-
sus frequency performance. Similarly, Microchip's Windows-based FilterLab lets you design
Sallen-Key or multiple-feedback low-pass

fi

filters with either Butterworth, Chebyshev, or
Bessel responses using their MCP60x family of single-supply op-amps.
Maxim also of

fi

filter ICs , the MAX274 and MAX275. These
ICs have independent cascadable second-order sections that can each implement all-pole
bandpass or low-pass

ff ers a line of state-variable fi

filter responses, such as Butterworth, Bessel, and Chebyshev, and is
programmed by four external resistors. The MAX274 has four second-order sections, per-
mitting eighth-order

fi

filters to be realized with center frequencies up to 150 kHz. The
MAX275 has two second-order sections, permitting fourth-order

fi

fi filters to be realized with

center frequencies up to 300 kHz. Both fi filters operate from a single 5-V supply or from

dual filter design program is available from Maxim to

support the development of applications based on the MAX274 state-variable

5-V supplies. A free DOS-based fi

fi filter IC.

filter realizations have the distinct advantage that they provide simulta-
neous low-pass, bandpass, and high-pass outputs from the same

State-variable fi

fi filter circuit. In addition,

the fi filter parameters are independent of each other. For example, the cutoff ff frequency of

the active-feedback state-variable fi filter circuit of Figure 2.14 is given by

1
)(C1)

f C 2

π (R3

where R3 R4 and C1 C2. As shown in the ac-sweep PSpice simulation analysis of

Figure 2.15, this fi filter yields simultaneous low-pass and high-pass responses with a -3-dB

cuto frequency f C and a bandpass response centered at the same frequency. In this example,

the resistor values selected for R4 and R6 give the

ff

fi filter a cutoff ff frequency of approximately

50 Hz. The Butterworth response on a state-variable fi filter gives it a value Q -3 dB and an

in-band gain of the bandpass fi filter equal to Q ( 0.707), making all curves cross at the

same point.
Since the cutoff filter depends on the value of two resis-

tors (R3 and R4 in the prior example), it is relatively easy to design a tunable

ff frequency of a state-variable fi

fi filter by sub-

stituting these resistors by two tracking variable resistors. The fi filter can also be made to

have a cuto frequency that is proportional to a control voltage by using circuits that pres-

ent a variable resistance as a function of an input voltage.
Although FETs and variable transconductance ampli

ff

ers can be used as voltage-
dependent resistors, better results are easier to achieve using analog multipliers in series
with a resistor as the control elements. The circuit of Figure 2.16 shows how R3 and R4
of the circuit of Figure 2.14 have been replaced by two Analog Devices AD633 precision
analog multipliers. The transfer function of the AD633 is given by

fi

( x 1 x

1

)(
V

y 2 ) y 1

2

V out z

0