BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 79

3. National Semiconductor MF4 (or Texas Instruments TLC04) . This IC is an easy-to-
use, fourth-order Butterworth low-pass fi filter. The ratio of the clock frequency to the low-

pass cuto frequency is set internally to 50 : 1. A Schmitt trigger clock input stage allows

two clocking options, either self-clocking (via an external resistor and capacitor) for stand-
alone applications, or for tighter cutoff

ff

frequency control an external TTL or CMOS logic
compatible clock can be applied. The MF4-50 applies a 50 : 1 clock to cutoff

ff

frequency
ratio, while the MF4-100 applies a 100 : 1 ratio. The clock frequency of the circuit shown
in Figure 2.28 is given by

ff

1
1)(C1)

f clock 1.69(R

4. National Semiconductor and Maxim MF10. This IC consists of two independent and
general-purpose switched-capacitor active

filter building blocks. Each block, together with
an external clock and three to four resistors, can produce various second-order functions.
Each building block has three output pins. One of the outputs can be con

fi

gured to perform
either an all-pass, high-pass, or notch function; the remaining two output pins perform
low-pass and bandpass functions. The center frequency of the low-pass and bandpass
second-order functions can be either directly dependent on the clock frequency, or they can
depend on both clock frequency and external resistor ratios. The center frequency of the
notch and all-pass functions depends directly on the clock frequency, while the high-pass
center frequency depends on both the resistor ratio and the clock. Any of the classical

fi

filter
transfer functions can be implemented by selecting the right component values. The
speci

fi

c design methods and equations for the MF10 are available in the datasheet for these
products as well as in National Semiconductor Application Note 307.

fi

filters are Linear Technologies' LTC1068
and the LTC1562 as well as Maxim's MAX 265 and MAX 266. Maxim also o

Other useful universal switched-capacitor fi

ff ers two

microprocessor-programmable universal switched-capacitor fi filters: the MAX261 and

R1

IC1

8 5

IN FILTIN FILTOUT

OUT

1 2

INPUT
(8Vp-p max .)

CLKIN CLK R

OUTPUT

3

LSH

INGND OUTGND

7
4

6

V+
V-

AGND

C1 MF4

C2 C3

0.1uF 0.1uF

C 4 C 5

+ +

10uF 10uF

+5V -5V

Figure 2.28 National Semiconductor's MF4 IC is a fourth-order Butterworth low-pass filter with a clock-to-cutoff frequency ratio of 50 : 1.
The clock frequency is set via a capacitor and a resistor according to f clock ≈ 1/1.69(R1)(C1).