BANDPASS SELECTION FOR BIOPOTENTIAL AMPLIFIERS - Design and Development of Medical Electronic Instrumentation - page 78

A series resistor (R2) can be added to trim the oscillation frequency. In this case, the new
clock frequency is given by

f R2 0
clock
1

f clock

4(R2)(C2) f R2 0
clock

where f R2 0

clock is the oscillator frequency when R2 is not present (obtained through the prior
equation). Pin 5 of the MAX280 can also be driven from an external clock. In addition, the
state of pin 4 must be taken into account to determine the e ff ective clock frequency. If pin

4 is tied to V , the fi filter has a cutoff ff frequency ratio of 100 : 1; when tied to ground,

200 : 1; and when tied to V-, 400 : 1.
2. Maxim MAX29x. This is a family of easy-to-use eighth-order low-pass filters that can

be set up with corner frequencies from 0.1 to 25 kHz (MAX291/MAX292) or 0.1 Hz to
50 kHz (MAX295/MAX296). The MAX291 and MAX295

fi

filters provide a Butterworth
response, while the MAX292 and MAX296 yield a Bessel response. The clock frequency-
to-cuto

fi

frequency ratio for the MAX291 and MAX292 is 100 : 1; a ratio of 50 : 1 is used
in the MAX295 and MAX296. As shown in Figure 2.27, an external capacitor is used to
set the clock frequency of an internal oscillator according to

ff

1
1

0
(p

5

f clock (kHz) 3C

F)

Of course, the internal clock can also be driven externally. The MAX29 x family has an
internal uncommitted op-amp that has its noninverting input tied to ground. This op-amp
can be used to build a continuous-time low-pass fi filter for pre fi filter antialiasing or clock

attenuation at the switched-capacitor's fi filter output.

IC1

8 5

IN OUT

IN OUT

4 3

INPUT
(8Vp-p max.)

OPIN- OPOUT

OUTPUT

1

CLK

INGND OUTGND 7

V+

2 6

V- AGND

MAX291

C1

C2

C3

0.1uF

0.1uF

C 4

C5

+ +

10uF 10uF

+5V -5V

Figure 2.27 Maxim MAX29 x ICs are eighth-order low-pass filters that can be set up with corner frequencies from 0.1 to either 25 or
50 kHz with either a Butterworth or a Bessel response. The clock frequency-to-cutoff frequency ratio for the MAX291 and MAX292 is
100 : 1; a ratio of 50 : 1 is used in the MAX295 and MAX296. An external capacitor is used to set the clock frequency of an internal oscil-
lator according to f clock (kHz) 10 5 /3C1(pF).