Biomedical Engineering Reference
In-Depth Information
With these warnings in mind, let us look at some of the most popular switched-capacitor
fi
filter choices for processing biopotential signals.
filter with no dc error,
making it an excellent choice for processing low-frequency biopotential signals. As shown
in Figure 2.26, the
1. Maxim MAX280.
This IC is a
fi
fifth-order all-pole low-pass
fi
fi
filter IC uses an external resistor and capacitor to isolate the fourth-
order
filter implemented within the IC from the dc signal path. The external resistor and
capacitor are used as part of the
fi
fi
filter's feedback loop and also form one pole for the over-
all
fi
filter circuit. The values of these components should be chosen such that
1.
1
6
)
2
(C1)
f
cutoff
2
π
(R
where R1 is usually chosen to be around 20 k
.
The chip's internal four-pole switched-capacitor
Ω
fi
filter is driven by an internal clock that
determines the
fi
filter's cutoff
ff
frequency. For a maximally
fl
flat amplitude response, the clock
should be 100 times the desired cutoff
frequency. The internal oscillator runs at a nominal
frequency of 140 kHz that can be modi
ff
ed by connecting an external capacitor (C2)
between pin 5 and ground. The clock frequency is given by
fi
33 pF
p
F
C2
f
clock
140 kHz
R1
IN
OUT
INPUT
(6Vp-
p m
ax.)
OUTPUT
INGND
OUTGND
C1
IC1
1
8
FB
BOUT
5
CLK
4
+5V
DIVRTO
7
OUT
C2
6
3
V+
V-
2
AGND
MAX280
C
3
C
4
R2
0.1uF
0.1uF
C5
C6
+
+
10uF
10uF
+5V
-5V
Figure 2.26
The Maxim MAX280 IC is a fifth-order all-pole low-pass filter with no dc error, mak-
ing it an excellent choice for processing low-frequency biopotential signals. It uses an external resis-
tor and capacitor to isolate the fourth-order filter implemented within the IC from the dc signal path.
The internal switched capacitor filter is driven by an internal clock that determines the filter's cutoff
frequency. If pin 4 is tied to V
, the filter has a cutoff frequency ratio of 100 : 1; when tied to ground,
200 : 1; and when tied to V
, of 400 : 1.
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