Biomedical Engineering Reference
In-Depth Information
+5V
R2
56
C7
0.01uF
R1
IC2
8
7
IC1
3
+
6
OUT
8
5
2
2
IN
FILTIN FILTOUT
-
1
HIGH-PASS OUTPUT
CLKIN
CLK R
4
1
5
AD620
INPUT
3
LSH
OUTGND
INGND
7
4
V+
V-
6
AGND
C6
0.01uF
MF4
C1
R3
56
C 2
C 3
-5V
0.1uF
0.1uF
C 5
C 4
+
+
LOW-PASS
FILTER
10uF
10uF
+5V
-5V
Figure 2.29 An instrumentation amplifier can be used to convert a low-pass filter into its high-pass counterpart. In this example, the input
signal is fed to the AD620 instrumentation amplifier both directly and through the low-pass filter. This results in a high-pass function, since
the output of the amplifier is the original signal minus the attenuated low-frequency components.
MAX262. One thing that you won't
filter.
chips (such as the MAX280 or the MF4). However, there is an alternative to using a universal
fi
fi
find with ease is a high-pass version of the low-pass
fi
filter IC. Figure 2.29 shows how an instrumentation ampli
fi
er can be used to convert a low-
pass
filter into its high-pass counterpart. The input signal is fed to the AD620 instrumenta-
tion ampli
fi
fi
er both directly (to the noninverting input) and through the low-pass
fi
filter (into
the inverting input). At low frequencies, both inputs of the instrumentation ampli
fi
er see the
same signal since the low-pass
fi
filter passes the signal una
ff
ected. The output of the ampli
fi
er
should thus be zero. At high frequencies, however, signals are attenuated by the
fi
filter and the
ampli
fi
er outputs the di
ff
erence. This results in a high-pass function, since the output of the
ampli
er is the original signal minus the attenuated low-frequency components. If you
decide to use this technique with a switched-capacitor
fi
fi
filter, take precautions to avoid clock
noise from a
ff
ecting the performance of the
fi
filter. If possible,
fi
filter the ampli
fi
er's power
input lines (the
fi
figure shows these lines
fi
filtered through R2/C7 and R3/C6), and use a
continuous-time low-pass
fi
filter at the output of the switched-capacitor low-pass
fi
filter.
SLEW-RATE LIMITING
Sometimes, artifacts that obscure or distort a biopotential signal have signi
cant spectral
components within the bandpass of interest. However, there are times when the morphology
of the artifact is su
fi
ciently di
ff
erent from the signal of interest as to allow for its automatic
 
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