Biomedical Engineering Reference
In-Depth Information
Oscilloscope
Biopotential Amplifier
+IN
V
out cm
OUT
Input 2
Input1
-IN
V
in cm
5V p-p
60Hz
Figure 1.24 To measure the CMR of a biopotential amplifier, the differential inputs should be shorted and a relatively large common-mode
signal (e.g., 60-Hz 5-V p-p sinusoidal) injected between the shorted differential inputs and the biopotential amplifier's common reference
input. The common-mode rejection is then calculated as CMR (dB) 20 log 10 ( G differential / G CM ).
Oscilloscope
Biopotential Amplifier
R1
10M
+IN
1mV p-p
OUT
Input 1
Input 2
-IN
Figure 1.25
To measure the input impedance of the biopotential ampli
fi
er, inject an in-band test signal of known amplitude (e.g., 1 mV) to
the biopotential ampli
fi
er's inputs through a 10-M
potentiometer. Adjust the potentiometer until the output voltage is half of the amplitude
obtained with the potentiometer set to 0
. The resistance of the potentiometer at the half-output point is equal to the input impedance of the
biopotential ampli
er at the test frequency. This measurement should be repeated for a number of in-band frequencies to compute the capac-
itive and resistive components of the input.
fi
5. Without changing the setting of the potentiometer, measure its resistance with the
ohmeter. This value is equal to the input impedance of the biopotential ampli
fi
er at
ed frequency.
6. Repeat the experiment for various frequencies from 0.1 Hz to 20 kHz. Use appro-
priate settings for the oscilloscope's time base. Compute the capacitive and resis-
tive components of the input impedance based on the data obtained.
the speci
fi
OP-AMP INSTRUMENTATION AMPLIFIERS
An alternative to the simple di
ff
erential ampli
fi
er is the multiple op-amp con
fi
guration pre-
sented in Figure 1.26. This di
ff
erential con
fi
guration, known as an instrumentation
 
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