Biomedical Engineering Reference
In-Depth Information
-the-shelf components, however, the construction of such a circuit would intro-
duce parasitic capacitances in the same order of magnitude as the sample-and-hold capaci-
tances, resulting in errors that the internal charge-balancing circuitry within the integrated
circuit cannot cancel. For this reason, this design includes two noninverting ampli
Using of
ff
ers IC1B
and IC1D which present the switched-capacitor block with a signal level that is compatible
with a larger-valued sampling capacitor, e
fi
ectively eliminating the problems related to par-
asitic capacitances. The output of each of these ampli
ff
fi
ers is given by
R
R
2
1
(R
R
2
)
V
2
V
A
1
V
1
1
and
R
R
3
1
(R
R
3
)
V
1
V
B
1
V
2
1
Thus, if R2
R3, the theoretical di
ff
erential voltage presented to the sampling capacitor is
R
R
2
1
V
A
V
B
(
V
2
V
1
)
1
2
CMOS op-amps IC1A and IC1C are con
ers and serve as ultra-
high impedance to low-impedance transformers so that the biopotential signal may be car-
ried with negligible loss and contamination to the instrumentation stage. In critical
applications, these could be mounted in close proximity to the electrodes used to detect the
biopotentials. In addition, if the biopotential ampli
fi
gured as unity-gain bu
ff
fi
er can be mounted close enough to the
subject, IC1A and IC1C may be omitted.
In order not to reduce the high common-mode rejection that may be achieved through
use of a switched-capacitor instrumentation block, the use of high-precision components
is mandatory, so that the gain of the chain formed by IC1A and IC1B will closely match
that of IC1C and IC1D. In addition, an adequate layout of the printed circuit board or
breadboard, using guard rings and shielding the sampling capacitor from external parasitic
capacitances, is necessary to preserve the common-mode rejection from being degraded.
This also helps maintain the inherent ultrahigh impedance of the CMOS input bu
ff
ers.
An additional high-performance CMOS operational ampli
fi
er IC2, con
fi
gured as a nonin-
verting follower, ampli
es the single-ended output of the instrumentation stage. The ultrahigh
input impedance of this ampli
fi
fi
er ensures that the performance of the switched-capacitor stage
is not a
ff
ected by the output load. The dc gain of the noninverting follower is given by
R
R
5
4
G
IC2
1
which is multiplied by its own transfer function, the dc gain of the input ampli
fi
ers and
bu
ers, their transfer function, and the transfer function of the switched-capacitor instru-
mentation block to yield the frequency-dependent gain of the complete system. However, the
fl
ff
er is by far wider than that of
biopotential signals, and by selecting a very high sampling frequency and the correct capac-
itance ratio, a virtually
flat-response bandwidth of any modern operational ampli
fi
flat frequency response within the bandwidth of interest is achievable.
Figure 1.32 shows an array of these switched-capacitor instrumentation ampli
fl
fi
ers used to
detect myoelectric signals from muscle
fibers stimulated by an electrical current. Artifacts
induced by the high-voltage surface neuromuscular stimulation can be rejected by extending
the even-phase switching interval during stimulation. To do so, an external clock drives the
switched-capacitor timing logic. Just prior to stimulation, the clock is isolated from the
ampli
fi
ers by a logic AND gate, and all switched-capacitor blocks are set unconditionally to
even-phase mode. Shortly after stimulation ceases, switching at clock speed is restored.
fi
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