Biomedical Engineering Reference
In-Depth Information
Electrodes
Channel 1
Channel 2
4.92kHz
5kHz
Target
Tissue
Limb
Stimulation Current is “beat tone” at 80 Hz
Figure 7.21
Interferential current therapy (IFC) is based on the summation of two ac signals of
slightly di
erent frequency that are delivered using two pairs of electrodes. Each of the few-kilohertz
“carriers” on their own do not cause skin sensations or stimulation of the underlying tissues. However,
the tissue causes the signals to mix or interfere with each other, resulting in a low-frequency beat cur-
rent capable of stimulating the tissue.
ff
RMS. The most common beat-frequency therapy ranges are 1 to 10 Hz for edema, 1 to
150 Hz for rehabilitation of muscle, and 80 to 150 Hz for the control of pain. Figure 7.22
shows a simple circuit that can generate interferential audio-frequency currents. The power
ampli
fi
er sections are based around the ST Microelectronics TDA2005 10 W
10 W audio
ampli
fi
er IC. This IC is intended speci
fi
cally for use in bridge ampli
fi
er designs in car audio
systems. It is a class B dual audio power ampli
er with a high output current capability of
up to 3.5 A, ac and dc output short-circuit protection (one wire to ground only), and thermal
shutdown protection, and is capable of driving very inductive loads. Although it can be used
as a dual ampli
fi
fi
er by operating each half of the device separately, in this application it is
con
er. With this mode, the current and voltage swings
in and around the IC are twice that of a single ampli
fi
gured for operation as a bridge ampli
fi
fi
er, which results in a power output four
times greater than that of a single ampli
er with the same load connected to the output pins.
Two audio-output transformers are used in reverse to step up the ampli
fi
er's output volt-
age all the way up to the 150 V
pp
necessary to push up to 100 mA RMS through the body.
We used Hammond 1615 audio-output transformers in reverse; that is, the 8-
fi
Ω
speaker
outputs were connected to the outputs of the power ampli
fi
ers, and the 5-k
Ω
inputs were
connected to the electrodes. As with all bridge ampli
er designs powered from a single
supply, both output terminals of the TDA2005 are held at half the supply voltage, which
eliminates the need for the usually large-value dc blocking capacitor in series with the output
transformers. The power ampli
fi
ers are driven through audio isolation transformers. The
complete circuit should be powered from an IEC601-compliant power supply rated
for 12 V at 5 A or more, for example using a Condor model MD12-6.8-A medical-grade
linear supply.
fi
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