Biomedical Engineering Reference
In-Depth Information
current-regulating elements of the current source. In a nutshell, the circuit works as follows:
IC1B and Q2 will drive current through either the LEDs of IC2A and IC2D or those of
IC2B and IC2C, depending on the polarity of the input signal. This will cause current from
the battery to be driven into the load connected to the electrodes by the H-bridge transistors
of multichannel optocoupler IC2. The current delivered to the bridge is sensed by optocou-
pler IC3B. The current-control loop is closed via IC1A, which receives setpoint and feed-
back currents through IC3A and IC3B, respectively. Q1 and LED D1 turn on when IC1A
drives excessive current through the bridge, providing an indication that the voltage com-
pliance of the source has been reached.
Op-Amp Bridge Stimulators
Another stimulator output stage con
guration that is worth exploring is the op-amp bridge.
The bridge connection of two op-amps provides output voltage swings twice that of one op-
amp. This makes it possible to reach higher stimulation or compliance voltages in applica-
tions with low supply voltages or applications that operate ampli
fi
fi
ers near their maximum
voltage ratings in which a single ampli
fi
er could not provide su
cient drive.
2
In addition,
the bridge con
fi
guration is the only way to obtain bipolar dc-coupled drive in single-supply
applications.
Figure 7.14
a
shows a generic voltage-output bridge connection of two op-amps. Ampli-
fi
er
accepts the input signal and provides the gain necessary to develop full output swing from
the input signal. The total gain across the load is thus twice the gain of the master ampli
fier A1 is commonly referred to as the
master
and A2 as the
slave
. The master ampli
fi
fi
er.
The master ampli
fi
er can be set up as any op-amp circuit: inverting or noninverting, diff-
ff-
erential ampli
er, or as a current source such as a Howland current pump. In the latter, only
the master ampli
fi
gured as a current source. As shown in Figure 7.14
b
, the slave
remains as an inverting voltage ampli
fi
er is con
fi
er.
This is, however, not the only current-source circuit that can be built in bridge
con
fi
guration. In fact, U.S. patent 4,856,525 to van den Honert [1989] shows how to use
two current sources per channel of a multichannel stimulator to minimize the cross-
coupling of stimulating currents between channels. As shown in Figure 7.15
a
, each
channel of a multichannel stimulator may have a current source that drives current
between one electrode and a common ground electrode. However, a single current
source for each channel requires only that the current in the loop containing the elec-
trode and electrical ground be equal to the value of the current source and not neces-
sarily that the current passing to ground go through any particular one of electrodes
coupled to electrical ground. If the electrical power supplies of the channels of the elec-
trical stimulator are not isolated electrically, a cross current from one electrode pair to
another electrode pair could occur while maintaining the loop current requirements of
each channel's single current source. Van den Honert invented a multichannel current
source con
fi
guration with improved channel isolation that does not require electrically
isolated power supplies.
As shown in Figure 7.15
b
, each channel of the electrical stimulator has a pair of cur-
rent sources that work in concert. One current source is coupled to each electrode of each
channel's electrode pair. The current sources operate harmoniously. When one current
source is sourcing (or sinking) a certain current, its complementary current source is sink-
ing (or sourcing) an almost identical current. As such, the current passing between the
fi
2
Op-amp bridge current sources are not only limited to low-voltage applications in direct stimulation of tissues.
There are a number of op-amps that will operate with very high supply voltages, making it possible to use the same
techniques in the design of high-voltage external stimulators. For example, the PA89 by Apex Microtechnology is
rated for a total supply voltage of 1200 V.
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