Biomedical Engineering Reference
In-Depth Information
(a)
(b)
-V
+V
7
4
Rin
3
2
2
Iout
Iout
+
-
+
6
6
3
-
Vin
Vin
7
4
-V
+V
LOAD
LOAD
Rf
Vin
Vin Rf
______
Iout =
RSense
RSense
_________
-
Iout =
RSense
RSense Rin
(d)
(c)
Rin
Rf
Rin
Rf
-V
-V
Vin
4
4
2
2
RSense
RSense
Iout
Iout
-
+
-
+
6
Rin
6
Rin
3
3
7
7
+V
Vin
+V
LOAD
LOAD
Rf
Rf
Vin Rf
_________
Vin Rf
Iout =
-
___________
RSense Rin
Iout =
RSense Rin
Figure 7.11
Symmetrical biphasic current waveforms are often generated through voltage-to-current converters: (
a
) floating-load nonin-
verting configuration; (
b
) floating-load inverting configuration; (
c
) grounded-load noninverting improved Howland current pump; (
d
) invert-
ing configuration of the Howland pump.
As an example, Figure 7.12 presents the output stage of a backpack stimulator based on
the one developed by Livnat et al. [1981] for animal experiments involving chronic stim-
ulation of the central nervous system with implanted electrodes. Pulses are generated by
IC1 at a frequency set by R3 and R4 (e.g., 160 Hz). IC2A, IC3A, IC4A, IC4B, and IC5
reshape the output of timer IC1 into a symmetric bipolar square-pulse train. The voltage
waveform at the output of IC5 is converted into a constant-current waveform by the invert-
ing Howland pump built around IC6. The stimulation current can be selected by the set-
ting of R1. In Livnat's stimulator, power for the output stage was turned on and of
through
a set of CMOS switches driven by a low-frequency sequencer that selected the burst dura-
tion and frequency at which bursts were delivered to the animal. The stimulator output was
designed to deliver up to 1 mA into electrodes with impedances of up to 10 k
ff
.
Another way of producing biphasic current pulses is to use a unipolar current source that
is switched through an H-bridge so that the polarity presented to the tissue can be reversed.
Take, for example, the optically isolated bipolar current source of Figure 7.13. This very
neat circuit con
Ω
guration was proposed by Woodward [1998] as a laboratory stimulator for
neurophysiology research. In this circuit, the four optocoupler transistors of the H-bridge
(IC2) are used not only to switch the polarity of the current source, they are themselves the
fi
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