IEEE Solid-State Circuits Magazine - Fall 2016 - 54
Circuit Design Considerations
for Stimulation
is more like a resistor with chopping), and any dc offset at the
electrodes shows up at the amplifier output, with the gain defined
by the capacitor ratio, which may
saturate the amplifier.
Generally, two electrodes are needed
for stimulation, with one as the reference and the other as the stimulator.
The goal of stimulation is to deliver a
When an electrode, which is usually metal, is
placed inside a physiological medium (e.g.,
tissue or body fluid, which contain electrolytes),
electrochemical activity will occur, and an
interface will form between the two phases.
charge into the tissue that influences the potential outside the neuron
membrane so as to initiate spikes/action potential. A typical stimulation
waveform is shown in Figure 8, with
the x-axis representing time and the
y-axis representing current. The positive and negative signs indicate the
direction of the charge flow, whether
2) Low input impedance. The input impedance of the amplifier decreases
from 1/j~C to 1/fCC, where ~ is the
signal frequency of interest and fC
is the chopping frequency.
Improvements to address these issues
include a dc servo in both the analog
and digital domains for 1) and input
switch compensation for 2) [2].
Tissue/Body Fluid
(Electrolyte)
Na+ K+
Stimulation
Electrode 1 (Metal)
NT
Ca++
Stimulation
Electrode 2
NT
NT
NT
NT
NT
H+
1/f
Into Tissue
Charge Flow
Q (a )
t (a )
0
Q (c )
t (c )
Out of Tissue
Stimulation Phase
Figure 8: A typical stimulation waveform.
54
fa l l 2 0 16
IEEE SOLID-STATE CIRCUITS MAGAZINE
Recharge Phase
in or out of the tissue. The waveform
most often consists of a stimulation
phase followed by a recovery phase.
It could be made an active recharge
by using the same or a scaled current
in the reversed direction or via passive recovery by shorting the electrodes, as indicated by the dashed
line, which follows the resistorcapacitor discharge curve.
In the case of sensing, the electrode-tissue interface can be considered as a pure capacitor, in which no
charge goes across the double-layer
structure. However, in the case of
stimulation (wherein the typical stimulation voltage could vary from 1 to
10 V), there could be actual charge
transfer with electrochemical reaction. The leakage through the double-layer capacitor is usually called
Faradeic impedance. The model of
the electrode interface for stimulation is shown in Figure 9. Note that
the value of the Faradeic impedance
may vary with stimulation amplitude
and other factors.
Table of Contents for the Digital Edition of IEEE Solid-State Circuits Magazine - Fall 2016
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