STIMULATION OF EXCITABLE TISSUES - Design and Development of Medical Electronic Instrumentation - page 318

VDD

ANODE

Q1
BSS84

ACTIVE DISCHARGE

R1
100k

OUTPUT

R2
100 Ohms

C1
10uF

+ CATHODE

Q2
BSS138

STIMULUS

VDD

Q3
BSS84

C2
15 u F

HIGH AMPLITUDE STIMULUS

VDD

+

Q4
BSS138

R3
15k

D1
HSMS2824

3V

HIGH
AMPLITUDE
STIMULUS

ACTIVE DISCHARGE

STIMULUS

OUTPUT
VOLTAGE INTO
TISSUE

3V

10-50ms 10-50ms

6V

<2ms

<2ms

Figure 7.7 This capacitor-discharge output stage suitable for portable or implantable stimulators is capable of delivering 3- or 6-V stimuli
using a single lithium battery as the energy source. The circuit can sustain sucient stimulation currents for up to 2 ms when delivered into
implanted electrodes that present a load impedance of approximately 500 Ω .

the positive terminal of capacitor C2 to be connected with the battery's negative terminal.
When a 3-V stimulation is desired, HIGH AMPLITUDE STIMULUS is set low, which
connects C2's positive terminal to the battery's positive terminal (VDD). In the

fi rst case,

the potential di erence between the negative terminal of C2 and VDD is 6 V, while in the

second case the potential di

ff

erence is 3 V.
The STIMULUS line is set high to deliver the stimulus to the tissue, which closes Q2
and connects the negative terminal of C2 to C1 (which is discharged). As such, the lead-
ing edge voltage of the pulse appearing across electrode terminals V

ff

is equal to
the voltage selected (3 or 6 V). This voltage decays throughout the stimulus pulse as C2
discharges and C1 charges. To terminate current delivery to the tissue, the control logic
places all stimulus-related lines back to their rest condition.
Once the pulse has been delivered, coupling capacitor C1 remains charged. The delivery
of a new stimulation pulse will require this capacitor to be discharged. This is accomplished

and V