Operator's Manual
DS7A - Constant Current Stimulator
Appendix 2 .. DS7AH - High Output Current
The following information details a factory modification of the basic design of peripheral nerve stimulator to
the higher maximum current at reduced pulse duration provided by the DS7AH. This option is offered to
overcome the difficulty of stimulating deep peripheral nerves with large area electrode pads.
In this situation the capacity associated with the electrodes limits the rate of increase of stimulating voltage
when the stimulating machine is of constant current type as the DS7A design. There are two possible
solutions, increase the stimulus time or increase the stimulus current. The latter alternative has been
favoured on the basis of reduced patient discomfort since the human pain receptors are relatively slow
acting in comparison to the motor nerves.
Since the stimulus energy and charge available are limited in the DS7A design any increase in stimulus
current will be achieved by reduction of stimulus time. The DS7AH option will provide a maximum stimulus
current of 1 amp at up to 400 volts compliance at pulse durations up to 200 µs maximum compared to the
standard DS7A specification of 100 mA at pulse durations up to 2 ms.
Performance Differences over standard DS7A
As described above the DS7AH has an increased stimulus current pulse output providing maximum currents
of 100 mA on the x1 range and 1000 mA on the x10 range compared to 10 mA and 100 mA for the standard
DS7A. The current output of the DS7AH can be checked on a resistance load of maximum value 3.5k ohms
for the x1 range and 350 ohms for the x10 range. By comparison the maximum values of load resistance on
the DS7A are 35 k ohms and 3.5 k ohms.
Safety Considerations
Since there are no changes to the isolation components and clearances nor is there any increase in
maximum output voltage, the safety features of the DS7A are maintained in the DS7AH. In operation
although the DS7AH can provide higher stimulus current, this is dependent on achieving a low electrode
impedance by the use of large area electrodes.
There is no increase in the maximum available stimulus charge and energy with the DS7AH option. These
two parameters dictate the stimulus operating conditions relative to patient safety and are unchanged from
the DS7A to the DS7AH. The DS7AH option is accordingly used for short stimulus pulse time investigations
with large area electrode pads to reduce patient discomfort.
Due to the possibility of generating high current densities, the use of needle electrodes is NOT
recommended.
Current densities for any electrode exceeding 2 mA/cm
2
may require the special attention of the operator.
Digitimer Ltd.
48 of 58
Copyright © 2014
File Ref: N:\Docs\Company\Manuals\DS7A\Issue-14\DS7A-iss14.odt
Last Revised: February 1, 2017
