Introduction - Digitimer DS4 Operator's Manual

DS4 Bi-phasic Stimulus Isolator DS4 Operator's Manual Issue 1.1

Introduction

The DS4 has been developed to meet the needs of laboratory scientists who require a stimulus isolator that
can output a bi-phasic isolated constant current stimulus in response to an external analogue command voltage
signal, provided by a computer DAC via software or a signal generator. Such a requirement is already met by
our NeuroLog System in the form of the NL512 Biphasic Buffer and NL800A Stimulus Isolators, but the DS4
provides our first standalone device to meet this need.
The DS4 accepts a variety of voltage input ranges (±1V, ±2.5V, ±5V and ±10V) and produces a constant
current stimulus output in 4 overlapping ranges (±10µA, ±100µA, ±1mA and ±10mA) from a compliance
voltage of approximately ±44V. In addition, the DS4 has a GATE input which allows multiple DS4's to be
connected to a single analogue voltage source, with each DS4 being digitally enabled, separately.
One of the problems with stimulators that make use of an external voltage source to define a stimulus
waveform is that small offsets or noisy baseline signals from the DAC's used to drive them can result in
unwanted battery drain or perhaps worse, low amplitude stimulation. The DS4 uses a special "inactivity
sensor" to monitor the input voltage and disable the DS4 output if this voltage falls below 0.15% of the full
scale value (in -ve or +ve directions) for a user selectable time period of 100ms, 200ms, 1s or 2s. Unlike
other devices which only produce an output when the input voltage exceeds a threshold value, this "inactivity
sensor" reduces battery usage and damaging "leak currents" during infrequent stimulation, while at the same
time maintaining low levels of zero crossing distortion for repetitive waveforms.
The DS4 uses an external power supply to power the input control circuitry and readily available/inexpensive
batteries to provide the opto-isolated stimulus voltage source. Batteries are only used when a stimulus is
commanded by an input voltage waveform.
Figure 1 The DS4 Bi-phasic Stiimulus Isolator.
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