Brief pulses of electricity are used in various biomedical research applications as a stimulus to excite nerve or muscle fibres. In order to minimise artefacts introduced into electrophysiological data, it is desirable that the stimulator/stimulus isolator should be electrically isolated both from ground and from the trigger device. The voltage required to send current through tissues can vary greatly, making it important to have control over the stimulus driving force. Large impedance variations during an experiment can result in a lack or reproducibility or total loss of the stimulus. In these circumstances, a constant current stimulator like our DS3 would be recommended over our DS2A.
The DS3 provides a precise Constant Current stimulus (up to 32mA) controllable in Pulse Duration and Amplitude and as with the constant voltage DS2A this output comes from self-contained batteries. The DS3 also features a “clamp” or discharge circuit which discharges the output between stimuli, preventing a charge build up on the preparation. In other constant current devices this charge build up can lead to a loss of stimulus. The DS3 can be triggered by an external device such as our new DG2A Train/Delay Generator. The DS3 can be fitted into a 19″ rack mounting frame (D121-11) which can hold up to two DS2A’s, DS3’s DS4’s or DG2A’s.
- Low noise battery power supply with 90V compliance
- Internal (20µs to 2s) or external TTL “gated”control of pulse duration
- Four current ranges allow precise reproducible control of stimulus output between 2µA and 32mA
- Polarity reversal switch
- Battery test sockets
- Single-shot trigger button
- Output discharge (Clamp) circuit prevents capacitance build-up during stimulus trains, which is important to prevent stimulus loss
- Current is only drawn from batteries when a stimulus is being delivered
1. What is the maximum trigger rate of the DS3?
The unit can be triggered fast enough to allow >99% “on” time, but the “off” time must >20 microseconds – so, very fast!
2. Can the DS3 be used to stimulate human subjects?
The DS3 does not have medical device directive (MDD) certification and has not been designed for safe human use. We offer a range of MDD compliant stimulators for human/patient connection.
3. Can I control the DS3 using a computer?
Yes and no. The DS3 features an external “gate” input allowing externally derived trigger pulses to define stimulus duration. However, the output amplitude is always determined by the front panel setting of the stimulator. We recommend our NeuroLog system to those interested in using a computer generated waveform to define stimulus timing/polarity and strength.
4. I want to stimulate with a longer pulse duration than 2 seconds, in order to lesion some tissue, is it possible to do this?
Yes, there are two simple methods.
(i) The DS3 has the ability to be gated by an external pulse i.e. when external control of pulse duration is selected the output stimulus timing will be controlled by the trigger-in stimulus pulse. It should be noted that (with a 5V input pulse) there is only a very short delay of <2µs for the turn-on and <6µs for the turn-off. When a typical TTL pulse is used, which will only be about 3V, these are matched at 4µs each.
(ii) Alternatively, if a trigger pulse is not available or exact timing is not crucial (for instance when lesioning or injecting dye, there is a jumper which can be set to allow the unit to deliver a current for the duration the single shot button is held down. The internal “Single” jumper determines the output delivered when the “SINGLE” button is pressed (when “EXTERNAL” Duration is selected).
“MON” (factory setting) produces a very short pulse ~10µs when pressed.
“CONT” produces an active output for the duration of the press. This could be useful in dye marking etc.
5. I can’t decide between the constant current DS3 and constant voltage DS2A MkII- how should I choose?
The visible difference between the two units (apart from the colour!) is that the output control of the DS2A is defined in Volts while the output of the DS3 is defined in Amps. The actual stimulus passing through your preparation is measured in Amps in both cases and is dependent upon Ohms Law (V = IR). If your preparation has a variable impedance (R) and you are using a constant voltage (V) source such as the DS2A, then the actual current (I) passing through the tissue may vary considerably between each stimulus, which may not be a good idea if you want to apply reproducible stimuli. With the DS3, the constant current circuitry prevents variations in tissue impedance from altering the size of current applied (within the 90V compliance limit of the unit), leading to the stimulator equivalent of WYSIWYG – “What You Set Is What You Get”. Unfortunately there are no well defined rules governing the circumstances under which either stimulator should be used. If you are not sure and feel the need to evaluate either or both please contact us.
6. I have an old DS2 constant voltage stimulator and I recall that Digitimer offer a conversion to constant current. Is this still possible?
No, Digitimer no longer offers this service. Since the introduction of the new DS3 constant current stimulator, this unit is now offered to customers requiring a constant current version of the DS2A.
7. Is it possible to use two DS3 stimulators in parallel in order to stimulate through the same electrodes with different pulse types?
Yes, this is quite simple to setup and has been summarized in an application note.
DS3 Isolated Stimulators
Using two DS3 stimulators in parallel allows stimulation through the same electrodes with different pulse types.
Using a DG2A Train/Delay Generator & Two DS3 Isolated Stimulators to Produce a Biphasic Constant Current Stimulus
Publications which cite use of the Digitimer DS3 can be found on Google Scholar.
- Operator’s Manual
- One set of batteries (fitted)
- One Pair of output plugs (NL985P)
- NL985P – Output Plugs