Dual Biphasic Version 3 Dual Biphasic Version 3 Created: 05 Mar 2011 Updated: 07 Jun 2011 This page documents a small 2-channel biphasic current stimulator. This is the older version, dating from 2009 through 2010. A newer version can be found at this URL - http://bionic.gsbme.unsw.edu.au/~philp/HTML_Docs/BiphasicV4/index.html The main parts of this board are shown in this diagram. The Eagle design files are here: schematic - biphasic-3.sch pcb layout - biphasic-3.brd A PDF of all the diagrams is here: layout.pdf A printout of this PDF might be useful if doing a rewire or odd patch job. Specifications The specs for V3 pretty well match those for V4, and V4 may be more uptodate. Current is controlled with an 8-bit DAC, meaning there are 256 current levels Each channel can supply up to 25mA at a DAC value of 255. In this range the smallest current will be about 0.1mA Each channel can supply as low at 10uA at a DAC value of 255 - suggesting a minimum current in this range of ~40nA. Practically 1uA is as low as you are likely to be able to measure and it will be noisy as well. A very common range is 0-2.5mA Maximum phase time 44ms Minimum phase time ~10us Maximum interphase delay can be ~2us Minimum interphase delay can be 44ms Note that sometimes the firmware is modified and there is no control over interphase delay. Sorry if this catches you out. Maximum interstim delay can be 44ms Time resolution is better than 1us The external trigger can be set to phase 1, phase 2 or interstim delay The essential features of a biphasic signal, as applied to this device, are shown below Software The stimulator connects by a USB cable to some host computer. This cable provides a virtual com port. The stimulator provides a simple text interface. To talk to the stimulator, something like hyperterminal or teraterm under Windows can be used. Under Linux something like minicom. Here at UNSW we use our own program called 'comms', which can also be used to reprogram the firmware if needed. Comms can be found at this URL http://bionic.gsbme.unsw.edu.au/~philp/HTML_Docs/Comms/ A manual for the firmware and a lot of discussion of usage can be found in this document 0-Manual.txt. This manual is the best user documentation for most purposes. The firmware for the stimulator is written in AVR-GCC, and can be found here Code.tgz. Assuming everything hooks up correctly and operates, the device should respond to simple commands. 'h' for help should give something like this:
| Help:
| v: version h: help i: info
| r: Ramp DAC R:I=100 L: load all l: display all
| t: trig loc D: set delay T: test trig
| m: stim n: stim N N: set N
| c: #1 current C: #2 current p: phases
| d: ISdelay e: IPdelay
| A: arm trig a: dis-arm trig
| X: short x: no short
| g: galvanostat G: set current j: set duration
| E: polymerisation
while 'v' for version should give something like this:
| >
| BIPHASIC-DUAL V0.16 Jul 18 2010 20:52:59
|
but do read the manual for all the details. The biphasic stim board is designed to connect to a simple isolated USB-Serial adapter board, which also supplies isolated power. There are two versions of this board - an older one and a newer one. Most V3 Dual Biphasic boards will be wired to an older style adapter. That is what is shown here. This is a simple wiring diagram for connecting the biphasic to the usb-adapter Calibration The calibration notes for the V4 board are probably more up to date - use those notes. The boards are functionally identical, changes are simple PCB shape changes, so many the notes for V4 apply to V3. Calibration involves two steps: 1) setting the range resistors 2) adjusting the calibration trimpot First calculate the range resistor. A nominal 1 volt is applied to the resistor to create a current. Maximum current is 25mA (30mA if you really want to test your luck with smoke ...). Minimum current is low as small micro-amps. Current is set in software in 255 steps. Decide what maximum current you want - lets say 2.5mA. Assume 1 volt as the driving voltage Resistor value = 1 volt divided by 2.5mA = 1/.0025 = 400 ohms. The nearest value for normal resistors is 390 ohms. get one and install in the socket Now install a 1K resistor as a load for the device, and using an oscilloscope to view the current. You should see a sawtooth waveform Start the device using the "r" for Ramp command, and view the current across the 1K resistor. Adjust the trimpot to give 2.5 volts peak-to peak Done A similar calculation for 25mA suggests a 39 ohm resistor A similar calculation for 250uA suggests a 3.9K resistor A similar calculation for 25uA suggests a 39K resistor
