DIGITAL OUTPUT ADJUST SUBMENUS:DIGITAL OUTPUTS (DigOut):Use the DigOut Adjust item on the main menu page to enter the adjustment submenu system. ESC will return to the main page.All 8 digital output bits (bit pages A0-A7, for example) are output as a single byte for each sample. Each bit may be independently programmed to output either a single pulse of arbitrary polarity and duration, or a train of multiple pulses. Use CTRL-Pg keys to move between bit pages. If the current Pg Mode is PAIR or EACH and Dig Pg is Ind, use CTRL-ALT-Pg keys to move between sets of pages.
On / Off:This determines whether the bit page (0-7) will be active in the output when DigOut is active. Setting a bit page to Off forces that bit to remain at zero... it does not remain unchanged if it was previously set high.
PULSE:The Lag, High, Low, and Polarity parameters in this section define a single pulse interval, which may be repeated as a pulse train according to parameters in the Train section.
+5 .---------.
| |
0 ------' `------------ Polarity Normal
Lag High Low
+5 ------. .------------ Polarity Invert
| |
0 `---------'
Lag:Specifies the number of samples to wait from the start of the pulse interval to the actual pulse onset.
High:The pulse duration, or number of samples that the output bit is to remain high (low with Polarity Invert) within each pulse interval.
Low:The number of samples that the output is to remain low (high with Polarity Invert) after the pulse offset until the end of the pulse interval.
Polarity:If Polarity is set to Invert, each of the above phases of the pulse is inverted, as shown in the diagram.
TRAIN:The pulse interval defined in the PULSE section above may be repeated to form a pulse train:
.--. .--. .--. .--. .--.
| | | | | | | | | |
--------------' `----' `----' `----' `----' `--
| | Pulse | | | | |
1 2 3 4 5
|-- Start --|----------- Cycles --------------------|
Start:Delay, in samples, from the start of the stimulus to the start of the pulse train. If the pulse Polarity is set to Invert, the output will be high during the Start interval.
Cycles:Number of pulse intervals in the total stimulus.Note that in Sequential mode there will be an additional interval between trains, due to processing and display time. This behaves like an extension of the Start duration. Since the display time may be variable depending upon the activity of the trace, you will probably want to use the Trigger Cycle control to provide a fixed interval. You will need to insure that the Cycle setting is long enough to include the longest display and processing time. This is best done by increasing Cycle until you get SPEC on the Speed display when the trace is as large and as busy as you anticipate, and then add some "safety margin" as desired. If you have several different pulse trains on different output bits, you can use Start to control their relative timing and Trigger Cycle for overall timing. In RTime mode, there is no delay for processing and display, since that takes place in the background. You can thus use Start to control the time between pulse trains. (There is no Trigger Cycle option in RTime mode unless you set the Trigger Source to Intern.)
BIPHASIC STIMULI:Electrophysiological investigations often use biphasic stimulus pulses to avoid delivering a net charge to the subject:
+ .--. .--. .--.
| | | | | |
---' | .------' | .------' | .-----
| | | | | |
- `--' `--' `--'
Typically, the stimulus is given as a current, instead of a
voltage, so you would need a special voltage-to-current
source. Even more importantly, the controlling computer must
be electrically isolated from the subject. Special
transformers or optical isolators may be used to provide the
electrical isolation, and a separate high voltage battery
supply is often required to provide the current.
BIPHASIC STIMULI FROM DIGITAL OUTPUTS:If the isolator / current source accepts a proportional analog input, this biphasic pattern can be generated by summing two TTL digital outputs together and using a capacitor to remove the resulting DC level:
+5 .--. .--. .--.
| | | | | | Output
0 ---' `---------' `---------' `-------- A0
+5 ------. .---------. .---------. .----- A1
| | | | | |
0 `--' `--' `--'
+10 .--. .--. .--.
| | | | | |
+5 ---' | .------' | .------' | .----- Sum
| | | | | |
0 `--' `--' `--'
+V .--. .--. .--.
| | | | | | AC
---' | .------' | .------' | .----- Coupled
| | | | | |
-V `--' `--' `--'
To create the A0 output, set the appropriate High and Low sample counts to get the desired pulse width and spacing. The A1 output will use these same values, with a Lag value equal to the High value and Polarity set to Invert. A simple circuit to sum these together might be something like:
A0 ------R0-------.-------Co-------- AC Coupled
| Output
A1 ------R1-------|
|
Rg
|
|
Gnd
If you want both phases to be equal but opposite (the usual case), make resistor R0 = R1. Typical values might be 10K. Resistor Rg attenuates the sum. If it is the same size as the other two, the output will be 1/3 of the TTL levels from the digital outputs. Note that TTL outputs are usually specified as a "nominal" 5 Volts, but may in reality be somewhat less. They may also contain some low-level high-frequency "trash", which wouldn't cause problems in digital circuits but might here if the proportional isolator / source you are driving can't deal with it. Capacitor Co should be made as large as needed to produce pulses with tops of acceptable flatness. If it's too small, the flat tops and bottoms of the pulses will tilt like this:
|\ |\ |\
| \ | \ | \
---' | .------' | .------' | .-----
| / | / | /
|/ |/ |/
However, proportional isolator / current sources are rare, due to the difficulty of isolating analog signals. Isolating digital signals is much easier, using standard opto-isolators. A source using this approach would typically use one digital signal to turn the pulse on, and one to invert its polarity. Alternately, it might have a separate input for each polarity, though this requires special protection against both being active at once. You can use two Digital Output bits to drive either type of device to produce a biphasic stimulus:
Desired Stimulus from Current Source:
+ .--. .--. .--.
| | | | | |
---' | .------' | .------' | .----- Output
| | | | | |
- `--' `--' `--'
Driving Pulse / Invert Source Inputs:
.-----. .-----. .-----.
| | | | | |
---' `------' `------' `----- Pulse On
.--. .--. .--.
| | | | | |
------' `---------' `---------' `----- Invert
Driving Separate Polarity Source Inputs:
.--. .--. .--.
| | | | | |
---' `---------' `---------' `-------- Positive
.--. .--. .--.
| | | | | |
------' `---------' `---------' `----- Negative
BIPHASIC STIMULI FROM ANALOG (DAC) OUTPUTS:If your board has analog output capabilities and you have a "proportional" type isolator / current source, you can easily create biphasic stimuli from a DAC output, with independent control over the relative amplitudes of each phase.To generate classic biphasic pulses, use a separate tone component for each phase, and set the frequency of each to 0 Hz. The Rise and Fall are each set to 0 samples. The positive pulse is created by setting the tone Phase control to 90 degrees, and the negative by setting Phase to 270 degrees. Use Start and Sustain to control the position and width of each pulse. Alternately, you may wish to experiment with non-zero Rise and Fall durations to create shaped biphasic pulses with reduced transients. One problem with using DAC outputs to create pulses is that it offers poor control of pulse trains. Since you only have four tone components to work with and two are needed for each biphasic pulse, the "train" is limited to two pulses in Sequential mode or a continuous train in RTime mode. If you don't have digital output capability, you may be able to use normal tone bursts with Rise and Fall set to zero, instead of a rectangular biphasic pulse train. If needed, an external amplitude limiter may be used to clip the tops and bottoms of the sinusoid to make it look more rectangular, with a subsequent amplification stage to set the desired level. A simple limiter may be made from a pair of back-to-back diodes:
DAC ------- R ----.-------------.-------- To
Output | | Amplifier
`-->|--.-->|--' and level
| control
Gnd ---------------------^---------------
Use a value for R of about 10K. The output of this will
be about +/- 0.6 Volts, which can then be amplified to
the required level.
|