The Stimulator System

The stimulator system consists of an RZ real-time processor with a DSP that connects to a Subject Interface via fiber optic cable. The Subject Interface supports three different kinds of boards, PZA for analog recording, PZD for digital recording, and IZV for electrical stimulation. Each Subject Interface can have up to 8 boards. The IZV can deliver arbitrary electrical stimulation waveforms of up to 20 kHz bandwidth (depending on the RZ processor sampling rate) through a passive or switching headstage.

IZV Circuit Detail

Each IZV bank has four unique hardware "voices" that can be sent to any of its 16 channels, so four independent channels can stimulate on any given sample of the real-time clock on each bank. You can assign multiple voices to the same channel to increase maximum current for one or two channels at a time.

IZV Bank Voices

Each voice starts with a low voltage waveforms from the D/A converters. This is converted to constant voltage or constant current waveforms. An optional DC Block (RC circuit) removes any constant DC current from the output. You can also switch passive discharge shunts to removes excess voltage in between stim pulses.

IZV Voice Detail

The signal path switches can be flipped on each sample of the real-time processor controlling the IZV. The Electrical Stim Driver gizmo in Synapse is typically used to send the control signals to the IZV gizmo. The Electrical Stim Driver controls what happens between pulses in a burst (Inter-Pulse Action) and between bursts (Inter-Stim Action). The four options are described below.

IZV Inter-Stim and Inter-Pulse time periods

Option	Description
Channel Hold	Electrode remains connected to voice output (zero current / voltage)
Channel Release	Electrode is disconnected from voice output (open circuit)
Discharge	Electrode is switched to 10K discharge impedance
Ground	Electrode is switched to low impedance ground (voltage mode 0 V)

Channel Release minimizes any potential charge collection in between bursts, but switching it in between short pulses is not recommended. Therefore the default configuration is Channel Hold for the Inter-Pulse Action and Channel Release for the Inter-Stim Action.

One common use case is DBS where two electrodes use a 'local ground'; essentially the second channel acts as the return path. You might see charge build up on the electrodes and not get the desired waveform. The output voltage might look something like this:

Inter-Stim Action set to Channel Hold

Use Discharge in between pulses and Channel Release (open circuit) in between bursts to get rid of the excess charge on the electrode. The output voltage for that same stimulation pattern then looks like this:

Inter-Stim Action set to Discharge

IZV Configuration

The banks of the IZV can be separated into isolated 'Sub Stimulators'. Each sub stimulator must be configured independently and has its own multi-channel input signal. For example, the first sub stimulator uses the gizmo input called "VoiceIn-A", the second sub stimulator uses "VoiceIn-B" gizmo input, and so on.

IZV Stimulator Configure Tab Options

The input signal format is a multi-channel signal where the odd channels contain the actual stim waveforms and the even channels contain the target stim channel. For an input that has 4 unique stim signals, it will be an 8-channel signal where channels 1, 3, 5, 7 are the stim signals and channels 2, 4, 6, 8 are the corresponding target stim channels. Information about the state of the channels between stimulation pulses (Channel Hold, Channel Release, Discharge, or Ground) is also encoded in the target stim channel signals. It is generally expected that this input signal comes from an Electrical Stim Driver gizmo so that it has the proper format. If you are trying to send arbitrary waveforms to the IZV, please contact TDT support for more information.

Sub Stimulator Options

The sub stimulator options in the Configure tab are described below. For more information on how to apply these options to your application, see Stimulation Types.

Option	Description
Output Type	Select whether this sub amp is current-controlled or voltage-controlled. AC Mode adds a DC block to the output signal (an in-line 20 MOhm resistor and 22 uF capacitor in parallel).
Output Mode	The settings in this group determine how the input VoiceIn signals are mapped to a particular hardware voice on a board of the sub stimulator. It handles this channel mapping for you, so the stim channels you give it from the Electrical Stim Driver gizmo make sense for how the headstage is physically connected. Parallel assumes boards in this sub stimulator are wired in parallel, so you can get more than 4 unique voices per 16 channels or up to 4 higher current voices per 16 channels. Typically use an SI-MUX4 adapter for shorting 4 banks together, or the SI-MUX2 adapter for a 2 bank configuration. The VoiceIn input to the sub stimulator must contain enough voices/channels. Serial assumes channel 16 on all sub stimulator boards is physically shorted together. Allows one channel of bipolar stimulation with a higher compliance range. Repeated repeats the same input voices on all boards of the sub stimulator.
Number of Boards	Number of boards in sub stimulator.
Current Doubling	None - all four hardware voices on each board are unique. By two - connect pairs of hardware voices together on each board to achieve 2x max current compliance. By four - connect all four hardware voices together on each board to achieve 4x max current compliance.
Voices per Board	How many unique stim signals for each stimulator board.
Range per Board	How many channels are available to stim on each board.

The data table at the bottom of the Configure tab shows the mapping between the gizmo input voices and the physical hardware voices. The first column is the voice of the "VoiceIn" for this sub stimulator. The middle column is the board number within the sub stimulator that it uses and the hardware voice on that board it uses (a, b, c, d). The last column is the range of channel numbers that the VoiceIn stim channels should address.

For example, an input from an Electrical Stim Driver gizmo with two voices on it targeting banks 1 and 2 on a sub stimulator would read as:

Chan-1 | B1a, B2a | Chans: 1-32
Chan-2 | B1b, B2b | Chans: 1-32

Safety

When Safety Mode is enabled in Synapse, the Subject Interface IZV must be armed by the user before any current can flow (see Runtime Interface below). The hardware ensures that maximum output current is not exceeded. The Subject Interface is battery powered and thus isolated from any mains power, when not charging. The safety configuration options for the IZV (Compliance and Safety) are below.

IZV Stimulator Compliance and Safety Tab Options

Option	Description
Safety Mode	If enabled, IZV checks total output current and will disarm if system is outside specs. The device requires arming before it will operate. To arm, run your experiment and make sure no stim presentation is sent to the device. The physical IZV card will light all LEDs red if it is not ready to be armed. It will light all LEDs green when it is ready to be armed. When armed, it will light the LEDs on the stimulation channels during stimulation.
Remote Arming	Enable this to force the user to arm the device via the Subject Interface touchscreen interface. Otherwise, the user can arm the device via a button in Synapse at run-time.
Max Output Voltage	Set the maximum allowed output voltage for this sub stimulator. Max is 15-18 V, depending on the current battery level. If Safety Mode is Enabled and any channel goes beyond 1 V of this maximum, a Fault occurs and disarms the device.
Grounding	Isolated - each bank of 16 uses its own ground connection for return path. Shared - all banks in sub stimulator share the same ground return path (ground on first bank of sub stimulator). To Banana Jack - connect sub stimulator ground to the external banana jack on physical Subject Interface device. Caution: When using multiple sub stimulators make sure they aren't all sharing the Banana Jack connection or else they won't be isolated!
Monitoring	Off - disable compliance voltage monitoring. View Only - visualize the compliance voltage for each voice in the Flow Plot at runtime but do not save it to disk. Store - visualize and save the compliance voltage for each voice.
Impedance Checking	Enable runtime impedance checking interface and optionally log each impedance check to a CSV text file.

Runtime Interface

Safety Mode

States of the IZV Runtime Interface

If Safety Mode is enabled on the Safety and Compliance Tab, then an LED will show the state of the compliance monitor. If Remote Arming is disabled, the Arm/Disarm button is shown and the device can be armed when ready.

Important

The system can only Arm if the fiber is connected and a zero current command is sent to the IZV, so make sure the gizmo controlling stimulation is muted. In Safety Mode, you can ensure the stimulation is muted when you start the recording by setting Mute Control to Default Muted in the Electrical Stim Driver gizmo 'Misc and Saving' tab.

Impedance Check

If Impedance Checking is enabled, the IZV provides an interface for impedance checking on all channels.

IZV Impedance Check Runtime Interface

The display represents the stimulation channels divided into sub stimulators. Set the test stimulus frequency and amplitude, and define the high and low impedance threshold targets for visualization.

Running the Check

Run an impedance check on the currently selected sub stimulator. The test signal (sine wave of frequency defined by Freq (Hz) parameter and amplitude defined by Target (uA) parameter) is presented iteratively on each channel in the currently selected sub stimulator for ~500 ms and the impedance is measured using the IZV built in compliance monitor.

Check All. Run an impedance check for all IZV channels using the test signal as described above.

Stop Checking. Stops the impedance checking prematurely.

Double-click on any individual LED to run the impedance check on that channel only.

Results of impedance check are indicated by color: below low impedance target (green), above high impedance target (red), between low and high impedance targets (yellow). The actual impedance values (in KOhm) are displayed beneath each indicator.

Impedance results are optionally saved in a CSV text file located in the data block folder.

Testing

Each IZV bank comes with an RB10 resistor block to verify the IZV output signal. The RB10 has a 10 kOhm resistor on each channel to ground.

RB10 Resistor Block

Plug this directly into the Subject Interface IZV stim boards instead of a headstage and run an impedance check. The LEDs on the impedance check interface should indicate ~10 (kOhm) for each channel.

Each SW switching headstage comes with a ZCA32-RB10 or ZCA64-RB10 resistor block. If using a switching headstage, clip the switching headstage onto this resistor block and run the impedance check. The LEDs on the impedance check interface should indicate ~10 (kOhm) for each channel.

Zif-Clip® Resistor Block