Complete Guide to Voltage-To-Current Stimulators
In advanced brain stimulation and neuromodulation research there is no device more flexible then a voltage-to-current stimulator. In this review, Caputron will explain why, as well as clarifying some of the terminology around these types of devices.
What is voltage-to-current stimulator used for?
In electrical stimulation, “current controlled” approaches are preferred as they are more reliable in how the body is activated. So most stimulation devices including tDCS devices and also implanted devices like Spinal Cord Stimulation are current controlled. This is why stimulation intensity is reported in units of current, as in 1 mA.
In electrical stimulation the waveform is the shape of the stimulation, how intensity changes over time. For example a DC waveform for tDCS, or an pulsed waveform for CES. It is also important to control when stimulation happens (stimulation timing). When you get a stand-alone stimulator, like a tDCS device, the device sets the waveform (for example 2 mA for 20 minutes with a 30 second ramp on for tDCS) and make it current control.
There can be situations where you want a lot of flexibility on the waveform. For example, if you are experimenting with a fancy new waveform or if you want to start the stimulation based on some event like a change in brain activity (EEG). There exist some some high end stand-alone stimulators that can provide this flexibility, systems like the Soterix Medical MxN devices. But let’s face it, these high end medical-grade devices are pricey and don't always fit the researchers needs. The most flexible way to stimulate is by using a voltage-to-current stimulator.
How does a voltage-to-current stimulator work?
1) The waveform is generated by a voltage source. This can be for example a function generator. You set the function generator to create the waveform you want (for example a sinusoid, or pulse train, or noise, or kHz, or anything…). Note these function generators create the waveform in voltage.
2) The voltage function generator (the waveform generator) is connected to the INPUT of the voltage-to-current stimulator, and the OUTPUT of the voltage-to-current stimulator is connected to the body, brain tissue sample, or whatever you want to stimulate.
3) The voltage-to-current stimulator converts the waveform from voltage controlled to current controlled. So basically, you can create any waveform you want with the function generator and the voltage-to-current stimulator then converts it to a current controlled waveform.
What features are important in a voltage-to-current stimulator?
The job of a voltage-to-current stimulator is to receive a command voltage at its INPUT and reliably convert it to a current controlled waveform at its OUTPUT. So, a good voltage-to-current stimulator does this reliably and accurately. A voltage-to-current stimulator that distorts the waveform, so that the current output does not track the voltage command is a bad device.
The kind of features you look for in a voltage-to-current stimulator is its time-response (basically how fast of a voltage change the device can track and how high of a frequency the device can track) and its range (basically how much voltage and current is can take before its clips out).
Below is data comparing these features for different voltage-to-current stimulator. The High Precision Linear Current Isolator from Caputron is by some margin the most superior voltage-to-current stimulator.
What is an Isolator? What is a Voltage-to-Current Isolator
In electrical stimulation sometimes devices are called “isolators” or “Voltage-to-Current Isolator”. Isolator can also be voltage-to-voltage. A voltage-to-current isolator is functionally much the same as a voltage-to-current converter. But in some cases (and certainly not all) voltage-to-current isolators provide more electrical isolation between the INPUT and OUTPUT of the device. Which in some applications can add a safety buffer. But beware, some voltage-to-current isolator don't provide much protection at all, and can even cause trouble. For example, the AM Systems Isolators (like the AM Systems 2200) will surge whenever they are powered on or off or under some other conditions, producing uncontrolled output. In some medical application “isolation” is important and a big deal, but for Voltage-to-Current Isolator these are not typically to medical standards. Bad voltage-to-current isolators can also “leak” DC current. You can see in the table above that the High Precision Linear Current Isolator from Caputron provides best-in-class performance in many categories.