Constant amplitude current output

The Constant Amplitude Current Output (Current Clamp)... 

The optimal output characteristic is linked with the very variable load resistance. Tissue resistivity increases when coagulated. Fat has higher resistivity than muscles and blood, and the contact geometry is very dependent on the electrode chosen and the way it is held by the operator. If constant amplitude current is chosen, power would be proportional to load resistance, and tissue would quickly be carbonized in high resistance situations. If constant amplitude voltage were chosen, power would be inversely proportional to load resistance, and when tissue layers around the electrode coagulate, current stops flowing. Modern instrumentation therefore measures both... 

The pulses may be of unidireetional eurrent (interrupted DC, monophasic), which implies that the current has a DC component. High-voltage pulsed galvanic stimulation is also used, with pulse currents up to some amperes, but pulse duration only a few microseconds. If DC effects are to be avoided (e.g., to reduce electrolytic effects or electrode metal corrosion), the current is biphasic. Faradic currents are biphasic currents of the type generated by an induction coil. If the pulses are slowly increased in amplitude, then reduced, and after a pause again increased, we have a ramp or surged current. Because many effects are current controlled, it is often better to use a constant amplitude current mode than a constant amplitude voltage mode of the stimulator output. 

The output pulses of the CFDs are used as start and stop pulses of a time-to-amplitude converter, TAC. The TAC generates an output signal proportional to the time between the start and the stop pulse. Conventional TACs use a switched current source charging a capacitor. The start pulse switches the current on, the stop pulse off If the current in the start-stop interval is constant, the final voltage at the capacitor represents the time between start and stop. This principle works with remarkable accuracy, and time differences of a few ps can be clearly resolved. 

The BPB is a single-channel, constant-current, charge-balanced stimulator. The stimulation output is capacitance-coupled, which also prevents direct connection between the battery or battery-generated DC voltages and the tissue. Stimulation pulse amplitude, width, and frequency can be independently adjusted. In addition, triggering events can cause the stimulation to be dehvered continuously or in a pulse burst, which can be ramped up and down with a variety of Start/Stop times. 

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