Arbitrary Waveform Generator for a High Frequency Arbitrary Waveform Neural Stimulator
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Abstract
The goal of this Bachelor graduation project is to make an electrical stimulator that can be used to help people empty their urinary bladder. Patients that are unable to relax the urethral sphincter are most commonly treated by mechanically emptying the bladder or by sacral root stimulation where the roots are selectively cut.
The stimulator to be made must send a high-frequency signal that cancels the blocking of the urethral sphincter. This method should be able to empty the bladder without the use of mechanical devices or selectively cutting nerves.
The whole project is divided into three parts: Control and Interface, Arbitrary Waveform Generator and Safety Module. These parts have been performed by three different subgroups. In this report, the Arbitrary Waveform Generator is discussed. The other parts are explained in the respective reports [1, 2].
The requirements for this waveform are to generate a biphasic pulse with frequencies ranging from 1 to 15 kHz. The amplitude range of this pulse should be adjustable between 0 and 10 mA and the pulse width and interphase delay should be fully adjustable. In order to generate this signal, a power management system was necessary. In addition to the power management system, the LPC1343 microcontroller was chosen to control the system. One of its functions is to control a DAC by communication using the SPI protocol. The DAC can linearly control the output voltages between 0 and the offered reference voltage, in this case 3.3 V by sending 10 bits of data. Using a voltage to current converter, made by the Interface and Control subgroup, the output voltage is converted to a current between 0 and 10 mA [1]. Three additional signals from the microcontroller operate an H-bridge. This is a switching circuit that is able to direct the generated current through a load. Using a timer and four interrupt moments, the three signals are generated that can make a cathodic pulse, anodic pulse and can disconnect the current source.
The chosen DAC has a close to ideal behavior. Therefore, the conversion from the microcontroller to the voltage to current converter is very precise. The H-bridge works best at low frequencies. At 1 kHz, around 2% of a total pulse of 127.2 μs is needed to reach 63% of the cathodic or anodic amplitude. At high frequencies, the time increases. At 15 kHz, 24% of a total pulse of 8.4 μs is needed to reach the amplitude.