A Multichannel Synthesizer and Voltmeter for Electrical Impedance Tomography

This paper describes the signal generator and voltmeter design for a high-speed, high-precision electrical impedance tomography (EIT) and its implementation in a field programmable gate array (FPGA). Since the EIT inverse problem is highly ill-posed it requires that the applied and measured signals have very high precision. In our application we need 16 bits of control for the sources and 16 to 19 bits of precision for the voltmeters. The signal generator design reported here includes a Direct Digital Synthesizer (DDS) and a complex modulator to generate a sinusoidal waveform with controllable amplitude, frequency and phase. The modulated output is then sent to a D/A converter (DAC) to produce an analog signal. The voltmetering is performed using a complex matched filter that produces estimates of both the real and quadrature parts of the voltage. Many samples are used to produce a single voltage measurement, producing measurement precision greater than that of the A/D converter (ADC). The digital sinusoid signal is used as a coherent reference by the matched filter. The implementation of 8 signal generators and voltmeters in a single Xilinx Virtex-II Field Programmable Gate Array (FPGA) is presented.