FPGA implementation of a fully digital demodulation technique for biomedical application

Widely used demodulation technique in EIT/EIS systems is the direct conversion (zero-IF) method in which measured single tone current (or voltage) signal is mixed with source waveform at the same frequency, so that the magnitude and phase of the measured current (or voltage) can be determined after a low pass filtering stage. This is especially true for an EIS system which might need to operate in a low frequency range as small as 100 Hz. In the system presented here, the amplitude and phase of the IF signal can be measured after digitizing the signal by an ADC in digital domain with transferring the IF signal to the baseband (zero frequency). In this system, a lowpass FIR filter and a variable frequency digital oscillator have been used. In our designed system there is no need for zero crossing technique which suffers from jitter noise and delay of different path, for signal mixing. We have implemented digital demodulation part of system on a Virtex-4 LX25 FPGA from Xilinx.