Design of a 2-Bit Sigma-Delta Modulator for Scintillation Waveform Capture

Sigma-delta modulation is a common technique used in analog-to-digital conversion. It has been shown that it can be implemented in a field-programmable gate array (FPGA) using minimal external analog components. Zhao et al. demonstrated that energy of scintillation pulses can be computed by adding up all the elements on the output from a conventional 1-bit sigmadelta modulator. However, for parameters selected for a suitable resolution and dynamic range, the system passes through a period of saturation. During saturation, information about the shape of the pulse is lost even though the integrated energy remains preserved. We present the design of a 2-bit version of the sigmadelta modulator that makes use of a nonlinear adaptive digital-toanalog converter (DAC) that keeps the system unsaturated. Each output bit is scaled by one of the four possible values the DAC can take, resulting in a 2-bit modulation. We have demonstrated that the 2-bit modulation retains information about the pulse shape.