Noise Spectroscopy, a.k.a. Z-determination by Statistical Count-rate ANalysis (Z-SCAN), is a statistical technique to
determine a quantity called the “noise figure” from digitized waveforms of pulses of transmitted x-rays in cargo
inspection systems. Depending only on quantities related to the x-ray energies, it measures a characteristic of the
transmitted x-ray spectrum, which depends on the atomic number, Z, of the material penetrated. The noise figure can
thus be used for material separation. In an 80-detector prototype, scintillators are used with large-area photodiodes
biased at 80V and digitized using 50-MSPS 12-bit ADC boards. We present an ultra-compact low-noise preamplifier
design, with one high-gain and one low-gain channel per detector for improved dynamic range. To achieve adequate
detection sensitivity and spatial resolution each dual-gain preamplifier channel must fit within a 12.7 mm wide circuit
board footprint and maintain adequate noise immunity to conducted and radiated interference from adjacent channels.
The novel design included iterative SPICE analysis of transient response, dynamic range, frequency response, and noise
analysis to optimize the selection and configuration of amplifiers and filter response. We discuss low-noise active and
passive components and low-noise techniques for circuit board layout that are essential to achieving the design goals,
and how the completed circuit board performed in comparison to the predicted responses.