PCRD08 - Analogue backend for SiPM detectors with scintillation crystals

Analogue front-end and ADC module for radiation detectors based on silicon photomultipliers (SiPM) coupled to scintillation crystals. The module amplifies the SiPM pulses, detects a threshold crossing, and digitizes the signal for a controlling MCU over SPI.

Principle of function

The detector signal enters through the MCX connector J201 and is AC-coupled into a non-inverting amplifier (U201, AD8691). A second AD8691 stage (U204) provides additional gain before the signal reaches the LTC1865-MS ADC (U207), which is read out over SPI. A comparator (U203, TS3021ILT) together with a D flip-flop (U205, SN74LVC2G74) latches a digital flag whenever the signal crosses a threshold, which the controlling MCU can use to time the ADC conversion (CONV/RST/SCK/SDI/SDO are supplied by the host over the module's socket interface).

Difference from PCRD07

PCRD08 reuses the general architecture of the older PCRD07 analogue backend (dual AD8691 gain stages, TS3021ILT comparator, LTC1865-MS ADC), but the analogue signal processing was reworked for SiPM+scintillator pulses, which are positive-going:

• Input polarity - PCRD07 (both revision A and B) amplifies the input signal with an inverting first stage (signal fed into the op-amp's - input, gain set by the feedback/input resistor ratio). PCRD08's input stage (U201) instead feeds the signal into the op-amp's + input, giving a non-inverting, unity-gain-at-high-frequency response suited to positive pulses.
• No analogue sample-and-hold switch - PCRD07 uses an analogue switch (TS5A4594) driven by its comparator/flip-flop pair to freeze the peak voltage on a storage capacitor, i.e. a true analogue peak-detector/memory circuit. PCRD08 has no such switch or hold capacitor; its comparator/flip-flop pair (U203/U205) only produces a digital threshold-crossing flag, and the ADC conversion is triggered directly by the host rather than sampling a held analogue voltage.

(Verified by comparing hw/sch_pcb/analogue_backend.kicad_sch of both modules.)