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Diode Coupled Charge Division Readout
for Silicon Photomultipliers and Photodetectors


Introduction

Our patented diode-coupled charge division readout was developed in 2011 to improve the image quality of multiplexed large-area silicon photomultiplier (SiPM) arrays used in imaging applications. It provides superior image uniformity, lower noise, and faster timing than traditional resistor coupling techniques.

*US Patent No. 9,106,191



Summary

A special fast diode is placed between the photodetector and a fast amplifier, normally a transimpedance amplifier. For a positive polarity photodetector signal, the diode’s low forward resistance allows the photocurrent to flow from the photodetector to the amplifier. The diode’s high reverse resistance blocks current flow from the amplifier to the photodetector. Diodes also block current flow between individually diode-coupled photodetectors which isolates photodetector capacitance and improves signal quality. If the photodetector signal polarity is negative then the diode orientation would be reversed.



Advantages


Improved image uniformity
Diodes reduce undesirable leakage current between photodetectors connected in a row-and-column grid. Reduced leakage current improves spatial uniformity and position resolution in imaging applications. Recent studies reported no image distortion within the entire imaging area.

Attenuation of low-amplitude signals (amplitude “thresholding”)
The nonlinear forward resistance will attenuate low-amplitude signals, similar to amplitude thresholding, resulting in reduced image noise.

Faster response
When each photodetector in a group of photodetectors is separately connected through a diode to an amplifier, the diode isolates the individual photodetector capacitance from the group. This reduces the effects of accumulating capacitance and reduces charge loss between photodetectors. The signal rise time of a group of diode-coupled photodetectors is substantially lower than resistor-coupled photodetectors.

Timing resolution measurements:
Onsemi ArraySB-8 using the AB4T-ARRAY8 was measured to be 1.5ns.
Onsemi ArraySL-4P9 using the AB4T-SPMARRAY4P9 was measured to be 2.6ns at 14°C.
Onsemi ArrayC-30035-16P using the AB4T-ARRAY16P was measured to be <500ps.



Standard Diode Coupling Configurations


Row & Column Readout with Optional 4-Channel Encoder

  • SiPM current is divided equally into one row and one column
  • Each row and column has a separate transimpedance amplifier
  • Optional 4-channel encoder further multiplexes the rows and columns into two X and two Y signals
  • Often referred to as "symmetric charge division"

(Diodes oriented for positive SiPM signals)

Row+Column 4-Channel SiPM Diode Readout



Group Readout for Summing Applications

  • Every SiPM is connected through a separate fast diode to one transimpedance amplifier
  • Faster timing response than resistor coupling when forming a sum of many SiPMs

(Diodes oriented for positive SiPM signals)

Group SiPM Diode Readout