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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 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. For a positive polarity photodetector signal, the diode’s low forward resistance allows 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 allows isolates photodetector capacitance and improves signal quality. If the normal photodetector signal polarity is negative then the diode orientation is reversed.



Advantages

Reduced leakage current in a row-and-column grid of photodetectors

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 of some diodes will attenuate low-amplitude signals, similar to amplitude thresholding, resulting in reduced noise in the image.

Faster response from photodetectors

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 from a large group of diode-coupled photodetectors can resemble a single photodetector.

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




Standard Diode Coupling Configurations


Row & Column Readout with Optional 4-Channel Encoder
(diodes oriented for positive SiPM signals)

SiPM current is divided equally into a row and a column.
Each row and column has a current-to-voltage amplifier.
4-channel readout further encodes the rows and columns into two X and two Y signals.
Often referred to as "symmetric charge division".



Row+Column 4-Channel SiPM Diode Readout


Group Readout
(diodes oriented for positive SiPM signals)

Each SiPM is connected to the amplifier through a fast diode.
Achieves the fastest timing response when forming a sum of a large-area array.



Group SiPM Diode Readout