Evaluation of transimpedance amplifiers for readout of a position sensitive avalanche photodiode

BetaBox, a direct detection beta camera utilizing a 13.5×13.5mm2 active area position sensitive avalanche photodiode (PSAPD) (Radiation Monitoring Devices) is currently being developed to image distributions of charged particles in microfluidic chips. The previously reported signal processing chain incorporated charge sensitive amplifiers (Cremat Inc., CR-110) in the frontend readout design. Charge sensitive amplifiers, particularly the CR-110 product, are commonly used as preamplifiers when reading out PSAPDs. However, the 140µs pulse decay of this device suggests a possible count rate limitation in the performance of the beta camera. To address this limitation, the frontend readout was redesigned to replace the charge sensitive amplifiers with op-amps configured as transimpedance amplifiers. We hypothesize that, since the equivalent circuit of the PSAPD appears more like a charge division network loaded by a large semiconductor junction capacitance (∼150 pF) than an APD, this signal source can be optimally processed using the transimpedance amplifier approach. Results show that the designed signal processing chain using a transimpedance amplifier with 100ns RC time constant displays not only comparable image quality to the previous signal chain using the CR-110 but also reduced image distortion at high count rates.