An integrated readout circuit for personal dosimetry with phase-shift compensation technique to improve stability

Abstract

In this paper an integrated CMOS readout circuit for a radiation detector in a personal dosimeter is presented. High counting rate and low power requirements make the stability of the conventional high-pass pulse shaper a big problem. A novel phase-shift compensation method is proposed to improve the phase margin. The principle of the compensation circuit and its influence on noise performance are analyzed theoretically. A readout chip with two channels of conventional structure and one channel of the proposed structure has been implemented in a 0.35 μm CMOS technology. It occupies an area of 2.113×0.81 mm². Measurement results show that the proposed channel can process up to 1 MHz counting rate and provide a conversion gain of about 170 mV/fC at a power dissipation of 330 μW with a 3.3 V power supply. Ac-coupled to a silicon PIN detector, it successfully detects β-rays.

Introduction

A personal dosimeter is of great importance for individual protection against radiation and is widely used in space applications, nuclear research, medicine, military, etc. [1], [2], [3], [4], [5]. A typical personal dosimeter consists of a radiation detector and a corresponding readout circuit, microcontroller, memory, display board, wireless transceiver, alarms and so on. The detector and the readout circuit have the greatest influence on the resolution, counting rate and power consumption performance. Many kinds of semiconductor detectors have been developed for this purpose, such as surface barrier junctions, PIN or passivated diode, to take advantage of the fast response, small geometry and the facility to integrate with front-end electronics [6], [7]. A silicon PIN detector is used in this personal dosimeter.

Several aspects must be considered when designing the circuit. The amount of charges generated in the silicon radiation detector is so small that high gain and low noise properties are essential for the readout circuit. As a battery charged portable device, a personal dosimeter has a stringent restriction on the power dissipation, most of which is consumed by the analog part of the readout circuit [3]. However, low noise is usually achieved by adopting a large current to reduce thermal noise and a large gate area to reduce flicker noise. These methods conflict with low power and high frequency requirements, respectively. Besides, low power means a limited bandwidth of the amplifier. If a high counting rate is also required, stability becomes a big issue for the high-pass shaper as explained below. Parameters of power, speed and noise trade with each other for a practical readout circuit, while stability must be guaranteed.

A charge sensitive amplifier (CSA) is usually used as the preamplifier due to its insensitivity to detector capacitance. A pulse shaper follows the preamplifier to filter out noise and to further amplify the signal [8], [9], [10], [11]. We adopt this conventional configuration, but a compensation circuit is inserted between the preamplifier and the pulse shaper for stability consideration. The principle of the compensation circuit and its influence on noise performance are analyzed theoretically. A readout circuit for β-radiation detection with this phase-shift compensation technique has been designed and fabricated in a 0.35 μm CMOS technology. The test results show that the counting rate can be up to 1 MHz without any oscillation tendency, and the power consumption is 330 μW.