Optimisation of the Signal to Noise Performance of Photodiode Receivers in Near-infrared Absorption Tomography

P Wright, K B Ozanyan, S J Carey and H McCann

Electrical & Electronic Engineering, UMIST, Manchester, UK, H.McCann@umist.ac.uk All authors are members of the Virtual Centre for Industrial Process Tomography

ABSTRACT

The design of instrumentation hardware for tomographic systems must take careful account of measurement noise. This is especially true in absorption tomography, where the signal of interest is typically only a few percent of the total signal at the detector.

In this paper, the monitoring of photodiodes in near-IR absorption tomography is examined. This application typically involves the frequency region 50 kHz to 2 MHz, which lies above that utilized in the majority of radiometric and sensing systems, yet substantially below telecoms bit rates. The problem is further distinguished by the use of phase-sensitive detection schemes, which make local noise density more relevant than wideband noise performance and relax the requirement for DC precision.

Three alternative transimpedance circuit configurations are analysed with a view to signal:noise optimisation. Typical values of photodiode capacitance and shunt resistance are shown to result in significant noise gain, greatly increasing the importance of amplifier voltage noise relative to other noise sources. It is shown that, for applications of this type, viable alternatives to the traditionally dominant FET amplifier do exist. The results of practical tests, involving class-leading operational amplifiers, are presented to support the analyses. These results also underline the need for careful circuit layout and shielding if the capabilities of these devices are to be fully exploited.

Keywords Photodiode, amplifier, transimpedance, tomography