Design of high-performance photodiode receivers for optical tomography

The design of instrumentation hardware for tomographic systems must take careful account of measurement noise. This is especially true in near-infrared absorption tomography, where the signal of interest is typically only a few percent of the total signal at the detector, and the available optical power may have to be shared among many measurement channels. In this paper, the monitoring of photodiodes in near-IR absorption tomography is examined in detail, but much of the material is applicable at wavelengths ranging from the UV to beyond 2.5 mu m. The authors' application involves the frequency region 50 kHz to 2 MHz, which lies above that utilized in the majority of radiometric 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. Alternative transimpedance circuit configurations, including both single-ended and differential topologies, are analyzed with a view to optimization of the signal-to-noise ratio. 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 intrinsic noise sources. It is shown that for applications of this type, viable alternatives to the traditionally dominant FET amplifier do exist. The relative susceptibility to coupled interference is also considered. 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.