Integrated synchronous receiver channel for optical instrumentation applications

A two-channel synchronous receiver circuit for optical instrumentation applications has been designed and implemented. Each receiver channel comprises a.o. transimpedance preamplifier, voltage amplifiers, programmable feedback networks, and a synchronous detector. The function of the channel is to extract the slowly varying information carrying signal from a modulated carrier which is accompanied by relatively high levels of noise. As a whole, the channel can be characterised as a narrow band filter around the frequency of interest. Medical applications include arterial oxygen saturation (SaO(2)) measurement and dental pulp vitality measurement. In both cases, two optical signals with different frequencies are received by a single photodiode. The measured performance of the optical receiver shows its suitability for the above mentioned applications. Therefore the circuit will be used in a small sized, battery-operated sensor prototype to test the sensing method in a clinical environment. Other applications include the signal processing of optical position-sensitive detectors (PSD). A summary of measured receiver channel performance: input reduced noise current spectral density between 0.20 and 0.30 pA/root Hz at all relevant frequencies, total programmable channel transimpedance between 7 M Omega and 500 M Omega, lower -3 dB frequency of at least 50 Hz, upper -3 dB frequency of 40 kHz, maximum voltage swing at the demodulator output of 2.4 V.