Signal to Noise Ratio and Current Consumption in LED-LED Photoplethysmography

The signal-to-noise ratio (SNR) and the current consumption of a photoplethysmography (PPG) system based on a LED-LED configuration were estimated. The emitting LED was powered in switched mode using frequencies between 100 Hz and 400 Hz and duty cycles (D) between 5% and 30%. The signal from the detector LED was analog conditioned using synchronous demodulation implemented with a zero-order hold circuit that worked at the same frequencies and D. An important change in the SNR was observed when the switching frequency and D varied. This change was mainly due to the change in the amplitude of the PPG signal because of the changes in the light emitted by the emitting LED, and also by the changes in the frequency response of the synchronous demodulator. Regarding current consumption, the greatest contribution came from the conditioning circuit of the detector LED, since the maximum current consumed by the emitting LED was 51 μA. At 100 Hz and D = 5%, a SNR of 67 dB and current consumption of 2.71 mA were obtained. With the system used in this study, no tradeoff between the SNR and power consumption was observed when the frequency and D were changed.