Final answer:
Designing a low noise transresistance amplifier with filtering for a photoplethysmogram system involves a transresistance stage, RC filter, and gain stage. Component values and opamp selection are based on noise performance and the desired frequency range of 0.1–15 Hz. The circuit amplifies and filters the photodiode signal while maintaining fidelity for ADC conversion.
Step-by-step explanation:
The design of a low noise transresistance amplifier for a photoplethysmogram system involves using an operational amplifier (opamp) to convert the current signal from the photodiode (ranging from 100pA to 10nA) into a voltage signal. To achieve the required signal frequency range of 0.1–15 Hz and maintain a voltage level of approximately 1Vpp for the subsequent analog-to-digital converter (ADC) stage, careful component selection and circuit design are necessary. A feedback resistor in the transresistance amplifier sets the gain to convert the current to a voltage level suitable for the ADC. The amplifier should then be followed by an RC filter to remove noise outside the 0.1–15 Hz range, ensuring signal integrity. Lastly, an additional gain stage further amplifies the signal to the required voltage level.
Real component values would be chosen based on the desired signal bandwidth and noise characteristics of the opamps in use. Opamps with low input bias current and low noise characteristics are preferable, especially for handling the small currents from the photodiode. Power supply voltages would typically be within the range of ±15V. Each stage of the circuit must be designed to minimize the introduction of additional noise and maintain signal fidelity.
It is also important to ensure that the amplifiers have appropriate bandwidth and slew rate to handle the frequency range of interest without distortion. This is essential to maintain the fidelity of the signal throughout the amplification and filtering process.