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ADPD2211 Arkusz danych(PDF) 11 Page - Analog Devices |
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ADPD2211 Arkusz danych(HTML) 11 Page - Analog Devices |
11 / 14 page ADPD2211 Data Sheet Rev. 0 | Page 10 of 13 APPLICATIONS INFORMATION The current output of the ADPD2211 provides flexibility in interfacing to external circuitry. POWERING THE DEVICE The ADPD2211 is powered from a single positive 1.8 V to 5.0 V supply. The ADPD2211 features high PSRR, but proper circuit layout and bypassing is recommended to provide maximum sensitivity, especially in situations where the ADPD2211 may share reference nodes with transmitters in pulse mode applications. Above the quiescent current of the integrated current amplifier, there is a linear relationship to incident light as the current amplifier amplifies the photodiode output by a factor of 24. In typical battery-powered operation, the output of the source LEDs is dynamically reduced to save power based on the received signal strength of the photosensor. The extremely low noise floor of the ADPD2211 provides very high SNR, allowing accurate signal extraction with minimal source power and at low incident optical power. POWER-DOWN MODE The ADPD2211 is optimized for battery-powered operation by the inclusion of an extremely low power standby mode that can be quickly switched to provide ultralow power consumption during dark periods in pulsed or mode locked applications, where the light source is cycled to improve ambient light rejection and reduce transmitter power consumption. The power-down pin is not internally pulled up or down, and must be connected to an external logic level for proper operation of the ADPD2211. PULSE MODE OPERATION The ADPD2211 is optimized for battery-powered operation by the inclusion of a power-down pin (PWDN). When sensing is inactive, the ADPD2211 can be quickly switched into standby mode, reducing the supply current to 1 µA during dark periods for pulsed or mode locked applications, where the light source is cycled to improve ambient light rejection and reduce transmitter power consumption. For multiple wavelength systems, sequentially pulsing the optical emitters removes the need for multiple narrow bandwidth sensors. For both multiple wavelength (SpO2) and single wavelength (heart rate monitoring) systems, pulsed operation can provide significant power savings for battery-powered systems. Pulsed mode operation provides a calibration signal that is necessary to compensate for ambient light diffused throughout the tissue, which can be extracted by measuring the sensor output while the system emitters are off. Advanced algorithms can then extract the signal of interest from dc offsets, noise, and interferer signals such as motion artifacts. OUTPUT CONFIGURATION The output of the ADPD2211 allows different configurations depending on the application. The current gain of the ADPD2211 reduces the effect of surrounding interferers but, for best perfor- mance, careful design and layout is still necessary to achieve the best performance. The effect of capacitance on the output must be considered carefully regardless of configuration as bandwidth and response time of the system can be limited simply by the time required to charge and discharge parasitics. Because the ADPD2211 is effectively a current source, the ADPD2211 output voltage drifts up to its compliance voltage, approximately 1.2 V below VCC, when connected to an interface that presents a high impedance. The rate of this drift is dependent on the ADPD2211 output current, parasitic capacitance, and the impedance of the load. This drift can require additional settling time in circuits following the ADPD2211 if they are actively multiplexing the output of the ADPD2211 or presenting a high impedance due to power cycling. For multiplexed systems, a current steering architecture may offer a performance advantage over a break-before-make switch matrix. 3-WIRE CABLE VOLTAGE CONFIGURATION The ADPD2211 can be used in a minimal 3-wire voltage configuration, offering a compact solution with very few components (see Figure 13). A shunt resistor (RS) sets the transimpedance gain in front of the analog-to-digital converter (ADC). This configuration allows flexibility in matching the ADC converter full-scale input to the full-scale output of the ADPD2211. The dynamic range of the interface is limited to the compliance voltage of the ADPD2211. No additional amplification is needed prior to the ADC. Response time at the lower end of the range is limited by the ability of the output current to charge the parasitic capacitance presented to the output of the ADPD2211. 3-WIRE CURRENT MODE CONFIGURATION When used in the 3-wire current mode configuration with a photodiode (see Figure 14), the ADPD2211 is insensitive to load resistance and can be used when the signal processing is further from the sensor. EMI noise and shielding requirements are minimized; however, cable capacitance has a direct effect on bandwidth, making the 3-wire current mode configuration a better choice for unshielded interfaces. The feedback capacitance (CF) value must be chosen carefully to eliminate stability and bandwidth degradation of the ADPD2211. Large capacitance around the feedback loop of the TIA has a direct effect on the bandwidth of the system. |
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