ADPD2212 (Rev. 0)
Document overview
- Manufacturer or author: Analog Devices, Inc.
- PDF pages: 13
Technical content
Low Noise, High Sensitivity Optical Sensor Data Sheet ADPD2212 Rev. 0 Document Feedback Information furnished by Analog Devices is believed to be accurate and reliable. However, no responsibility is assumed by Analog Devices for its use, nor for any infringements of patents or other rights of third parties that may result from its use. Specifications subject to change without notice. No license is granted by implication or otherwise under any patent or patent rights of Analog Devices. Trademarks and registered trademarks are the property of their respective owners. Tel: 781.329.4700 ©2016 Analog Devices, Inc. All rights reserved. Technical Support www.analog.com
FEATURES
Ultrahigh detectivity photodetector 90 fA/√Hz (typical) ultralow noise floor Signal-to-noise ratio (SNR) near shot noise limit 137 µA (typical) of supply current when active (EE = 0 µW/cm2) 1 µA (typical) of supply current in standby High speed, deep junction photodiode Nominal linear output current: 240 µA (typical) Flexible output configuration Excellent pulse response High ambient light rejection Space-saving, 3 mm × 4 mm LFCSP package
APPLICATIONS
Heart rate, pulse oximetry monitoring (photoplethysmography) Battery-powered medical sensors Chemical analysis FUNCTIONAL BLOCK DIAGRAM Figure 1. GENERAL DESCRIPTION The ADPD2212 is an optical sensor optimized for biomedical applications. Very low power consumption and near theoretical signal-to-noise ratio (SNR) are achieved by packaging an ultralow capacitance deep junction silicon photodiode operated in zero bias photoconductive mode with a low noise current amplifier. The ADPD2212 offers a typical 400 kHz bandwidth performance, which is well suited for use with pulsed excitation. The ADPD2212 uses very little power during operation and incorporates a power-down pin, enabling power cycling to optimize battery life in portable applications. The ADPD2212 provides shot noise limited performance, making it an excellent choice for measuring signals with the highest possible fidelity in low light conditions. This combination of low power, very high SNR, and electromagnetic interference (EMI) immunity enables low power system solutions not possible with traditional photodiode (PD) and transimpedance amplifier (TIA) systems. 13721-001GNDPWDN OUT ADPD2212 VCC CURRENT AMPLIFIER
Rev. 0 | Page 2 of 13 TABLE OF CONTENTS
REVISION HISTORY
4/16—Revision 0: Initial Version
Rev. 0 | Page 3 of 13 SPECIFICATIONS VCC = 3.3 V, TA = 25°C, λ = 528 nm, unless otherwise noted. IPD is the photodiode current, IMOD is the modulation current, EE is irradiance, IOUT is output current, VBIAS is the bias voltage, RFEEDBACK is the TIA feedback resistor, and RLOAD is the load resistance. Table 1. Parameter Symbol Test Conditions/Comments Min Typ Max Unit GAIN Gain (Current Amplifier) βTLA 24 DYNAMIC PERFORMANCE Frequency Response Peaking <6 dB Rise Time tR 10% to 90% full scale (FS) (IOUT = 24 µA) 1.24 µs Fall Time tF 90% to 10% FS (IOUT =24 µA) 1.27 µs Bandwidth BW IPD = 10 nA, IMOD = 1 nA 400 kHz OPTICAL PERFORMANCE Diode Active Area 2.5 mm2 Saturation Irradiance 1600 µW/cm2 NOISE PERFORMANCE Current Noise, Output Referred1 EE = 0 µW/cm2 1920 fA/√Hz IPD = 10 nA to 300 nA 1.4 × NSHOT fA/√Hz IPD > 300 nA 1.15 × NSHOT fA/√Hz Current Noise Floor, Input Referred EE = 0 µW/cm2, at 1 kHz 90 150 fA/√Hz Noise Equivalent Power NEP At 1 kHz 100 fW/√Hz EE Required for SNR = 10000:1 At 1 kHz 144 nW/cm2 POWER AND SUPPLY Supply Voltage VCC 1.8 3.3 5.0 V Power Supply Rejection Ratio PSRR VCC = 1.8 V to 5.0 V, EE = 1600 µW/cm2 120 nA/V Current Standby ISTANDBY PWDN > VIH 1 µA Supply at EE = 0 µW/cm2 IFLOOR 137 µA Supply2 ISUPPLY IOUT = 10 µA 166 µA IOUT = 240 µA 857 µA OUTPUT CHARACTERISTICS Amplifier Static Bias Current Input Referred EE = 0 µW/cm2 10 nA Output Referred EE = 0 µW/cm2 240 nA Maximum Output Voltage VOUT_MAX VCC − 0.75 V Nominal Linear Output Current IOUT_FS 240 µA Linearity into TIA VBIAS = 1.3 V, RFEEDBACK = 25 kΩ 60 dB Linearity into Resistive Load IOUT < 100 µA , RLOAD = 5 kΩ 60 dB Peak Output Current3 300 µA Output Capacitance COUT From OUT to GND 5 pF Output Resistance ROUT From OUT to GND 1000 MΩ POWER-DOWN LOGIC Input Voltage High Level VIH VCC − 0.2 V Low Level VIL 0.2 V Leakage Current High IIH PWDN = 3.3 V 0.2 nA Low IIL PWDN = 0 V −8.5 µA OPERATING AMBIENT TEMPERATURE RANGE −40 +85 °C 1 NSHOT refers to photon shot noise. Photon shot noise is the fundamental noise floor for all photodetectors in photoconductive mode. 2 ISUPPLY = IFLOOR + (3 × IOUT). 3 Outputs greater than IOUT_FS may have degraded performance.
Figure 3. Pin Configuration Table 5. Pin Function Descriptions 1 PWDN Power-Down Input. Must be connected. Pull this pin high to disable the device. 3 NIC Not Internally Connected. This pin can be grounded. 6 NIC Not Internally Connected. This pin can be grounded. 7 NIC Not Internally Connected. This pin can be grounded. 8 NIC Not Internally Connected. This pin can be grounded. 9 NIC Not Internally Connected. This pin can be grounded. 10 NIC Not Internally Connected. This pin can be grounded. can be left blank to facilitate this requirement.
- NIC = NOT INTERNALLY CONNECTED.
- THE EXPOSED PAD MUST BE LEFT FLOATING.
10 NIC
9 NIC
8 NIC
7 NIC
6 NIC
Rev. 0 | Page 8 of 13 TERMINOLOGY Optical Power Optical power is defined as the photon energy per unit of time measured as radiant flux (Φ) or radiant power, which is radiant energy (Q) per unit of time. Responsivity Photodiode responsivity, ρ, is a constant that correlates incident optical power (POPT) with photodiode current (IPD) and is typically expressed in units of amperes per watt (A/W). Responsivity is essentially the quantum efficiency of the ability of the sensor to convert light into electron/hole pairs and is highly dependent upon the wavelength of the incident light as well as sensor material and temperature. Photodiode Area Photodiode area is a measure of the photosensitive area of the diode. In PIN diodes, this is the photosensitive area of intrinsic silicon between the positive and negative doped diffusion areas. In general, larger photodiodes demonstrate greater sensitivity as the output signal increases linearly with photosensitive area while noise increases at the sum of the square of the photosensitive area. A larger photodiode area has a higher capacitance and longer carrier diffusion paths adversely affecting bandwidth. Photoconductive Mode Photoconductive operation of a photodiode occurs when photons entering the silicon generate electron/hole pairs that are swept by the electric field to the opposite terminal. These carriers are presented at the terminals of the photodiode as a current proportional to the luminous flux incident on the junction of the photodiode. Shot Noise Shot noise is a statistical fluctuation in any quantized signal such as photons of light and electrons in current. The magnitude of the shot noise is expressed as a root mean square (rms) noise current. Shot noise is a fundamental limitation in photodetectors and takes the form of Shot noise = √(2qI PD) where: q is the charge of an electron (1.602 × 10−19 Coulomb). IPD is the photodiode current. Photoplethysmography (PPG) Photoplethysmography uses light to measure biological functions by sensing changes in the absorption spectra of soft tissue due to changes in hemoglobin volume and composition. Linearity Linearity is a measure of the deviation from an ideal change in output current relative to a change in optical power falling on the sensor. Linearity is specified as the deviation from a best straight line fit of the current output of the sensor over a speci- fied range of optical power. Linearity is a critical specification in PPG measurements due to the requirement of sensing small ac signals impressed upon large dc offsets. Static Bias The ADPD2212 has an internal 10 nA bias that linearizes the input current mirror at low input levels and prevents transient reverse bias of the amplifier input stage. This bias is fixed and appears on the output as a 240 nA typical offset. Noise Equivalent Power (NEP) Noise equivalent power is the amount of incident light power on a photodetector, which generates a photocurrent equal to the total noise current of the sensor. The noise level is proportional to the square root of the frequency bandwidth; therefore, NEP is specified with a 1 Hz bandwidth. NEP is the fundamental baseline of the detectivity of the sensor.
Rev. 0 | Page 9 of 13 THEORY OF OPERATION OVERVIEW The ADPD2212 is an integrated, low power, optical sensor composed of a deep junction silicon photodiode coupled to a low noise current amplifier in an optically transparent chip scale package. The ADPD2212 is optimized for battery- powered, wearable, medical, and industrial optical sensing applications requiring low power and high SNR. SHOT NOISE LIMITED PERFORMANCE The on-board photodiode of the ADPD2212 is operated in photoconductive mode with a zero bias voltage. This mode of operation allows the diode to operate with no dc dark current caused by leakage across the depletion area of the diode, providing a fundamental limit of shot noise. The noise level is proportional to the square root of the frequency bandwidth. SENSITIVITY AND SNR SNR is a measure of the ability of the sensor to separate the signal of interest from spurious signals that occur from the surrounding environment of the device, such as ambient light, nonlinearity, and noise within the device itself. The ADPD2212 operates its integrated photodiode in a zero biased photoconductive mode to provide near zero dark current and, therefore, no dark shot noise component contribution from the photodiode. The integrated current amplifier requires an internal bias current of 10 nA to improve bandwidth and linearize response at low light levels. This bias generates a shot noise component of 90 fA/√Hz at the output of the current amplifier and establishes the noise floor of the ADPD2212. To optimize the sensitivity of the ADPD2212, it is important to ensure that the optical signal is concentrated on the photoactive area of the integrated photodiode. The on-board precision current amplifier is shielded and is not significantly affected by light hitting its surface, but device sensitivity is based solely on the optical power incident to the photodetector. LINEARITY Linearity is critical to PPG due to the need to accurately extract a small amplitude, pulsatile ac signal modulated onto the large dc component, which is caused by nonpulsatile tissue absorption and ambient light. In pulsed light applications, bandwidth is a critical component of the linearity because fast recovery of the device from dark and/or power-down conditions can have a profound effect on the ability of the sensor to extract the signal of interest. The ADPD2212 is production trimmed to ensure 60 dB linearity at an irradiance of up to E E = 1600 µW/cm2, λ = 528 nm, at a supply voltage of 3.3 V. PACKAGE CONSIDERATIONS The ADPD2212 is packaged with a transparent epoxy molding compound. To maintain optimum sensitivity, take care in handling the device to prevent scratches or chemicals that may affect the surface finish above the photodiode. Due to the lack of stabilizing fillers (typically up to 70% silica) used in opaque molding compounds, the maximum storage temperature of the ADPD2212 is 105°C. The temperature profile for soldering is shown in Figure 2. EPAD CONNECTION The EPAD on the ADPD2212 acts as a common electrical, thermal, and mechanical platform for the photodiode and amplifier and must not be connected externally. External cooling is not required due to the extremely low power consumption of the ADPD2212. Analog Devices, Inc., recommends removal of traces beneath the device to eliminate potential coupling of external signals into the sensitive internal nodes of the ADPD2212.
Rev. 0 | Page 10 of 13 APPLICATIONS INFORMATION The current output of the ADPD2212 provides flexibility in interfacing to external circuitry. POWERING THE DEVICE The ADPD2212 is powered from a single positive 1.8 V to 5.0 V supply. The ADPD2212 features high PSRR, but proper circuit layout and bypassing is recommended to provide maximum sensitivity, especially in situations where the ADPD2212 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 ADPD2212 provides very high SNR, allowing accurate signal extraction with minimal source power and at low incident optical power. POWER-DOWN MODE The ADPD2212 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 ADPD2212. PULSE MODE OPERATION The ADPD2212 is optimized for battery-powered operation by the inclusion of a power-down pin (PWDN). When sensing is inactive, the ADPD2212 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 ADPD2212 allows different configurations depending on the application. The current gain of the ADPD2212 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 ADPD2212 is effectively a current source, the ADPD2212 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 ADPD2212 output current, parasitic capacitance, and the impedance of the load. This drift can require additional settling time in circuits following the ADPD2212 if they are actively multiplexing the output of the ADPD2212 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 ADPD2212 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 ADPD2212. The dynamic range of the interface is limited to the compliance voltage of the ADPD2212. 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 ADPD2212. 3-WIRE CURRENT MODE CONFIGURATION When used in the 3-wire current mode configuration with a photodiode (see Figure 14), the ADPD2212 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 F) value must be chosen carefully to eliminate stability and bandwidth degradation of the ADPD2212. Large capacitance around the feedback loop of the TIA has a direct effect on the bandwidth of the system.
Figure 18. 10-Lead Lead Frame Chip Scale Package [LFCSP]
0.203 REF
0.20 MIN
0.050 MAX
0.035 NOM
0.50 BSC
registered trademarks are the prop erty of their respective owners.