Precision Pulse Oximeter LED Current Sinks Using the ADA4505-2 Op Amp, the ADR1581 Shunt Reference, and the ADG1636 Switches (cn0125)
Current sources or current sinks are key circuits used broadly in medical, industrial, communication, and other types of equipment for sensor excitation. A good example is the current sources (sinks) needed to drive the internal IR (infrared) and R (red) LEDs in a pulse oximetry sensor.
A pulse oximeter is a noninvasive medical device used for continuously measuring the percentage of hemoglobin (Hb) saturated with oxygen and the pulse rate of a patient. Hemog-lobin that is carrying oxygen (oxyhemoglobin) absorbs light in the infrared (IR) region of the spectrum; hemoglobin that is not carrying oxygen (deoxyhemoglobin) absorbs visible red (R) light. In pulse oximetry, a clip containing two LEDs (sometimes more, depending on the complexity of the measurement algorithm) and the light sensor (photodiode) is placed on the finger or earlobe of the patient. One LED emits red light (wavelengths of 600 nm to 700 nm), and the other emits light in the near IR (wavelengths 800 nm to 900 nm) region. The clip is connected by a cable to a processor unit. The LEDs are rapidly and sequentially excited by two current sources (one for each LED) whose dc levels depend on the LED being driven, based on manufacturer requirements. The detector is synchronized to capture the light from each LED as it is transmitted through the tissue.
Low power, precision current sources (if the current flows into the load) or current sinks (if the current flows out of the load) used in pulse oximeter designs are required to deliver a few decades of milliamps (hundreds of milliamps for legacy products). The active elements in these circuits are a low power precision operational amplifier, a precision shunt voltage reference, and a MOSFET or a bipolar transistor. To save power an analog switch can be added to power off the current source/sink when it is in the standby mode. If ultraprecision design is required, then an ultraprecision series voltage reference can be utilized instead of the shunt voltage reference.
An excellent low power, low cost precision amplifier to use in this medical application is the dual 10 μA rail-to-rail zero crossover distortion ADA4505-2. A good ultralow power, low cost precision shunt voltage reference to accompany the amplifier is the 1.25 V ADR1581 (A grade). An excellent choice for the analog switch is the 1 Ω on-resistance ADG1636 dual SPDT switch.
The maximum quiescent currents for the devices are as follows when these devices are operated from a power supply rail of 5 V and working under the industrial temperature range of –40°C to +85°C: one-half the ADA4505-2, 15 μA; ADR1581, 70 μA; ADG1636, 1 μA. These numbers add up to a total of 86 μA current consumption per circuit, which is suitable for portable and battery-powered instruments.
Current sources (sinks) implemented using the low power, low cost ADA4505-2 amplifier; the micropower, low-cost ADR1581 shunt voltage reference; and the ultralow power ADG1636 SPDT analog switch are characterized by being precise, low power, cost effective, flexible, and small in PCB footprint.
Figure 1. Pulse Oximeter Red and Infrared Current Sources Using the ADA4505-2, ADR1581 Voltage Reference, and ADG1636 Switch (Simplified Schematic: Decoupling and All Connections Not Shown)
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