The development of medical devices is revolutionizing the home health care market, people do not need to leave the house will be able to diagnose various health conditions. The development of technology has made portable self-care care systems a reality that can help people monitor important indicators such as blood pressure, blood sugar and body temperature.
Home medical surveillance and surveillance systems can help people control their health, but these medical devices must be fast and efficient, and work at the most important times. With the development of portable medical sensors, the need for longer battery life and smaller form factors has become increasingly critical for non-tissue invasive care.
Medical measurement equipment generally needs to integrate a variety of signal conditioning circuits, including amplifiers, filters, reference sources, and analog-to-digital converters (ADCs) to distinguish and identify sensor signals. In addition to small size, it is important to read the analog output of the sensor circuit to require low power operation to provide longer battery life and more reads. With the introduction of smaller and faster analog ICs, small, low-power medical devices powered by wall outlets are becoming more popular.
Examples of medical devices that require small size and low power solutions include blood analysis systems, pulse oximeters, digital X-rays, and digital thermometers.
Analog circuit used for medical measurements
Some medical measurements require analog circuits to run continuously and take thousands or even millions of readings per second. Some applications only need to be read once a day. For these contingency tests, the analog circuit only needs to be powered up once and then idle for the rest of the day, allowing it to enter a low-power "sleep" mode.
The choice of analog IC depends on how often the sensor readings. At the heart of the analog circuit is an ADC that ultimately converts the analog readings from the sensor into digital results, which can be stored in memory or displayed on the screen. For most portable medical sensor applications, the best choice for a data converter would be a successive approximation register (SAR) ADC.
There are many reasons to choose such an ADC. First, SAR ADCs are ideal for measuring signals from zero hertz (steady state) up to a few megahertz. These ADCs also feature fast response and low latency, making them ideal for measuring a single input or multiple inputs. Another key factor is power. Unlike flash or pipeline ADCs, the power consumption of a SAR ADC will vary with the sample rate. Therefore, the power consumption of an ADC running at 10,000 samples per second (10ksps) will be lower than the power consumption at 100ksps, and the power savings are significant. For example, a SAR ADC that converts data at millions of samples per second (Msps) may consume several milliamps of current, while the same SAR ADC may consume only a few tens of microamps when operating at 1ksps or lower.
Pulse oximeter
The pulse oximeter is an example of a medical application that benefits from the SAR ADC. This device is used to measure blood oxygen levels comparable to hemoglobin in the patient's blood. The pulse oximeter detects blood pulsations in the artery and therefore calculates the patient's heart rhythm. A pair of light-emitting diodes (LEDs) face a photodiode through a translucent portion of the patient's body (usually a fingertip). A light emitter triggers a red LED with a wavelength of 660 nm while triggering an infrared LED with a wavelength of 940 nm. The photodiode receives these two signals and converts the photoinduced current into a voltage. This voltage is then measured by the ADC to read the percentage of blood oxygen based on the absorbance of each wavelength of light as it passes through the patient's body (see Figure 1). The next step is usually to send the digital data to a data acquisition system for storage or display on a monitor across an isolation device.
The Linear Technology LT6202 amplifier shown in Figure 1 provides a good combination of gain bandwidth (100MHz) and low voltage noise (1.9nV/Hz) while consuming only 2.5mA. It also features low current noise of 0.75pA/Hz and ultra-low overall noise and distortion power in small signal applications. This amplifier is specified to operate with 3V, 5V and ±5V supplies.
Figure 1: Application of an ultra-small ADC in a pulse oximeter.
The sampled LT6202 outputs a 12-bit 3Msps SAR ADC. The LTC2366 is a family of micro ADCs with sampling rates from 100ksps to 3Msps, fully pin and software compatible. This series of ADCs consumes only 7.8mW at 3Msps, 1.5mW at 100ksps, and only 0.3μW in sleep mode. The LTC2366 features no data pass delay, so sampled data is available in the same clock cycle. The device provides sample results via a 3-wire SPI/Microwire compatible interface.
The LTC2366 is available in a ThinSOT 6-pin or 8-pin package (8.1mm2) to help minimize the overall solution size of the pulse oximeter. At 3Msps, it has ample sampling bandwidth and is capable of correctly sampling the voltage through the amplifier and photodiode currents. The LTC2366 operates from a single 3V supply and can be powered by a single-cell Li-Ion battery, multiple AA batteries, or a wall-powered system that wants to operate at low power.
The LTC236x family consists of five ADCs, including the LTC2366 at 3Msps, the LTC2365 at 1Msps, the LTC2362 at 500ksps, the LTC2361 at 250ksps, and the LTC2360 at 100ksps. The above sampling rate is the highest sampling rate per ADC. For applications that do not need to run at 3Msps, each LTC236x ADC can save even more power at lower sample rates. Figure 2 details the supply current vs. sample rate for the three lower speed versions of the ADC. Based on the core design of the SAR ADC itself, as the sampling rate decreases, the power consumption will drop rapidly.
Figure 2: The LTC236x ADC power consumption decreases rapidly as the sample rate decreases.
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