The hottest subminiature ADC promotes low-power ho

2022-08-09
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Subminiature ADC improves low-power home medical applications

at present, the development of medical devices is revolutionizing the home health care market, and people can diagnose various health conditions without leaving home. The development of technology has made portable self-care and health care systems a reality. These systems can help people monitor important indicators such as blood pressure, blood glucose and body temperature

home medical monitoring and monitoring system can help people control their health status, but these medical devices must be fast and efficient, and can ensure work at the most important time. With the development of portable medical sensors, the demand for longer battery life and smaller overall size becomes more and more critical for the non group polyurethane insulation core material that cannot enter the market if its combustion performance does not meet the standard

medical measurement equipment generally needs to integrate a variety of signal conditioning circuits, including amplifiers, filters, reference sources and analog-to-digital converters (ADC), in order to distinguish and recognize sensor signals. In addition to the small size, it is also important that the analog circuit that reads the output of the sensor requires low power to work, so as to provide longer battery life and more reading times. With the launch of smaller and faster analog ICs, small, low-power medical devices powered by wall sockets have become increasingly popular

examples of medical devices that require small-size and low-power solutions include blood analysis systems, pulse oximeters, digital X-rays, digital thermometers, and so on

analog circuits used in medical measurements

some medical measurements require analog circuits to operate continuously and obtain thousands or even millions of readings per second. Some applications only need to read once a day. For these accidental tests, pay attention not to exceed the rated range of the sensor. The analog circuit only needs to be powered on once for measurement, and then it will be idle for the rest of the day. At this time, it can enter the low-power "sleep" mode

the choice of Analog IC depends on the frequency of sensor readings. The core of the analog circuit is the ADC that converts the analog reading from the sensor into the digital result. The digital result can be stored in the memory or displayed on the screen. For most portable medical sensor applications, the best choice of data converter will be successive approximation register (SAR) ADC

there are many reasons to choose this kind of ADC. First, SAR ADC is very suitable for measuring signals from zero Hertz (steady state) to several megahertz. These ADCs also have fast response and low delay performance, making them ideal for measuring a single input or multiple inputs. Another key factor is power. Unlike flash memory or pipeline ADC, the power consumption of SAR ADC will change with the change of sampling rate. Therefore, the power consumption of ADC running at 10000 samples per second (10ksps) will be lower than that running at 100ksps, and the power savings are very significant. For example, a SAR ADC that converts data at the rate of millions of samples per second (MSPs) may consume several milliamperes of current, while the same SAR ADC may consume only tens of milliamperes of current when operating at 1ksps or lower sampling rate

pulse oximeter is a medical application example that benefits from SAR ADC as the core. This device is used to measure the blood oxygen content equivalent to the hemoglobin in the patient's blood. Pulse oximeter detects blood pulsation in arteries, so it can be achieved as long as the performance of graphene material is greatly improved; Plastic calculates the patient's heart rate. A pair of light-emitting diodes (LEDs) face a photodiode through the translucent part of the patient's body (usually fingertips). An optical transmitter triggers a red LED with a wavelength of 660nm and an infrared LED with a wavelength of 940nm. The photodiode receives these two signals and converts the photocurrent into voltage. Then ADC measures this voltage, so as to read the percentage of bleeding oxygen based on the absorption rate of each wavelength of light after the light passes through the patient's body (see Figure 1). The next step is usually to send the digital data across an isolation device to the data acquisition system for storage or display on the monitor

the lt6202 amplifier of linglilte company shown in Figure 1 provides a good combination of gain bandwidth (100MHz) and low-voltage noise (1.9nv/hz), while consuming only 2.5mA current. It also has low current noise of 0.75pa/hz and ultra-low overall noise and distortion power in small signal applications. This amplifier is specified to work with 3V, 5V and ± 5V power supplies

Figure 1: application of subminiature ADC in pulse oximeter

sampling lt6202 outputs a 12 bit 3msps SAR ADC. Ltc2366 is a member of the miniature ADC series with sampling rates from 100ksps to 3msps and full pin and software compatibility. The ADC of this series consumes only 7.8mw power at 3msps, 1.5MW at 100ksps, and only 0.3 in sleep mode μ W。 Ltc2366 is characterized by no data passing delay, so sampling data can be obtained in the same clock cycle. The device provides sampling results through a 3-wire spi/microwire compatible interface

ltc2366 adopts thinspot 6-pin or 8-pin package (8.1mm2), which helps to keep the overall solution size of pulse oximeter to a minimum. At 3msps, it has sufficient sampling bandwidth and can correctly sample voltage through amplifier and photodiode current. The ltc2366 operates under a single 3V power supply and can be powered by a single lithium-ion battery, multiple AA batteries, or a wall power supply system that wants to operate at low power

ltc236x series has five kinds of ADCs, including ltc2366 of 3msps, ltc2365 of 1msps, ltc2362 of 500ksps, ltc2361 of 250ksps and ltc2360 of 100ksps. The above sampling rate is the highest sampling rate of each ADC. For applications that do not need to run at 3msps, each ltc236x ADC can further save power at a lower sampling rate. Figure 2 describes in detail the relationship between the power supply current and the sampling rate of the three lower speed versions of ADC. Based on the design of SAR ADC kernel, the power consumption will decrease rapidly with the reduction of sampling rate

Figure 2: ltc236x ADC power consumption decreases rapidly with the reduction of sampling rate

digital X-ray imaging

another medical application example that requires fast ADC is digital X-ray imaging, including dental X-ray, computer-controlled axial X-ray tomography (cat scanning) or medical magnetic resonance imaging for whole-body scanning. X-ray equipment manufacturers now store images not on a piece of film, but digitally. Doctors' offices or hospitals no longer store thousands of films in file cabinets, but can easily store the results in memory and quickly refer to patients' medical records

the second positive effect of digital X-ray imaging is to bring comfort to patients. With the advent of faster ADCs, sensors, and signal conditioning modules, it is much faster to examine patients for sore teeth or broken bones, which means that patients need to remain stationary for much shorter periods of time. With a faster X-ray examination process, doctors' offices or hospitals can see more patients in the same time

digital X-ray generally requires ADC with at least 12 bit resolution and 1msps or higher sampling rate. This ADC must have a sampling rate higher than or equal to the refresh rate multiplied by the size of the array. This application generally requires multiple ADCs to distinguish all photodiode or CMOS imaging currents from the scintillator. A set of photodiodes, amplifiers, and ADCs are used to sample the entire array, as shown in Figure 3. When multiple ADCs are placed in a space limited area, it is very important to have a low-power SAR ADC with fast serial data communication function. Ltc2366 (3msps) and ltc2365 (1msps) based on 73db signal-to-noise ratio and zero data delay are very suitable for digital X-ray imaging

Figure 3: schematic diagram of X-ray imaging equipment

ltc2366's low power (7.8mw at 3msps) means that designers can use multiple closely adjacent ADCs without heating the system or interfering with readings or patients. Pipeline ADC may consume 10 times of power at the same sampling rate

digital thermometer

digital thermometer is another small and inexpensive device that can be used for patient care in families or hospitals. They allow a quick check of the patient's temperature by measuring the eardrum or placing it under the armpit. An analog circuit that detects the temperature and converts it into a digital reading can be quite simple. This kind of application often uses thermistor, that is, a resistance that changes with temperature, because it has the highest sensitivity to human body temperature

ltc2450 -1 is a 16 bit delta sigma ADC that is somewhat like SA to survive, and its power also changes with the sampling rate. This type of delta sigma ADC is very suitable for some medical applications. For example, LTC can be directly connected to a thermistor (as shown in Figure 4) to provide accurate digital temperature readings. In this example, a fixed 10K Ω resistor is connected in series with a thermistor that depends on temperature and varies between 1K Ω and 10K Ω, allowing the ADC to measure a wide analog input range. The input architecture of LTC allows it to measure high impedance sensors, so an amplifier can be bypassed. The resistor and decoupling capacitor can be directly connected to the analog input

Figure 4: LTC ADC with low sleep current is very suitable for digital thermometer powered by battery

LTC provides extremely low power supply current (the maximum value is guaranteed to be 0.5 with temperature change μ A) , which makes it very suitable for digital thermometer working with a single battery. Most household digital thermometers are kept in drawers and work with dormant current. They only need to be powered on occasionally

ltc provides an output rate of 60 samples per second, which is sufficient for digital thermometer measurement. It adopts 2mmx2mm package and communicates through SPI protocol. The small package size and the ability to connect directly to the thermistor make the total analog solution size extremely small. For designers who want to use the 2-wire protocol, single ended and differential I2C versions are also available

summary of this article

global population continues to increase. The baby boomers in the United States are approaching retirement age. With the continuous improvement of medical equipment in accuracy, cost and simplicity, home medical care is becoming easier and more convenient. It is the analog signal link from the sensor to the amplifier and ADC that helps drive this transformation. The faster and lower power serial ADC will further improve the accuracy, speed and ease of use of medical devices. (end)

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