Using ultra-small ADC to design low-power medical equipment solutions
The development of medical equipment is revolutionizing the home healthcare market, and people can diagnose various health conditions without leaving their homes. The development of technology has made portable self-care health care systems a reality. These systems can help people monitor important indicators such as blood pressure, blood sugar and body temperature.Home medical monitoring and surveillance systems can help people control their health status, but these medical devices must be fast and efficient, and can ensure work when it matters most. With the development of portable medical sensors, the need for longer battery life and smaller form factor becomes increasingly critical for non-organized 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 (ADC), etc., in order to distinguish and identify sensor signals. In addition to the small size, it is also important that the analog circuit that reads the sensor output requires low-power operation so that it can provide longer battery life and more reading times. With the introduction of smaller and faster analog ICs, small, low-power medical devices powered by wall outlets 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, and digital thermometers.
Analog circuits for medical measurement
Some medical measurements require continuous operation of analog circuits, and thousands or even millions of readings are taken every second. Some applications only need to be read once a day. As far as these accidental tests are concerned, the analog circuit only needs to be powered up for measurement once, and then remains idle for the rest of the day, at which point it can be put into a 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 a digital result. The digital result can be stored in memory or displayed on the screen. For most portable medical sensor applications, the best choice for data converters will be the successive approximation register (SAR) ADC.
There are many reasons for choosing this type of ADC. First, SAR ADCs are very suitable for measuring signals from zero hertz (steady state) up to a few megahertz. These ADCs also have fast response and low latency 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 required for an ADC running at 10,000 samples per second (10ksps) will be lower than that running at 100ksps, and the power savings are significant. For example, a SAR ADC that converts data at a rate of millions of samples per second (Msps) may consume several milliamps of current, while the same SAR ADC operating at a sampling rate of 1ksps or lower may consume only tens of microamps.
Pulse oximeter
Pulse oximeter is an example of medical application that benefits from SAR ADC as the core. This device is used to measure blood oxygen content equivalent to hemoglobin in the patient's blood. The pulse oximeter detects the pulsation of blood in the artery, so it can also calculate the patient's heart rhythm. A pair of light-emitting diodes (LEDs) face a photodiode through the translucent part of the patient's body (usually the fingertip). A light emitter 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. This voltage is then measured by the ADC, so that after the light passes through the patient's body, the percentage of bleeding oxygen is read based on the absorption rate of the light at each wavelength (see Figure 1). The next step is usually to send digital data across an isolation device to the data acquisition system for storage or display on a monitor.
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 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 operate with 3V, 5V and ± 5V power supplies.
The output of sampling LT6202 is a 12-bit 3Msps SAR ADC. LTC2366 is a member of the miniature ADC family with sampling rates from 100ksps to 3Msps, fully compatible with pins and software. This series of ADCs consumes only 7.8mW at 3Msps, 1.5mW at 100ksps, and only 0.3μW in sleep mode. The characteristic of LTC2366 is that there is no data passing delay, so the sampled data can be obtained within the same clock cycle. The device provides sampling results through a 3-wire SPI / Microwire compatible interface.
The LTC2366 is available in a ThinSOT 6-pin or 8-pin package (8.1mm2), which helps the overall solution size of the pulse oximeter to be kept to a minimum. At 3Msps, it has a sufficiently rich sampling bandwidth to accurately sample the voltage through the amplifier and photodiode current. The LTC2366 operates on a single 3V power supply and can be powered by a single lithium-ion battery, multiple AA batteries, or a wall power system that wants to operate at low power.
There are 5 ADCs in the LTC236x series, including 3Msps LTC2366, 1Msps LTC2365, 500ksps LTC2362, 250ksps LTC2361 and 100ksps LTC2360. The above sampling rates are the highest sampling rates 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 details the relationship between the power supply current and sampling rate of the three lower-speed versions of the ADC. Based on the core design of SAR ADC itself, as the sampling rate decreases, the power consumption will drop rapidly.
Digital X-ray imaging
Another example of medical applications that require fast ADCs is digital X-ray imaging, including dental X-ray, computer-controlled axial tomography (CAT scan), or medical magnetic resonance imaging for full-body scanning. X-ray equipment manufacturers are now not storing images on a film, but digitally storing data. Doctors' offices or hospitals no longer store thousands of films in file cabinets, but can easily store the results in memory and quickly check the patient's medical records.
The second positive effect of using digital X-ray imaging is to bring comfort to patients. With the advent of faster ADCs, sensors, and signal conditioning modules, it can be much faster to check a patient ’s sore teeth or broken bones, which means that the patient needs to remain motionless for a much shorter period of time. With a faster X-ray examination process, the doctor's office or hospital can also see more patients in the same time.
Digital X-rays generally require an ADC with at least 12-bit resolution and a sampling rate of 1Msps or higher. This ADC must have a sampling rate higher than or equal to the refresh rate multiplied by the array size. This application generally requires multiple ADCs to resolve 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-constrained area, it is important to have a low-power SAR ADC with fast serial data communication. LTC2366 (3Msps) and LTC2365 (1Msps) based on the 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
The low power of the LTC2366 (7.8mW at 3Msps) means that designers can use multiple ADCs in close proximity without heating the system or disturbing the reading or the patient. Pipeline ADCs may consume 10 times the power at the same sampling rate.
Digital thermometer
Digital thermometers are another small and inexpensive device that can be used for patient care in homes or hospitals. They allow quick examination of the patient's body temperature by measuring the eardrum or placing it under the armpit. Analog circuits that detect temperature and convert it to digital readings can be quite simple. This type of application often uses a thermistor, a resistor that changes with temperature, because it has the highest sensitivity to human body temperature.
The LTC2450-1 is a 16-bit delta sigma ADC that is a bit like a SAR ADC. Its power also changes with the sampling rate. This type of delta sigma ADC is very suitable for certain medical applications. For example, LTC2450-1 can be directly connected to a thermistor (as shown in Figure 4) to provide accurate digital temperature readings. In this example, a fixed 10kΩ resistor in series with a temperature-dependent thermistor that varies between 1kΩ and 10kΩ allows the ADC to measure a wide analog input range. The input architecture of the LTC2450-1 allows it to measure high-impedance sensors, so an amplifier can be bypassed. The resistor network and decoupling capacitor can be directly connected to the analog input.
LTC2450-1 provides extremely low power supply current (guarantee a maximum of 0.5μA with temperature change), making it very suitable for digital thermometers working with a single battery. Most household digital thermometers will be placed in a drawer and work with sleep current, and only need to be powered up once in a while.
The LTC2450-1 provides an output rate of 60 samples per second, which is sufficient for digital thermometer measurements. It uses a 2mmx2mm package and communicates through the SPI protocol. The small package size and ability to connect directly to the thermistor make the overall analog solution extremely small. For designers who want to use the 2-wire protocol, single-ended and differential I2C versions are also available.
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