The care and feeding of ADCs in portable medical sensor apps - Embedded.com

The care and feeding of ADCs in portable medical sensor apps

This “Product How-To” article focuses how to use a certain product in an embedded system and is written by a company representative.

Home medical supervision and monitoring systems allow people to take control of their health, but these medical units must be quick, efficient and operate at the most important times. As portable medical sensors evolve, the need for longer battery life and smaller form factor becomes more critical for non-invasive care.

Medical measurement devices typically require some combination of signal conditioning, including amplifiers, filtering, a reference source and an ADC to resolve the sensor's signal. In addition to small size, the analog circuitry reading the sensor's output should feature low power to provide longer battery life and more readings over time.

Small, low-power medical devices powered by a wall socket are also gaining popularity due to the availability of smaller, faster analog ICs. Examples of medical units that require a small solution size and low power dissipation include blood analysis systems, pulse oximeters, digital X-rays and digital thermometers.

Analog circuitry
Some medical measurements require the analog circuitry to continuously run, taking thousands or even millions of readings per second. Other applications will require only a single reading per day. For these occasional tests, the analog circuitry only has to power up once, take the measurement and then sit idle running on a low-power “sleep” mode for the rest of the day.

The choice of analog ICs will depend on how often the sensor needs to take a reading. At the core of the analog circuitry is the ADC that converts the analog reading from the sensor into a digital result, which can be stored in memory or displayed on a screen. For most portable medical sensor applications, the best choice for the data converter will be a successive approximation register (SAR) ADC.

There are many reasons for choosing this type of ADC. 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. A SAR ADC's power dissipation varies with sample rate, unlike flash or pipeline ADCs.

Thus, the power dissipation from running the ADC at 10KSps will be lower than running at 100KSps. Power savings can be dramatic. For example, a SAR ADC converting data at a few million samples per second may dissipate a few milliamps while the same SAR ADC may dissipate only a few tens of microamps by running at 1KSps or less.

Portable electronics demand low-power electronics and small size. Low power is crucial as more designs migrate to lower voltage supplies and power budgets shrink. As board space becomes tighter with designers trying to integrate more electronics into less space especially in battery powered applications, a small package becomes vital.

Pulse oximetry
An example of a medical application that benefits from a SAR ADC at its core is a pulse oximeter, which measures the amount of oxygen compared to hemoglobin in a patient's blood. Pulse oximetry detects blood pulsing in the artery, thereby also calculating a patient's heart rate.

A pair of LED faces a photodiode through a translucent portion of a patient's body, usually on the finger tip. An optical transmitter triggers both a red LED (with a wavelength of 660nm) and an infrared LED (with a wavelength of 940nm). A photodiode receives the two signals and converts the current from the light into a voltage.

Figure 1: Shown is a pulse oximeter application that uses the LTC2366 12bit ADC.

An ADC then measures the voltage, thereby reading the percentage of blood oxygen based on the absorption rate from each wavelength of light after it passes through the patient's body (Figure 1 above ). Next, the digital data is transmitted – often across an isolation barrier – to the data collection system for storage or display on a monitor.

Linear Technology's LT6202 amplifier combines a gain bandwidth of 100MHz and low voltage noise, while consuming only 2.5mA. It also features low current noise, and it contributes low total noise and distortion power in small-signal applications.

The amplifier is specified for operation on 3-, 5- and 5V supplies. Sampling the output of the LT6202 is a 12bit 3MSps SAR ADC. The LTC2366 is part of a family of tiny ADCs sampling from 100KSps to 3MSps that are pin- and software- compatible.

The ADCs dissipate only 7.8mW at 3MSps, 1.5mW at 100KSps and 0.3 microwatts while in sleep mode. The LTC2366 features no data latency through the ADC, so data sampled is available within the same clock cycle. A threewire SPI/Microwire-compatible interface provides access to the sampled result.

Offered in a thin SOT 6-pin or 8-pin package (8.1mm2), the LTC2366 is good for keeping the total solution size of a pulse oximeter small. At 3MSps, it offers more than enough sampling bandwidth to correctly sample the voltage via the amplifier and photodiode current. Operating from a single 3V supply, the LTC2366 can be powered from a single Li+ cell, multiple AA cells or a wall-powered system wishing to run at a low power supply.

Figure 2: The LTC236x ADCs rapidly decrease their power consumption with sample rate.

The five ADCs in the LTC236x family include: LTC2366 at 3MSps; LTC2365 at 1MSps; LTC2362 at 500KSps; LTC2361 at 250KSps; and LTC2360 at 100KSps. The sample rate listed with each ADC is its maximum sample rate. For applications that don't need to run at 3MSps, each LTC236x ADC offers further power savings at lower sample rates.

Figure 2 above details the supply current compared with the sample rate for the three lower-speed versions, with power dissipation quickly decreasing with a slower sample rate due to the SAR ADC's inherent core design.

Digital X-rays
Another example of a medical application that requires a fast ADC is digital X-ray imaging, including usage such as dental X-rays, computed axial tomography or medical resonance imaging for complete body scans.

Rather than storing the image on a sheet of film, X-ray manufacturers are now storing the data digitally. Instead of storing thousands of sheets of film in a filing cabinet, a doctor's office or hospital can easily store the results in memory and quickly refer to a patient's file.

A second positive effect of switching to digital X-ray imaging is patient comfort. With the advent of faster ADCs, sensors and signal conditioning blocks, the patient's sore tooth or broken bone can be detected much more quickly. Thus, the patient needs to remain motionless for a much shorter period of time.

With a faster X-ray procedure, a doctor's office or hospital can also see more patients in the same amount of time. Digital X-rays typically require an ADC of at least 12bits of resolution and 1MSps or greater. The ADC must have a sample rate greater than or equal to the array size multiplied by the refresh rate. This application typically requires multiple ADCs to resolve all of the photodiode or CMOS imaging currents coming from the scintillator.

An array of photodiodes, amplifiers and ADCs are used to sample the full array (Figure 3 below ). A low-power SAR ADC with fast serial data communication is vital when multiple ADCs are placed in a space-constrained area.

LTC2366 (3MSps) and LTC2365 (1MSps) fit in these applications due to their 73dB SNR and zero data latency. The LTC2366's low power means that designers can use multiple ADCs in close proximity without the system heating up and disturbing the reading or the patient. At similar sampling rates, a pipeline ADC can draw 10 times as much power.

Figure 3: Shown is an example of digital X-ray imaging, which typically requires an ADC of at least 12bits of resolution and 1MSps or greater.

Digital thermometers
Another small and inexpensive device that can be used at home or in a hospital is the digital thermometer. They allow for a quick check of a person's temperature by measuring in the ear or under the arm.

The analog circuitry that senses temperature and converts to a digital reading can be rather simple. A thermistor, which is a resistor that varies with temperature, is often used because it offers the highest sensitivity for human body temperature.

The LTC2450-1 is a 16bit SigmaDelta ADC that somewhat resembles a SAR ADC, with its power scaling with sample rate. This type of SigmaDelta ADC is a good fit for some medical applications.

For example, the LTC2450-1 can be connected directly to a thermistor (Figure 4 below ), providing an accurate digital temperature reading. In this example, a fixed 10 kil-ohms resistor is placed in series with a thermistor that varies between 1-10 kil-ohm, depending on the temperature, allowing the ADC to measure a wide analog input range.

Figure 4: Shown is a battery-powered digital thermometer application with a fixed 10 k-ohm resistor placed in series with a thermistor that varies between 1-10k k-ohm, allowing the ADC to measure a wide analog input range.

The LTC2450-1's input architecture allows it to measure high impedance sensors. Thus, an amplifier can be bypassed. The resistor network, along with a decoupling capacitor, can be connected directly to the analog input.

The LTC2450-1 offers low supply current (0.5 microAmpere maximum guaranteed over temperature), making it a great fit for a digital thermometer operating from a single battery. Most digital thermometers for the home will sit in a drawer operating from the sleep current and only need to be powered up occasionally.

This ADC provides an output rate of 60Sps, more than enough for digital thermometer measurements. Packaged in a 2mm x 2mm package, it communicates via SPI protocol. The small package size and direct connection to the thermistor allow the total analog solution size to be extremely small.

The world's population continues to grow. With medical devices continuing to improve in accuracy, cost and simplicity, home medical care is becoming easier and faster. It is the analog signal chain that is helping to drive this transformation – from the sensor to the amplifiers and ADCs. With faster and lower power serial ADCs, medical devices will continue to improve in accuracy, speed and ease-of-use.

Steve Logan is Product Marketing Engineer at Linear Technology Corp.

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