Vital Signs Measurements

The AD8233 is an integrated signal conditioning block for electrocardiogram (ECG) and other biopotential measurement applications. It is designed to extract, amplify, and filter small biopotential signals in the presence of noisy conditions, such as those created by motion or remote electrode placement. This design allows an ultralow power analog-to-digital converter (ADC) or an embedded microcontroller to easily acquire the output signal.

The AD8233 implements a two-pole, high-pass filter for eliminating motion artifacts and the electrode half cell potential.This filter is tightly coupled with the instrumentation amplifier architecture to allow both large gain and high-pass filtering in a single stage, thereby saving space and cost.

An uncommitted operational amplifier enables the AD8233 to create a three-pole, low-pass filter to remove additional noise. The user can select the frequency cutoff of all filters to suit different types of applications.

To improve the common-mode rejection of the line frequencies in the system and other undesired interferences, the AD8233 includes a right leg drive (RLD) amplifier for driven electrode applications. The AD8233 includes a fast restore function that reduces the duration of the otherwise long settling tails of the high-pass filters. After an abrupt signal change that rails the amplifier (such as a leads off condition), the AD8233 automatically adjusts to a higher filter cutoff. This feature allows the AD8233 to recover quickly, and therefore, to take valid measurements soon after connecting the electrodes to the subject.

The AD8233 is available in a 2 mm × 1.7 mm, 20-ball WLCSP package and a 150 μm thin die for height constrained applications. Performance is specified from 0°C to 70°C and is operational from −40°C to +85°C.

Applications

• Fitness and activity heart rate monitors
• Portable ECG
• Wearable and remote health monitors
• Gaming peripherals
• Biopotential signal acquisition, such as EMG or EEG

The ADAS1000 measures electro cardiac (ECG) signals, thoracic impedance, pacing artifacts, and lead-on/lead-off status and output this information in the form of a data frame supplying either lead/vector or electrode data at programmable data rates. Its low power and small size make it suitable for portable, battery-powered applications. The high performance also makes it suitable for higher end diagnostic machines.

The ADAS1000 is a full-featured, 5-channel ECG including respiration and pace detection, while the ADAS1000-1 offers only ECG channels with no respiration or pace features. Similarly, the ADAS1000-2 is a subset of the main device and is configured for gang purposes with only the ECG channels enabled (no respiration, pace, or right leg drive).

The ADAS1000/ADAS1000-1/ADAS1000-2 are designed to simplify the task of acquiring and ensuring quality ECG signals. They provide a low power, small data acquisition system for biopotential applications. Auxiliary features that aid in better quality ECG signal acquisition include multichannel averaged driven lead, selectable reference drive, fast overload recovery, flexible respiration circuitry returning magnitude and phase information, internal pace detection algorithm operating on three leads, and the option of ac or dc lead-off detection. Several digital output options ensure flexibility when monitoring and analyzing signals. Value-added cardiac post processing is executed externally on a DSP, microprocessor, or FPGA. Because ECG systems span different applications, the ADAS1000/ADAS1000-1/ADAS1000-2 feature a power/noise scaling architecture where the noise can be reduced at the expense of increasing power consumption. Signal acquisition channels can be shut down to save power. Data rates can be reduced to save power.

To ease manufacturing tests and development as well as offer holistic power-up testing, the ADAS1000/ADAS1000-1/ADAS1000-2 offer a suite of features, such as dc and ac test excitation via the calibration DAC and cyclic redundancy check (CRC) redundancy testing, in addition to readback of all relevant register address space. The input structure is a differential amplifier input, thereby allowing users a variety of configuration options to best suit their application.

The ADAS1000/ADAS1000-1/ADAS1000-2 are available in two package options, a 56-lead LFCSP package and a 64-lead LQFP package. Both packages are specified over a −40°C to +85°C temperature range.

APPLICATIONS

ECG: Monitor & Diagnostic

• Bedside Patient Monitoring
• Portable Telemetry
• Holter
• AED
• Cardiac Defibrillators
• Ambulatory Monitors
• Pace Maker Programmer
• Patient Transport
• Stress testing

AD8237 - это микропотребляющий инструментальный усилитель с нулевым дрейфом, обладающий rail-to-rail диапазонами входных и выходных напряжений (размах напряжения до напряжений питания). Коэффициент усиления может быть установлен равным любому значению в диапазоне от 1 до 1000 путем изменения отношения номиналов двух резисторов. AD8237 обладает превосходной погрешностью коэффициента усиления, которая поддерживается для любых настроек усиления при использовании двух резисторов с точным согласованием отношения номиналов.

Для обеспечения rail-to-rail размаха входных и выходных напряжений в AD8237 применяется архитектура с косвенной обратной связью по току. В отличие от стандартных измерительных усилителей AD8237 способен усиливать сигналы, синфазные напряжения которых равны напряжениям питания, или даже слегка выходят за их пределы. Это позволяет уменьшить напряжения питания и сократить потребляемую мощность в схемах с высокими синфазными напряжениями.

AD8237 является превосходным выбором для портативных систем. Благодаря минимальному напряжению питания 1.8 В, типичному потребляемому току 115 мкА и широкому диапазону входных напряжений AD8237 позволяет в полной мере использовать ограниченный бюджет энергопотребления, обеспечивая при этом показатели ширины полосы и дрейфа, которые отвечают требованиям к стационарным системам.

Области применения

• Усиление сигналов мостовых схем
• Измерение давления
• Медицинские измерительные приборы
• Интерфейс с термопарами
• Портативные системы
• Измерение тока

The circuit shown in Figure 1 provides a completely isolated connection between the popular USB bus and an RS-485 or RS-232 bus. Both signal and power isolation ensures a safe USB device interface to an industrial bus or debug port, allowing TIA/EIA-485/232 bus traffic monitoring and the convenience of sending and receiving commands to and from a PC that is not equipped with an RS-485 or RS-232 port.

Isolation in this circuit increases system safety and robustness by providing protection against electrical line surges and breaks the ground connection between bus and digital pins, thereby removing possible ground loops within the system.

The TIA/EIA RS-485 bus standard is one of the most widely used physical layer bus designs in industrial and instrumentation applications. RS-485 offers differential data transmission between multiple systems, often over very long distances. RS-485 communication offers additional robustness through differential communication when compared to the RS-232 standard.

TIA/EIA RS-232 devices are widely used in industrial machines, networking equipment, and scientific instruments. In modern personal computers, which are often used for debugging network problems, USB has displaced RS-232 from most of its peripheral interface roles, and many computers do not come equipped with RS-232 ports. The circuit in Figure 1 offers a robust and compact solution for both RS-232 and RS-485 interfaces.

The circuit shown in Figure 1 is an ultralow power, multichannel data acquisition system that can be powered by a photovoltaic (PV) cell or thermoelectric generator (TEG). The circuit uses the industry’s lowest power, multichannel, 12-bit successive approximation analog-to-digital converter (SAR ADC), the AD7091R-5, along with an efficient energy harvesting circuit based on the ADP5090 boost regulator. The ADC has a typical power consumption of 100 μW on a single 3 V supply when sampling at 22 kSPS. Typical signal-to-noise ratio (SNR) is 68 dB for a 1 kHz input signal.

The low power consumption and small form factor make this combination of devices ideally suited for portable low power applications, particularly for wearable and self-powered devices.

Figure 1. Low Power Data Acquisition System with Energy Harvesting Circuit (Simplified Schematic: All Connections and Decoupling Not Shown)

The circuit in Figure 1 is a complete single-supply, low noise LED current source driver controlled by a 16-bit digital-to-analog converter (DAC). The system maintains ±1 LSB integral and differential nonlinearity and has a 0.1 Hz to 10 Hz noise of less than 45 nA p-p for a full-scale output current of 20 mA.

The innovative output driver amplifier eliminates the crossover nonlinearity normally associated with most rail-to-rail input op amps that can be as high as 4 LSBs or 5 LSBs for a 16-bit system.

This industry-leading solution is ideal for pulse oximetry applications where 1/f noise superimposed on the LED brightness levels affects the overall accuracy of the measurement.

Total power dissipation for the three active devices is less than 20 mW typical when operating on a single 5 V supply.

Figure 1. ±1 LSB Linear 16-Bit LED Current Source Driver (Simplified Schematic: All Connections and Decoupling Not Shown)

The AD5933 and AD5934 are high precision impedance converter system solutions that combine an on-chipprogrammable frequency generator with a 12-bit, 1 MSPS (AD5933) or 250 kSPS (AD5934) analog-to-digital converter (ADC). The tunable frequency generator allows an external complex impedance to be excited with a known frequency.

The circuit shown in Figure 1 yields accurate impedance measurements extending from the low ohm range to several hundred kΩ and also optimizes the overall accuracy of the AD5933/AD5934.

Figure 1. Optimized Signal Chain for Impedance Measurement Accuracy (Simplified Schematic, All Connections and Decoupling Not Shown)

The circuit shown in Figure 1 is an ultralow, power data acquisition system using the AD7091R 12-bit, 1 MSPS SAR ADC and an AD8031 op amp driver with a total circuit power dissipation of less than 5 mW on a single 3 V supply.

The low power consumption and small package size of the selected components makes this combination an industryleading solution for portable battery-operated systems where power dissipation, cost, and size play a critical role.

The AD7091R requires typically only 350 μA of supply current on the V_DD pin at 3 V, which is significantly lower than any competitive ADC offering currently available in the market. This translates to ~1 mW typical power dissipation.

The AD8031 requires only 800 μA of supply current, that results in 2.4 mW typical power dissipation at 3 V supply, making the total power dissipation of the system less than 5 mW when sampling at 1 MSPS with a 10 kHz analog input signal.

Figure 1. 12-Bit, 1 MSPS Low Power ADC with Driver (Simplified Schematic: All Connections Not Shown)

The combination of parts shown in Figure 1 provides an ultralow power, 3-axis, motion activated power switch solution capable of controlling up to 1.1 A of load current. The circuit is ideal for applications where extended battery life is critical. When the switch is off, the battery current is less than 300 nA, and when the switch is on, it draws less than 3 μA. The circuit provides an industry leading, low power motion sensing solution suitable for wireless sensors, metering devices, home healthcare, and other portable applications.

The 3-axis accelerometer controls the high-side switch by monitoring the acceleration in three axes and closes or opens the switch depending on the presence or absence of motion.

The ADXL362 is an ultralow power, 3-axis accelerometer that consumes less than 100 nA in wake-up mode. Unlike accelerometers that use power duty cycling to achieve low power consumption, the ADXL362 does not alias input signals by under sampling; it samples continuously at all data rates. There is also an on-chip, 12-bit temperature sensor accurate to ±0.5°.

The ADXL362 provides 12-bit output resolution and has three operating ranges, ±2 g, ±4 g, and ±8 g. It is specified over a minimum temperature range of −40°C to +85°C. For applications where a noise level less than 480 μg/√Hz is desired, either of its two lower noise modes (down to 120 μg/√Hz) can be selected at a minimal increase in supply current.

The ADP195 is a high-side load switch designed for operation between 1.1 V and 3.6 V and is protected against reverse current flow from output to input. The device contains a low on-resistance, P-channel MOSFET that supports over 1.1 A of continuous load current and minimizes power losses.

Figure 1. Ultralow Power Standalone Motion Switch
(Simplified Schematic: Decoupling and All Connections Not Shown)

Basic Operation of the ADXL362

The universal serial bus (USB) is rapidly becoming the standard interface for most PC peripherals. It is displacing RS-232 and the parallel printer port because of superior speed, flexibility, and support of device hot swap. There has been a strong desire on the part of industrial and medical equipment manufacturers to use the bus as well, but adoption has been slow because there has not been a good way to provide the isolation required for connections to machines that control dangerous voltages or low leakage defibrillation proof connections in medical applications.

The ADuM4160 is designed primarily as an isolation element for a peripheral USB device. However, there are occasions when it is useful to isolate a host device. Several issues must be addressed to use the ADuM4160 for this application. Whereas the buffers on the upstream and downstream sides of the ADuM4160 are the same and capable of driving a USB cable, the downstream buffers must be capable of adjusting speed to a full or low speed peripheral that is connected to it.

Unlike the case of building a dedicated peripheral interface where the speed is known and not changed, host applications must adapt. The ADuM4160 is intended to be hardwired to a single speed via pins; therefore, it works when the peripheral plugged into its downstream side is the correct speed, but it fails when the wrong speed peripheral is attached. The best way to address this is to combine the ADuM4160 with a hub controller.

The upstream side of a hub controller can be thought of as a standard fixed speed peripheral port that can be easily isolated with the ADuM4160, whereas the speed of the downstream ports is handled by the hub controller. The hub controller converts peripherals of different speeds to match the upstream port speed. The circuit shown in Figure 1 shows how a two-port hub chip can be used to isolate two downstream host ports in a design that can be made fully compliant with the USB specification. 

The universal serial bus (USB) is rapidly becoming the standard interface for most PC peripherals. It is displacing RS-232 and the parallel printer port because of superior speed, flexibility, and support of device hot swap. There has been a strong desire on the part of industrial and medical equipment manufacturers to use this bus as well, but adoption has been slow because there has not been a good way to provide the isolation required for connections to machines that control dangerous voltages or low leakage defibrillation proof connections in medical applications.

The application circuit shown is typical of many medical and industrial applications. 

 

The universal serial bus (USB) is rapidly becoming the standard interface for most PC peripherals. It is displacing RS-232 and the parallel printer port because of superior speed flexibility and support of device hot swap. There has been a strong desire on the part of industrial and medical equipment manufacturers to use the bus as well, but adoption has been slow because there has not been a good way to provide the isolation required for connections to machines that control dangerous voltages or low leakage defibrillation proof connections in medical applications.

The ADuM4160 is designed primarily as an isolation element for a peripheral USB device. However, there are occasions when it is useful to create an isolated cable function. Several issues must be addressed to use the ADuM4160 for this application. Whereas the buffers on the upstream and downstream sides of the ADuM4160 are the same and capable of driving a USB cable, the downstream buffers must be capable of adjusting speed to a full or low speed peripheral that is connected to it. The upstream connection must act like a peripheral, and the downstream connection must behave like a host.

Unlike the case of building a dedicated peripheral interface where the speed is known and not changed, host applications must adapt to detect whether a low or full speed device has been connected. The ADuM4160 is intended to be hardwired to a single speed via pins; therefore, it works when the peripheral plugged into its downstream side is the correct speed, but it fails when the wrong speed peripheral is attached. The best way to address this is to combine the ADuM4160 with a hub controller.

The upstream side of a hub controller can be thought of as a standard fixed speed peripheral port that can be easily isolated with the ADuM4160, whereas the downstream ports are all handled by the hub controller. However, in many cases, while it is not certifiable as fully USB compliant, a single speed cable is acceptable from a practical standpoint, especially if custom connectors are used so that it cannot be confused with a compliant device. The hub chip can be eliminated, and the design becomes very small and simple.

The ADuM4160 provides an inexpensive and easy way to implement an isolation buffer for medical and industrial peripherals. The challenge that must be met is to use this to create a bus-powered cable isolator by pairing the ADuM4160 with a small isolated dc-to-dc converter such as the ADuM5000. As with isolating any device, the services that the ADuM4160 provides are as follows: The goal of the application circuit shown in Figure 1 is to isolate a peripheral device that already implements a USB interface. It is not possible to make a fully compliant bus-powered cable because there are no 100% efficient power converters to transfer the bus voltage across the barrier. In addition, the quiescent current of the converter does not comply with the standby current requirements of the USB standard. This is all in addition to the speed detection limitations of the ADuM4160. What can be achieved is a fixed speed or switch-controlled speed cable that can supply a modest power to the downstream peripheral. However, it is a custom application that is not completely compliant with the USB standard.

 

The ADXL345 is a small, thin, low power, 3-axis accelerometer with high resolution (13-bit) measurement up to ±16 g. Digital output data is formatted as 16-bit twos complement and is accessible through either an SPI (3- or 4-wire) or I²C digital interface.

The ADXL345 is well suited for mobile device applications. It measures the static acceleration of gravity in tilt-sensing appli-cations, as well as dynamic acceleration resulting from motion or shock. Its high resolution (4 mg/LSB) enables measurement of inclination changes of about 0.25°. Using a digital output accelerometer such as the ADXL345 eliminates the need for analog-to-digital conversion, reducing system cost and real estate. Additionally, the ADXL345 includes a variety of built-in features. Activity/inactivity detection, tap/double-tap detection, and free-fall detection are all done internally with no need for the host processor to perform any calculations. A built-in 32-stage FIFO memory buffer reduces the burden on the host processor, allowing algorithm simplification and power savings. Additional system level power savings can be implemented using the built-in activity/inactivity detection and by using the ADXL345 as a “motion switch” to turn the whole system off when no activity is felt and on when activity is sensed again.

The ADXL345 communicates via I²C or SPI interface. The circuits described in this document demonstrate how to implement communication via these protocols.

Figure 2. ADXL345 and ADuC7024 in I²C Configuration (Simplified Schematic: Decoupling and All Connections Not Shown)

The circuit shown in Figure 1 is a cost effective, low power, multichannel data acquisition system that is compatible with standard industrial signal levels. The components are specifically selected to optimize settling time between samples, providing 18-bit performance at channel switching rates up to approximately 750 kHz.


The circuit can process eight gain-independent channels and is compatible with both single-ended and differential input signals.

The analog front end includes a multiplexer, programmable gain instrumentation amplifier (PGIA); precision analog-to-digital converter (ADC) driver for performing the single-ended to differential conversion; and an 18-bit, 1 MSPS PulSAR^® ADC for sampling the signal on the active channel. Gain configurations of 0.4, 0.8, 1.6, and 3.2 are available.

The maximum sample rate of the system is 1 MSPS. The channel switching logic is synchronous to the ADC conversions, and the maximum channel switching rate is 1 MHz. A single channel can be sampled at up to 1 MSPS with 18-bit resolution. Channel switching rates up to 750 kHz also provide 18-bit performance. The system also features low power consumption, consuming only 240 mW at the maximum ADC throughput rate of 1 MSPS.

The circuit shown in Figure 1 is a 16-bit, 100 kSPS successive approximation analog-to-digital converter (ADC) system that has a drive amplifier that is optimized for a low system power dissipation of 7.35 mW for input signals up to 1 kHz and sampling rates of 100 kSPS.

This circuit is a highly integrated electrocardiogram (ECG) front end for use in battery powered patient monitoring applications.

Figure 1 shows a top level diagram of the physical connections in a typical 5-lead (four limb and one precordial chest lead) ECG measurement system, including features such as respiration and pace detection. The configuration is typical for portable telemetry ECG measurements or a minimum lead set from a line powered bedside instrument.

An ECG signal has a small amplitude of typically around 1 mV when measured on the surface of the skin. Important information about the health or other characteristics of the patient is stored within that small signal; therefore, requiring measurement sensitivity in the μV level. At the system level, various medical standards call for a maximum of 30 μV p-p noise; however, designers typically target less than this. As a result, when designing a solution suitable for system level requirements, all noise sources must be considered.

The noise performance of the ADAS1000 is specified across various different operating conditions. The power supply must be designed to ensure that it does not degrade the overall performance. Selection of the ADP151 linear regulator was based on its ultralow noise performance (9 μV rms typical, 10 Hz to 100 kHz), coupled with the power supply rejection of the ADAS1000, ensures that the noise of the ADP151 does not degrade the overall noise performance.

Figure 1. Simplified Block Diagram Showing the ADAS1000 as Used in a Typical 4-Electrode + RLD or 5-Lead Configuration (All Connections and Decoupling Not Shown)

The circuit shown in Figure 1 is a high speed photodiode signal conditioning circuit with dark current compensation. The system converts current from a high speed silicon PIN photodiode and drives the inputs of a 20 MSPS analog-to-digital converter (ADC). This combination of parts offers spectral sensitivity from 400 nm to 1050 nm with 49 nA of photocurrent sensitivity, a dynamic range of 91 dB, and a bandwidth of 2 MHz. The signal conditioning circuitry of the system consumes only 40 mA of current from the ±5 V supplies making this configuration suitable for portable high speed, high resolution light intensity applications, such as pulse oximetry. 

The circuit in Figure 1 is a two-channel, bank isolated, wide bandwidth data acquisition (DAQ) system, implemented with a simultaneous sampling architecture using an analog-to-digital converter (ADC) per channel. The system achieves high channel density along with isolation between the bank and the digital backplane, all while delivering exceptional performance. The design also makes efficient use of isolation channels by configuring the ADCs in daisy-chain mode and utilizing an isolator product with a trimmed delay clock feature. Power generation is also simplified using an isolator with an integrated pulse width modulation (PWM) controller and transformer driver to perform dc-to-dc conversion across the isolation barrier. The system also includes many common features of a typical DAQ signal chain, including input circuit protection, programmable gain channels, high accuracy, and high performance.

The simultaneous sampling realizes multiple channels without sample rate limitations inherent in multiplexed DAQ signal chains. The analog front end (AFE) design is also simpler than the multiplexed option, because the settling performance requirements of the system are less demanding. Sampling occurs simultaneously for each channel, while sequential sampling systems have delays between channels.

Digital bank isolated DAQ designs provide protection for digital back end circuitry and reduce ground loop and common-mode interference between banks. They feature multiple DAQ signal chains per ground plane, and can be implemented with fewer digital isolation devices than channel-to-channel isolated systems.