Disease Management & Wellness

The ADuCM355 is an on-chip system that controls and measures electrochemical sensors and biosensors. The ADuCM355 is an ultralow power, mixed-signal microcontroller based on the Arm^® Cortex^™-M3 processor. The device features current, voltage, and impedance measurement capability.

The ADuCM355 features a 16-bit, 400 kSPS, multichannel successive approximation register (SAR) analog-to-digital converter (ADC) with input buffers, built-in antialias filter (AAF), and programmable gain amplifier (PGA). The current inputs include three transimpedance amplifiers (TIA) with programmable gain and load resistors for measuring different sensor types. The analog front end (AFE) also contains two low power amplifiers designed specifically for potentiostat capability to maintain a constant bias voltage to an external electrochemical sensor. The noninverting inputs of these two amplifiers are controlled by on-chip, dual output digital-to-analog converters (DACs). The analog outputs include a high speed DAC and output amplifier designed to generate an ac signal.

The ADC operates at conversion rates up to 400 kSPS with an input range of −0.9 V to +0.9 V. An input mux before the ADC allows the user to select an input channel for measurement. These input channels include three external current inputs, multiple external voltage inputs, and internal channels. The internal channels allow diagnostic measurements of the internal supply voltages, die temperature, and reference voltages.

Two of the three voltage DACs are dual output, 12-bit string DACs. One output per DAC controls the noninverting input of a potentiostat amplifier, and the other controls the noninverting input of the TIA.

The third DAC (sometimes referred to as the high speed DAC) is designed for the high power TIA for impedance measurements. The output frequency range of this DAC is up to 200 kHz.

A precision 1.82 V and 2.5 V on-chip reference source is available. The internal ADC and voltage DAC circuits use this on-chip reference source to ensure low drift performance for all peripherals.

The ADuCM355 integrates a 26 MHz Arm Cortex-M3 processor, which is a 32-bit reduced instruction set computer (RISC) machine. The Arm Cortex-M3 processor also has a flexible multichannel direct memory access controller (DMA) supporting two independent serial peripheral interface (SPI) ports, universal asynchronous receiver/transmitter (UART), and I²C communication peripherals. The ADuCM355 has 128 kB of nonvolatile flash/EE memory and 64 kB of single random access memory (SRAM) integrated on-chip.

The digital processor subsystem is clocked from a 26 MHz on-chip oscillator. The oscillator is the source of the main digital die system clock. Optionally, a 26 MHz phase-locked loop (PLL) can be used as the digital system clock. This clock can be internally subdivided so that the processor operates at a lower frequency and saves power. A low power, internal 32 kHz oscillator is available and can clock the timers. The ADuCM355 includes three general-purpose timers, a wake-up timer (which can be used as a general-purpose timer), and a system watchdog timer.

The analog subsystem has a separate 16 MHz oscillator used to clock the ADC, DACs, and other digital logic on the analog die. The analog die also contains a separate 32 kHz, low power oscillator to clock a watchdog timer on the analog die. Both the 32 kHz oscillator and this watchdog are independent from the digital die oscillators and system watchdog timer.

A range of communication peripherals can be configured as required in a specific application. These peripherals include UART, I²C, two SPI ports, and general-purpose input/output (GPIO) ports. The GPIOs, combined with the general-purpose timers, can be combined to generate a pulse-width modulation (PWM) type output.

Nonintrusive emulation and program download are supported via the serial wire debug port (SW-DP) interface.

The ADuCM355 operates from a 2.8 V to 3.6 V supply and is specified over a temperature range of −40°C to +85°C. The chip is packaged in a 72-lead, 6 mm × 5 mm land grid array (LGA) package.

Note that, throughout this data sheet, multifunction pins, such as P0.0/SPI0_CLK, are referred to either by the entire pin name or by a single function of the pin, for example, P0.0, when only that function is relevant.

Applications

• Gas detection
• Food quality
• Environmental sensing (air, water, and soil)
• Blood glucose meters
• Life sciences and biosensing analysis
• Bioimpedance measurements
• General Amperometry, voltammetry, and impedance spectroscopy functions

The AD5940 and AD5941are high precision, low power analog front ends (AFEs) designed for portable applications that require high precision, electrochemical-based measurement techniques, such as amperometric, voltammetric, or impedance measurements. The AD5940/AD5941 is designed for skin impedance and body impedance measurements, and works with the AD8233 AFE in a complete bioelectric or biopotential measurement system. The AD5940/AD5941 is designed for electrochemical toxic gas sensing.

The AD5940/AD5941 consist of two high precision excitation loops and one common measurement channel, which enables a wide capability of measurements of the sensor under test. The first excitation loop consists of an ultra low power, dual-output string, digital-to-analog converter (DAC), and a low power, low noise potentiostat. One output of the DAC controls the noninverting input of the potentiostat, and the other output controls the noninverting input of the transimpedance amplifier (TIA). This low power excitation loop is capable of generating signals from dc to 200 Hz.

The second excitation loop consists of a 12-bit DAC, referred to as the high speed DAC. This DAC is capable of generating high frequency excitation signals up to 200 kHz.

The AD5940/AD5941 measurement channel features a 16-bit, 800 kSPS, multichannel successive approximation register (SAR) analog-to-digital converter (ADC) with input buffers, a built in antialias filter, and a programmable gain amplifier (PGA). An input multiplexer (mux) in front of the ADC allows the user to select an input channel for measurement. These input channels include multiple external current inputs, external voltage inputs, and internal channels. The internal channels allow diagnostic measurements of the internal supply voltages, die temperature, and reference voltages.

The current inputs include two TIAs with programmable gain and load resistors for measuring different sensor types. The first TIA, referred to as the low power TIA, measures low bandwidth signals. The second TIA, referred to as the high speed TIA, measures high bandwidth signals up to 200 kHz.

An ultra low leakage, programmable switch matrix connects the sensor to the internal analog excitation and measurement blocks. This matrix provides an interface for connecting external transimpedance amplifier resistors (R_TIAs) and calibration resistors. The matrix can also be used to multiplex multiple electronic measurement devices to the same wearable electrodes.

A precision 1.82 V and 2.5 V on-chip reference source is available. The internal ADC and DAC circuits use this on-chip reference source to ensure low drift performance for the 1.82 V and 2.5 V peripherals.

The AD5940/AD5941 measurement blocks can be controlled via direct register writes through the serial peripheral interface (SPI) interface, or, alternatively, by using a preprogrammable sequencer, which provides autonomous control of the AFE chip. 6 kB of static random access memory (SRAM) is partitioned for a deep data first in, first out (FIFO) and command FIFO. Measurement commands are stored in the command FIFO and measurement results are stored in the data FIFO. A number of FIFO related interrupts are available to indicate when the FIFO is full.

A number of general-purpose inputs/outputs (GPIOs) are available and controlled using the AFE sequencer. The AFE sequencer allows cycle accurate control of multiple external sensor devices.

The AD5940/AD5941 operate from a 2.8 V to 3.6 V supply and are specified over a temperature range of −40°C to +85°C. The AD5940 is packaged in a 56-lead, 3.6 mm × 4.2 mm WLCSP. The AD5941 is packaged in a 48-lead LFCSP.

Applications

• Electrochemical measurements
• Electrochemical gas sensors
• Potentiostat/amperometric/voltammetry/cyclic voltammetry
• Bioimpedance applications
  
  • Skin impedance
  • Body impedance
• Continuous glucose monitoring
• Battery impedance

The ADPD4100/ADPD4101 operate as a complete multimodal sensor front end, stimulating up to eight light emitting diodes (LEDs) and measuring the return signal on up to eight separate current inputs. Twelve time slots are available, enabling 12 separate measurements per sampling period.

The data output and functional configuration utilize an I²C interface on the ADPD4101 or a serial port interface (SPI) on the ADPD4100. The control circuitry includes flexible LED signaling and synchronous detection. The devices use a 1.8 V analog core and 1.8 V/3.3 V compatible digital input/output (I/O).

The analog front end (AFE) rejects signal offsets and corruption from asynchronous modulated interference, typically from ambient light, eliminating the need for optical filters or externally controlled dc cancellation circuitry. Multiple operating modes are provided, enabling the ADPD4100/ADPD4101 to be a sensor hub for synchronous measurements of photodiodes, biopotential electrodes, resistance, capacitance, and temperature sensors. The multiple operation modes accommodate various sensor measurements, including, but not limited to, photoplethysmography (PPG), electrocardiography (ECG), electrodermal activity (EDA), impedance, capacitance, temperature, gas detection, smoke detection, and aerosol detection for various healthcare, industrial, and consumer applications.

The ADPD4100/ADPD4101 are available in a 3.11 mm × 2.14 mm, 0.4 mm pitch, 33-ball WLCSP and 35-ball WLCSP

Applications

• Wearable health and fitness monitors: heart rate monitors (HRMs), heart rate variability (HRV), stress, blood pressure estimation, SpO2, hydration, body composition
• Industrial monitoring: CO, CO2, smoke, and aerosol detection
• Home patient monitoring

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 ADXL362 is an ultralow power, 3-axis MEMS accelerometer that consumes less than 2 μA at a 100 Hz output data rate and 270 nA when in motion triggered wake-up mode. Unlike accelerometers that use power duty cycling to achieve low power consumption, the ADXL362 does not alias input signals by undersampling; it samples the full bandwidth of the sensor at all data rates.

The ADXL362 always provides 12-bit output resolution; 8-bit formatted data is also provided for more efficient single-byte transfers when a lower resolution is sufficient. Measurement ranges of ±2 g, ±4 g, and ±8 g are available, with a resolution of 1 mg/LSB on the ±2 g range. For applications where a noise level lower than the normal 550 μg/√Hz of the ADXL362 is desired, either of two lower noise modes (down to 175 μg/√Hz typical) can be selected at minimal increase in supply current.

In addition to its ultralow power consumption, the ADXL362 has many features to enable true system level power reduction. It includes a deep multimode output FIFO, a built-in micropower temperature sensor, and several activity detection modes including adjustable threshold sleep and wake-up operation that can run as low as 270 nA at a 6 Hz (approximate) measurement rate. A pin output is provided to directly control an external switch when activity is detected, if desired. In addition, the ADXL362 has provisions for external control of sampling time and/or an external clock.

The ADXL362 operates on a wide 1.6 V to 3.5 V supply range, and can interface, if necessary, to a host operating on a separate, lower supply voltage. The ADXL362 is available in a 3 mm × 3.25 mm × 1.06 mm package.

Applications

• Hearing aids
• Home healthcare devices
• Motion enabled power save switches
• Wireless sensors
• Motion enabled metering devices

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 ADuM4160 provides an inexpensive and easy to implement isolation buffer for medical and industrial peripherals. The challenges that need to be met are:

1. Isolate directly in the USB D+ and D− lines allowing the use of existing USB infrastructure in microprocessors.
2. Implement an automatic scheme for data flow of control that does not require external control lines.
3. Provide medical grade isolation.
4. Allow a complete peripheral to meet the USB-IF certifi-cation standards.
5. Support full speed (12 Mbps) and low speed (1.5 Mbps) signaling rates.
6. Support flexible power configurations.

The circuit shown in Figure 1 isolates a peripheral device that already supports a USB interface. Because the peripheral is not explicitly defined in this circuit, power to run the secondary side of the isolator has been provided as part of the solution. If the circuit is built onto the PCB of a peripheral design, power could be sourced from the peripheral’s off line supply, a battery, or the USB cable bus power, depending on the needs of the application.

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

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.

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.

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.

Other suitable applications for this circuit are as an analog opto-isolator. It can also be adapted to applications that require larger bandwidth and less resolution such as adaptive speed control systems.

This circuit note discusses the design steps needed to optimize the circuit shown in Figure 1 for a specific bandwidth including stability calculations, noise analysis, and component selection considerations.

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.

Basic Operation of the ADXL362

The ADXL362 is a three-axis, ultralow power acceleration measurement system capable of measuring dynamic acceleration (resulting from motion or shock) as well as static acceleration (that is, gravity).

The moving component of the sensor is a polysilicon, surface micromachined structure, also referred to as a beam, built on top of a silicon wafer. Polysilicon springs suspend the structure over the surface of the wafer and provide a resistance against acceleration forces.

Deflection of the structure is measured using differential capacitors. Each capacitor consists of independent fixed plates and plates attached to the moving mass. Any acceleration deflects the beam and unbalances the differential capacitor, resulting in a sensor output whose amplitude is proportional to acceleration. Phase-sensitive demodulation is used to determine the magnitude and polarity of the acceleration.

Modes of Operation

The three basic modes of operation for the ADXL362 are standby, measurement, and wake-up.

• Placing the ADXL362 in standby mode suspends measurement and reduces current consumption to 10 nA. Any pending data or interrupts are preserved; however, no new information is processed. The ADXL362 powers up in standby mode with all sensor functions turned off.

• Measurement mode is the normal operating mode of the ADXL362. In this mode, acceleration data is continuously read, and the accelerometer consumes less than 3 μA across its entire range of output data rates of up to 400 Hz using a 2.0 V supply. All described features are available while operating in this mode. The ability to continuously output data from the minimum 12.5 Hz to the maximum 400 Hz data rate while still delivering less than 3 μA of current consumption is what defines the ADXL362, as an ultralow power accelerometer. Under sampling and aliasing do not occur with the ADXL362. because it continuously samples the full bandwidth of its sensor at all data rates.

• Wake-up mode is ideal for simple detection of the presence or absence of motion at extremely low power consumption (270 nA at a 2.0 V supply voltage). Wake-up mode is useful particularly for implementation of a motion-activated on/off switch, allowing the rest of the system to be powered down until activity is detected. Wake-up mode reduces current consumption to a very low level by measuring acceleration only 6 times a second to determine whether motion is present. In wake-up mode, all accelerometer features are available with the exception of the activity timer. All registers are accessible, and real-time data is available from the part.

The CN0274 evaluation software uses the wake-up mode of the ADXL362. That is, the ADXL362 is asleep until it detects motion at which point it enters measurement mode.

Power/Noise Tradeoff

The ADXL362 offers a few options for decreasing noise at the expense of only a small increase in current consumption.

The noise performance of the ADXL362 in normal operation, typically 7 LSB rms at 100 Hz bandwidth, is adequate for most applications, depending upon bandwidth and the desired resolution. For cases where lower noise is needed, the ADXL362 provides two lower noise, operating modes that trade reduced noise for somewhat higher supply current.

Table 1 shows the supply current values and noise densities obtained for normal operation and the two lower noise modes, at a typical 3.3 V supply.

The CN0274 evaluation software uses the normal operation noise mode of the ADXL362.

Motion Detection

The ADXL362 has built-in logic that detects activity (acceleration above a certain threshold) and inactivity (lack of acceleration above a certain threshold).

Detection of an activity or inactivity event is indicated in the status register and can also be configured to generate an interrupt. In addition, the activity status of the device, that is, whether it is moving or stationary, is indicated by the AWAKE bit.

Activity and inactivity detection can be used when the accelerometer is in either measurement mode or wake-up mode.

Activity Detection

An activity event is detected when acceleration stays above a specified threshold for a user-specified time period. The two activity detection events are absolute and referenced.

• When using absolute activity detection, acceleration samples are compared to a user set threshold to determine whether motion is present. For example, if a threshold of 0.5 g is set, and the acceleration on any axis is 1 g for longer than the user defined activity time, the activity status is asserted. In many applications, it is advantageous for activity detection to be based not on an absolute threshold but on a deviation from a reference point or orientation. This is particularly useful because it removes the effect on activity detection of the static 1 g imposed by gravity. When an accelerometer is stationary, its output can reach 1 g, even when it is not moving. In absolute activity, if the threshold is set to less than 1 g, activity is immediately detected in this case.
• In the referenced activity detection, activity is detected when acceleration samples are at least a user set amount above an internally defined reference, for the user defined amount of time. The reference is calculated when activity detection is engaged, and the first sample obtained is used as a reference point. Activity is only detected when the acceleration has deviated sufficiently from this initial orientation. The referenced configuration results in a very sensitive activity detection that detects even the most subtle motion events.

The CN0274 evaluation software uses the referenced mode of operation when searching for activity.

Inactivity Detection

An inactivity event is detected when acceleration remains below a specified threshold for a specified time. The two inactivity detection events are absolute and referenced.

• In absolute inactivity detection, acceleration samples are compared to a user set threshold for the user set time to determine the absence of motion.
• In referenced inactivity detection, acceleration samples are compared to a user specified reference for a user defined amount of time. When the part first enters the awake state, the first sample is used as a reference point, and the threshold is applied around it. If the acceleration stays inside the threshold, the part enters the asleep state. If an acceleration value moves outside the threshold, this point is then used as a new reference, and the thresholds are reapplied to this new point.

The CN0274 evaluation software uses the referenced mode of operation when searching for inactivity.

Linking Activity and Inactivity Detection

The activity and inactivity detection functions can be used concurrently, and processed manually by a host processor, or they can be configured to interact in several ways:

• In default mode, activity and inactivity detection are both enabled, and all interrupts must be serviced by a host processor; that is, a processor must read each interrupt before it is cleared and can be used again.
• In linked mode, activity and inactivity detection are linked to each other such that only one of the functions is enabled at any given time. Once activity is detected, the device is assumed moving or awake and stops looking for activity: inactivity is expected as the next event so only inactivity detection operates. When inactivity is detected, the device is assumed stationary or asleep. Activity is now expected as the next event so that only activity detection operates. In this mode, a host processor must service each interrupt before the next is enabled.
• In loop mode, motion detection operates as previously described in linked mode; however, interrupts do not need to be serviced by a host processor. This configuration simplifies the implementation of commonly used motion detection and enhances power savings by reducing the amount of power used in bus communication.
• When enabling autosleep mode in linked mode or loop mode, it causes the device to autonomously enter wake-up mode when inactivity is detected, and reenter measurement mode when activity is detected.

The CN0274 evaluation software uses the autosleep and loop modes to demonstrate the functionality of the ADXL362.

The AWAKE Bit

The AWAKE bit is a status bit that indicates whether the ADXL362 is awake or asleep. The device is awake when it has seen an activity condition, and the device is asleep when it has seen an inactivity condition.

The awake signal can be mapped to the INT1 or INT2 pin and can thus be used as a status output to connect or disconnect power to downstream circuitry based on the awake status of the accelerometer. Used in conjunction with loop mode, this configuration implements a trivial, autonomous motion-activated switch.

If the turn-on time of the downstream circuitry can be tolerated, this motion switch configuration can save significant system-level power by eliminating the standby current consumption of the rest of the application. This standby current can often exceed the full operating current of the ADXL362.

Interrupts

Several of the built-in functions of the ADXL362 can trigger interrupts to alert the host processor of certain status conditions.

Interrupts may be mapped to either (or both) of two designated output pins, INT1 and INT2, by setting the appropriate bits in the INTMAP1 and INTMAP2 registers. All functions can be used simultaneously. If multiple interrupts are mapped to one pin, the OR combination of the interrupts determines the status of the pin.

If no functions are mapped to an interrupt pin, that pin is automatically configured to a high impedance (high-Z) state. The pins are placed in this state upon a reset as well.

When a certain status condition is detected, the pin that condition is mapped to is activated. The configuration of the pin is active high by default, so that when it is activated, the pin goes high. However, this configuration can be switched to active low by setting the INT_LOW pin in the appropriate INTMAP register.

The INT pins may be connected to the interrupt input of a host processor and interrupts responded to with an interrupt routine. Because multiple functions can be mapped to the same pin, the STATUS register can be used to determine which condition caused the interrupt to trigger.

The CN0274 evaluation software configures the ADXL362 such that when activity is detected, the INT1 pin is high, and when inactivity is detected, the INT1 pin is low.

Test Results

All testing was performed using the EVAL-CN0274-SDPZ and the EVAL-SDP-CS1Z. Functionality of the part is demonstrated by setting the activity threshold at 0.5 g, the inactivity threshold at 0.75 g, and the number of inactivity samples at 20. When looking for activity, only one acceleration sample on any axis is required to cross the threshold.

Starting with the circuit oriented so that the battery pack is flat against the table, the printed circuit board (PCB) can be slowly rotated 90° in any direction causing the acceleration to cross the threshold as it approaches perpendicular to the initial orientation.

Figure 2 shows a screen shot of the CN0274 evaluation software showing the ADXL362 first asleep, looking for activity. Then, when Sample 11 crosses the threshold, the ADXL362 enters the awake state and begins looking for inactivity. The thresholds adjust to show the device is now looking for inactivity.

For better visibility, the X-axis and Z-axis plots are disabled using the radio buttons above the chart.

The output of the ADP195, or the interrupt pin itself, was measured using a digital multimeter. When the ADXL362 is awake, the interrupt goes high and drives the EN pin of the ADP195, high, which in turn drives the gate of the MOSFET low, causing the switch to close, connecting any downstream circuitry to the power supply. Conversely, when the ADXL362 is asleep, the interrupt drives the EN pin of the ADP195 low, which in turn drives the gate of the MOSFET high, causing the switch to open.

PCB Layout Considerations

In any circuit where accuracy is crucial, it is important to consider the power supply and ground return layout on the board. The PCB should isolate the digital and analog sections as much as possible. The PCB for this system was constructed in a 4-layer stack up with large area ground plane layers and power plane polygons. See the MT-031 Tutorial for more discussion on layout and grounding, and the MT-101 Tutorial for information on decoupling techniques.

Decouple the power supply to the ADXL362 with 1 μF and 0.1 μF capacitors to properly suppress noise and reduce ripple. Place the capacitors as close to the device as possible. Ceramic capacitors are advised for all high frequency decoupling.

Power supply lines should have as large a trace width as possible to provide low impedance paths and reduce glitch effects on the supply line. Shield clocks and other fast switching digital signals from other parts of the board by digital ground. A photo of the PCB is shown in Figure 3.

A complete design support package for this circuit note can be found at www.analog.com/CN0274-DesignSupport.

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 approach is highly useful in portable battery powered or multichannel applications, or where power dissipation is critical. It also provides benefits in applications where the ADC is idle most of the time between conversion bursts.

Drive amplifiers for high performance successive approximation ADCs are typically selected to handle a wide range of input frequencies. However, when an application requires a lower sampling rate, considerable power can be saved because reducing the sampling rate reduces the ADC power dissipation proportionally.

To take full advantage of the power saved by reducing the ADC sampling rate, a low bandwidth, low power amplifier is required. For instance, the 80 MHz ADA4841-1 op amp (12 mW at 10 V) is recommended for operation with the AD7988-1 16-bit successive approximation register (SAR) ADC (0.7 mW at 100 kSPS). The total system power dissipation including the ADR435 reference (4.65 mW at 7.5 V) is 17.35 mW at 100 kSPS.

For input bandwidths up to 1 kHz and sampling rates of 100 kSPS, the 3 MHz AD8641 op amp (2 mW at 10 V) offers excellent signal-to-noise ratio (SNR) and total harmonic distortion (THD) performance and reduces total system power from 17.35 mW to 7.35 mW, which is a 58% power savings at 100 kSPS.

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.

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.