In Search of the Best Op Amp for Remote Devices

Portable and remote devices are integral to medical, home, and business systems that manage the collection of analog data. The trend today is to create smaller, more energy efficient devices to shrink equipment size and lengthen system battery life for performing tasks such as medical monitoring, room occupancy, or noxious gas detection.

This article identifies three portable/remote circuits which require an amplifier front-end. The task at hand will be to define the critical amplifier specifications for each circuit, to identify the best amplifier for all three applications.


Whether the portable or remote device is a medical, home, or business gadget, smaller size and lower power are becoming critical requirements for these designs (Figure 1).

Figure 1. Ultra-Mini Glucose Meters are Increasing in Capability While Shrinking in Size.

In these applications, the operational amplifier provides the critical gain and filtering functions for the sensor signal. The ideal operational amplifier that fits into these application spaces will complement these trends toward smaller size and ultra-low-power operation. For these systems there are several players in the game. So, the design challenge is to not only build in power and size enhancements, but to go beyond and find the amplifier with precisely the right specifications.

Whether sensing blood glucose level, counting occupancy numbers, or measuring the concentration level of gases, the front-end amplifier is an integral part of the entire system. The connection between the sensor’s output and the amplifier’s input terminal is a delicate juncture where the amplifier’s input characteristics dramatically affect the successful operation of the portable or remote device.

To find the optimal solution, we will discuss three systems:

  • Glucose monitor
  • Occupancy Sensor
  • Gas detector

Even though there are many brands of blood glucose test strips, each with their own technology, they all fundamentally function the same way. A two-electrode test strip is composed of several layers. As shown in Figure 2, the test strip layers channel the blood sample to a reaction center and add glucose reactant enzymes and a chemical mediator to speed-up electrons along the test strip’s interior.


Figure 2. Two-electrode, diabetes test strips capture the patient’s blood and transport it to the testing electrodes.

The bottom layer of the strip has a gold and palladium-coated trace to transfer the reaction electrons to the amplifier’s input (Figure 3). The test strip’s full-scale output current ranges from 10μA to 50μA, with a resolution less than 10nA.

Figure 3. A Single-Supply Dual Amplifier at the Front-End of the Blood Glucose Monitor

In Figure 3, the test strip output current, IS, flows through U1’s feedback resistor, RS, creating a voltage equal to RS times IS. The second amplifier (U2) implements a 5V/V gain and a 7Hz lowpass filter (RL/CL). The size of glucose monitors continues to shrink while increasing in functionality. Today these monitors provide an interface screen along with a data sync to mobile apps, a display, and data logging. Due to the battery-powered environment, these amplifiers must be nanopower-capable and housed in a small package.

These characteristics are important, but the amplifier must also have the precise characteristics that the test strip requires. The test strip’s ampere magnitude dictates the use of amplifiers with extremely low input bias current to complement the minimum 10nA resolution. The amplifier input must also have a common mode range that extends to the negative power supply as well as an output that swings rail-to-rail. A passive infrared sensor (PIR sensor) is an electronic sensor that measures infrared (IR) radiation emitted by objects in the PIR’s field of view. A home or business intrusion detector is a perfect application for the PIR sensor.

The PIR sensor produces a small millivolt signal by detecting radiated temperature changes such as those from the movement of humans. This sensor does not produce a voltage output for static incidences or temperatures. Depending on the proximity of the passing person, the PIR’s output voltage increases or decreases as the object enters or leaves the PIR’s field of vision, which happens within a 0.5Hz to 7Hz frequency range (Figure 4).

Figure 4. PIR Motion Detection Circuit Using a Dual Single-Supply Operational Amplifier

In Figure 4, the PIR’s bias DC voltage output is approximately 1V. The two amplifier stages are exact duplicates, with a gain of ~46.4 and a 0.5Hz (zero) to 7Hz (pole) second-order bandpass filter, which matches the expected frequency range of passing humans. With these two second-order bandpass filters, the DC signals, including the amplifier’s offset voltages, do not pass to the circuit output (OUT). The two second-order bandpass filters require an adequate amplifier bandwidth of 8kHz or greater.

The battery life of today’s motion detection systems is approximately one year. With future upgrades, devices with lower power consumption and smaller component sizes will be expected. Due to this battery-powered environment, the amplifiers for this solution must also be nanopower-capable and housed in a small package.

A gas detector is a device that senses the presence of gases in an area. As an example, carbon monoxide (CO) is an odorless, colorless gas produced by burning fuel. Sometimes, due to a stove, lantern, grill, or furnace, the CO’s concentration can build in the room or facility to harmful levels. The gas detector’s function is to sense the CO’s concentration level and interface with a control system that notifies the user and/or shuts down the offending system.

The gas detector front-end circuit shown in Figure 5 detects various types of gases, as determined by the gas sensor type. In Figure 5, U1 energizes the sensor with a constant DC voltage (VREF1) at the sensor reference electrode (RE). U2, configured as a transimpedance amplifier, changes the sensor output current (ISENSE) into a voltage (VOUT). The output voltage is equal to the sensor’s output current (ISENSE) times the amplifier’s feedback resistor (R3). The ISENSE polarity depends on the type of sensor. 

Figure 5. Gas Detection Circuit Using a Single-Supply Dual Operational Amplifier

Due to the transimpedance configuration, this circuit requires amplifiers with extremely low input bias currents in the picoamp range. Combined with the battery-powered environment, these amplifiers must be nanopower-capable and housed in an extremely small package.

As mentioned earlier, devices for portable and remote applications require power-conscious components in small packages while still meeting critical electrical performance specifications. The key amplifier specifications that our application circuits required were common-mode input ranges to the negative supply, rail-to-rail output swing, and ample bandwidth. An amplifier which meets the criteria for the three circuits discussed is the MAX40018, a nanoPower dual operational amplifier in a WLP package. This device consumes the lowest power with the smallest packaging in its class (Figure 6).

Figure 6. Dual Operational Amplifier Quiescent Current vs. Package Size

Figure 6 compares four dual operational amplifiers. Of the four, the bottom left dual amplifier outperforms the others with 400nA quiescent current per operational amplifier with a 1.488mm2 wafer level package (WLP) housing. The glucose meter, the occupancy sensor, and the gas meter are appropriate fits for the nanoPower, tiny dual amplifier.

Battery-powered portable and remote devices demand small, ultra-low power components. Operational amplifiers used in these applications must meet these power conditions without compromising key performance specifications. We examined how a dual nanopower operational amplifier fits directly into these requirements with ultra-low power consumption, a small chip-size package, a sub-pico ampere input bias current, and a unity gain bandwidth greater than 8kHz. Who could ask for more?


Related


Slimming program for medical operating devices

Operating devices in the medical sector are not only subject to strict controls and requirements. Nowadays design demands are becoming more and more important for developers of medical HMI devices. De...

 

DIN-Rail Embedded Computers from MEN Mikro

The DIN-Rail system from MEN is a selection of individual pre-fabricated modules that can variably combine features as required for a range of embedded Rail Onboard and Rail Wayside applications. The ...


Embedded Graphics Accelerates AI at the Edge

The adoption of graphics in embedded and AI applications are growing exponentially. While graphics are widely available in the market, product lifecycle, custom change and harsh operating environments...


ADLINK Optimizes Edge AI with Heterogeneous Computing Platforms

With increasing complexity of applications, no single type of computing core can fulfill all application requirements. To optimize AI performance at the edge, an optimized solution will often employ a...


Synchronized Debugging of Multi-Target Systems

The UDE Multi-Target Debug Solution from PLS provides synchronous debugging of AURIX multi-chip systems. A special adapter handles the communication between two MCUs and the UAD3+ access device and pr...


Smart Panel Fulfills Application Needs with Flexibility

To meet all requirement of vertical applications, ADLINK’s Smart Panel is engineered for flexible configuration and expansion to reduce R&D time and effort and accelerate time to market. The...


AAEON – Spreading Intelligence in the connected World

AAEON is moving from creating the simple hardware to creating the great solutions within Artificial Intelligence and IoT. AAEON is offering the new solutions for emerging markets, like robotics, drone...


ASIC Design Services explains their Core Deep Learning framework for FPGA design

In this video Robert Green from ASIC Design Services describes their Core Deep Learning (CDL) framework for FPGA design at electronica 2018 in Munich, Germany. CDL technology accelerates Convolutional...


Microchip explains some of their latest smart home and facility solutions

In this video Caesar from Microchip talks about the company's latest smart home solutions at electronica 2018 in Munich, Germany. One demonstrator shown highlights the convenience and functionalit...


Infineon explains their latest CoolGaN devices at electronica 2018

In this video Infineon talks about their new CoolGaN 600 V e-mode HEMTs and GaN EiceDRIVER ICs, offering a higher power density enabling smaller and lighter designs, lower overall system cost. The nor...


Analog Devices demonstrates a novel high-efficiency charge pump with hybrid tech

In this video Frederik Dostal from Analog Devices explains a very high-efficiency charge-pump demonstration at their boot at electronica 2018 in Munich, Germany. Able to achieve an operating efficienc...


Microchip demonstrates a flexible motion control platform at electronica

In this video Marcus from Microchip explains a motion control demonstration at their booth at electronica 2018 in Munich, Germany. The demonstration underscores the ability of the solution to rapidly ...


Infineon goes over their latest SiC devices for automotive systems

In this video an Infineon engineer goes over their latest Silicon Carbide (SiC) devices for automotive systems at electronica 2018 in Munich, Germany. Among the devices described are an inverter for a...


Bertrand Lombardo of Honeywell, Sensing requirements of IoT

Bertrand Lombardo, Sales director for EMEA for Honeywell SIOT discusses future sensing trends in relation to IoT at Electronica 2019 with Alix Paultre. Links to more information: Dynamic Hone...


Analog Devices updates their Silent Switcher technology

In this video an FAE from Analog Devices explains the latest version of their Silent Switcher technology, which addresses noise issues in power systems. He describes a live demonstration in their boot...


Western Digital talks about their automotive-grade memory solutions

In this video Martin Booth from Western Digital talks about the company's memory solutions specifically designed for automotive applications and the harsh environments involved. Systems such as ne...


Picotest demonstrates their latest advanced power test solutions

In this video Steve Sandler from Picotest shows us two of the company's latest test solutions at electronica 2018 in Munich, Germany. The first demo is of a micro-Ohm-resolution power rail measure...


STMicro describes their latest smart 48V DC brushless motor driver board

In this video an engineer from STMIcroelectronics explains a motor-driver board setup based on their L9907 smart power device at electronics 2018 in Munich, Germany. Based on BCD-6s technology. the de...


Microchip shows their newest PolarFire FPGAs at electronica 2018

In this video Microchip shows a one of the demos highlighting the capabilities of their newest low-power PolarFire FPGAs at electronica 2018 in Munich, Germany. The demonstration shown here is a kit f...


Western Digital discusses their memory solutions for Cloud-enabled devices

In this video Ze'ev Paas of Western Digital talks to Alix Paultre of Aspencore Media about their latest memory products at electronica 2018 in Munich, Germany. Depending on the application space, ...


Picotest explains a couple of power test systems at electronica 2018

In this video Steve Sandler from Picotest explains a couple of his power test systems at electronica 2018 in Munich, Germany. The first demonstration shows a micro-Ohm measurement system, and the seco...