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?


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