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Instrumentation Amplifier Captures IoT Low Level Signal

o-leading. o-leading.com 2019-02-18 16:19:08
The realization of the Internet of Things (IoT) and smart home is inseparable from induction, and it is also the first problem faced by enthusiasts, manufacturers and even professional designers. Many relatively inexpensive sensors, such as accelerometers, force sensors, strain gauges, and pressure transmitters, are designed around resistive Wheatstone bridges, so they output differential voltages in millivolts (mV).

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These further low-level signals need to be accurately captured and amplified before further processing to achieve levels compatible with the microprocessor analog-to-digital converter (ADC) without DC offset and noise. Similarly, using a high-voltage side shunt for current sensing requires an amplifier that has no ground reference input and can withstand large common-mode voltages.
To ensure accurate data capture, manufacturers and enthusiasts need to be familiar with instrumentation amplifiers (INAs). The INA is a balanced differential amplifier with easily controllable gain, small offset drift and noise cancellation. It complements low-cost transmitters in home control applications. At the same time, since the INA has two high-impedance inputs that are not ground-referenced, it is also suitable for various types of floating differential measurements.

This article describes the sensor-processor signal chain and the need for common mode rejection, accuracy, and stability in the amplifier stage. Specific sensors and INAs and how to use them are also described.

Piezoresistive transmitter
Transmitters using piezoresistive components are one of the most popular sensor families. They can be used to measure stress, force, acceleration and pressure, and more.

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Smaller piezoresistive components are connected to the mechanical components of the transmitter. These elements may be strips, plates, springs, or diaphragms. The expected parameters sensed cause the mechanical structure to deform. Piezoresistive components are subject to stresses that are proportional to the inductive parameters that change the resistance of the component.
Instrumentation amplifier

INA is a differential amplifier based on op amp technology with differential inputs and single-ended outputs. The amplifier is a differential amplifier and therefore attenuates the common-mode signal, and the degree of attenuation is the CMRR specification mentioned earlier. Therefore, INA is well suited for amplifying small signals in the presence of large common-mode signals or offsets. In addition, INA features: stable, accurate and easily adjustable gain, high input impedance and low output impedance.

INA has two common circuit topologies available. The most common is the triple op amp design shown in the figure below. In this circuit configuration, amplifiers U1 and U2 are non-inverting input buffers. Their output will be fed to differential amplifier U3. The gain of the INA is mainly set by the resistor RG. The reference input is normally grounded when not in use, controlling the output offset voltage level. The sense input can be used to vary the gain of the output differential amplifier. When not in use, it is associated with the output of the differential stage.

Reduce the number of op amps required with a dual op amp topology
High side current detection
Using a low value resistor shunt is one of the most common methods of measuring current. For power measurements with several amplifiers, a resistor of approximately 10 milliohms (mΩ) produces a voltage drop of 10 mV per amplifier (below).

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If the shunt resistor is placed between the load and ground, it is called low side current sense. Placing the sense resistor between the power supply and the load is called high side current sense. High side sensing has the advantage of eliminating ground interference. It can also be used to detect load ground faults.
When performing high-side current detection, it is necessary to carefully study the common-mode voltage applied to the instrumentation amplifier, which will be discussed later.

If RSENSE is 10mΩ, a current swing of 5A will produce a voltage of 50mV in the resistor. Adjusting the gain of the INA to 100 will cause a 5V output swing.
to sum up

In the implementation of the design process, enthusiasts and professional engineers quickly discovered that connecting the sensor to the IoT first requires a good understanding of how to acquire and amplify the low-level signal from the Wheatstone bridge, and then use the ADC to convert it to the digital domain.
INA is ideal for amplifying differential signals. They offer high gain, high common mode rejection and high input impedance. Because they come in a variety of configurations, it's important to understand how it works, key specifications, and precautions for use.