Many industrial and medical applications use instrumentation amplifiers (INAs) to condition small signals in the presence of large common-mode voltages and DC potentials. The three operational amplifier (three op amp) INA architecture performs this function, where the input stage provides high input impedance and the output stage filters common mode voltage and provides differential voltage. The combination of high impedance and high common mode rejection ratio is key to many sensor and biometric applications such as flow sensors, temperature sensors, weighing devices, electrocardiograms (ECG) and blood glucose meters.
This article describes the basic operation of the three-op INA, analyzes the advantages of zero-drift amplifiers, RFI input filters, monitor sensor health, and programmable gain amplifiers, and lists sensor health monitors and active shield drivers (acTIve shield guard drive). ) Application examples of circuits.
Three operational amplifier INA basic operation
The nature of INA itself makes it suitable for conditioning small signals. Its combination of high impedance and high common-mode rejection ratio is ideal for sensor applications. High input impedance can be achieved by using the non-inverting input of the input stage without any feedback technique (see Figure 1). The three op amp circuit eliminates the common-mode voltage and amplifies the sensor signal with very little error, but must consider the input common-mode voltage (VCM) and differential voltage (VD) to avoid saturating the input stage of the INA.
A saturated input stage may appear to be normal to the processing circuit, but it has catastrophic consequences. The maximum design margin is provided by using an amplifier with rail-to-rail input and output (RRIO) configuration to help avoid input stage saturation. The following discussion describes the basic operation of the three op amp INA and gives an example of how the amplifier handles common mode and differential signals.
Figure 1 is a block diagram of the three op amp INA. According to the design, the input is divided into a common mode voltage VCM and a differential voltage VD. Among them, VCM is defined as the common voltage of the two inputs, which is the average of the sum of INA+ and INA-, and VD is defined as the net difference between INA+ and INA- (see Equation 1).
Formula 1:
Figure 1. Three op amps and their voltage nodes
Equation 2 gives the node voltage (INA+, INA-) generated on the INA input pin due to the application of the common-mode voltage and the differential voltage.
Equation 2:
In the non-saturation mode, the A1 and A2 operators apply a differential voltage on the gain setting resistor RG to generate a current ID:
Equation 3:
Therefore, the output voltages of A1 and A2 are:
Equation 4:
Substituting Equation 3 into Equation 4 yields:
Equation 5:
among them
Equation 5 shows only the differential component VD/2 amplified by the gain G1, and the common mode voltage VCM passes through the input stage with unity gain and is subsequently cancelled by the common mode rejection of the amplifier A3. This action helps the INA eliminate the common-mode signal from the desired differential signal to get the results we want. Differential signals from various sensors are often amplified 100 - 1000 times to obtain the sensitivity required for the measurement. Examples include precision weighing devices, medical instruments, Wheatstone bridges, and thermopile sensors.
Advantages of zero drift amplifier
Regardless of the process technology and architecture, the input offset voltage of all amplifiers varies with temperature and time. The manufacturer will provide specifications on the input offset voltage as a function of temperature (expressed in volts per degree Celsius). This specification for conventional amplifiers is a few microvolts to several tens of microvolts per degree Celsius. This offset drift can be problematic in high precision applications and cannot be calibrated during initial manufacturing. In addition to drift with temperature, the amplifier's input offset voltage drifts over time and causes large product life errors. For obvious reasons, the product data sheet does not include technical specifications for this drift.
By continuously self-correcting the offset voltage, drift is minimized with temperature and time, which is an inherent characteristic of zero-drift amplifiers. Some zero-drift amplifiers correct the offset voltage up to 10,000 times per second. Input offset voltage (VOS) is a key parameter and can cause DC errors when using INA to measure sensor signals. Zero-drift amplifiers such as the ISL2853x and ISL2863x provide very low offset drift of 5nV/C.
Zero-drift amplifiers also eliminate 1/f noise, or flicker noise (see Figure 2). The 1/f noise is noise generated by low frequency phenomena caused by irregularities in the conduction path and currents in the transistors. This makes the zero-drift amplifier ideal for low-frequency input signals close to DC, such as those from strain gauges, pressure sensors, and thermocouples. Taking into account the sampling and holding function of the zero-drift amplifier transforms it into a sampled data system, which makes it easy to produce aliasing and folding effects due to subtraction errors, which causes the wide-band components to fold into the baseband. However, under low frequency conditions, the noise changes slowly, so subtracting two consecutive noise samples can achieve true noise cancellation.
Figure 2. Noise Density in Semiconductors: From 1/f Noise to White Noise
The importance of RFI input filters
The increased use of wireless transceivers in portable applications has led to greater attention to the ability of electronic circuits to operate in the vicinity of high frequency radio transmitters, such as Bluetooth. This requires RF suppression to ensure that the sensor is not disturbed. In applications that are sensitive to electromagnetic interference (EMI), high frequency RF signals may appear as rectified DC offsets at the output of the precision amplifier. Because the precision of the precision front end can reach 100 or more, it must not amplify any conducted or radiated noise that may be present at the input of the amplifier. An easy way to solve this problem is to set up the RFI filter at the input of the INA as shown in Figure 3.
Figure 3. Input stage of an INA with RFI input filter
Sensor health monitoring
Being able to monitor any changes in the sensor over time helps to improve the robustness and accuracy of the measurement system. Direct measurements on the sensor are likely to affect the reading. One solution is to use the INA's input amplifier as a high impedance buffer. The ISL2853x and ISL2863x instrumentation amplifiers allow the user to operate the output of the input amplifier for this purpose only. VA+ is referenced to the non-inverting input of the differential amplifier, while VA- is referenced to the inverting input. These buffered pins can be used to measure the input common-mode voltage to provide sensor feedback and health monitoring. By connecting two resistors on VA+ and VA-, the buffered input common-mode voltage can be extracted at the midpoint of the two resistors (see Figure 4). This voltage can be sent to an analog-to-digital converter (ADC) for sensor monitoring or feedback control to continuously improve the accuracy and accuracy of the sensor.
Figure 4. VA+ and VA- pins can be connected to the output of the input gain stage
Programmable Gain Amplifier Benefits
It is widely accepted that discrete components cannot be used to build precision differential amplifiers with good CMR performance or gain accuracy. This is due to the matching of the four external resistors used to configure the op amp as a differential amplifier. Analysis shows that the resistor tolerance causes the upper limit of the CMR range to be as high as the limit of the op amp, and the lower limit is as low as -24.17Db2.
The integrated solution improves on-chip resistor matching, but when used to set the gain of an amplifier, there is still an absolute match with the external resistor. The deviation between the on-chip precision resistor value and the external resistor value may reach 20% or even 30%. Another source of error is the difference in thermal performance between the internal and external resistors. Internal and external resistors may have opposite temperature coefficients.
The way the programmable gain amplifier solves this problem is to make all resistors internal. The gain error of such an amplifier (see Equation 6) may be less than 1% and has an adjustment capability of ±0.05% typical and ±0.4% maximum (gain up to 500) under temperature variations.
Equation 6:
Intersil's ISL2853x and ISL2863x Series Programmable INA (PGIA) provide single-ended (ISL2853x) and differential (ISL2863x) outputs with three different gain sets. There are nine different gain settings for each gain set, as shown in Table 1. As shown at the bottom of each column, these gain sets are suitable for a particular application.
Table 1: Programmable Gain Values
Sensor Health Monitor and Active Shield Drive Application Examples
Sensor health monitor
The bridge sensor uses four matched resistive components to build a balanced differential circuit. The bridge can be a combination of discrete and resistive sensors for quarter bridge, half bridge and full bridge applications. The bridge is driven by a low noise, high accuracy voltage reference source located on two branches. The other two branches are differential signals whose output voltage changes are similar to those of the sensed environment. In a bridge circuit, the common mode voltage of the differential signal is the "midpoint" potential voltage of the bridge excitation source. For example, in a single-supply system using a +5V reference as the excitation source, the common-mode voltage is +2.5V.
The concept of sensor health monitoring is to track the bridge impedance in a data acquisition system. Changes in the environment, wear over time, or faulty bridge-type resistive components can cause the bridge to become unbalanced, causing measurement errors. Since the bridge differential output common-mode voltage is half the excitation voltage, the sensor's impedance health can be monitored by measuring the common-mode voltage (see Figure 5). By periodically monitoring the common-mode voltage of the bridge, we can understand the health of the sensor.
Figure 5. Sensor Health Monitoring Application Circuit Diagram
Active shield drive
Sensors that are remote from the signal conditioning circuit are subject to noisy environments during operation, reducing the signal-to-noise ratio into the amplifier. Differential signaling and shielded cables are two techniques used to reduce the noise of sensitive signal lines. Reducing the noise that the instrumentation amplifier cannot suppress (high-frequency noise or common-mode voltage levels beyond the rail) improves measurement accuracy. Shielded cables provide excellent signal line noise coupling suppression. However, cable impedance mismatch can cause common mode errors to enter the amplifier. The drive cable is shielded to a low impedance potential to reduce impedance mismatch. The cable shield is usually connected to the chassis ground because it is a very good low impedance point and easy to operate. This approach is very effective for dual-supply applications, but for single-supply amplifiers, this may not always be the best potential voltage for the shield.
In some data acquisition systems, the sensor signal amplifier uses a dual supply voltage (±2.5V). Connecting the shield to the analog ground (0V) places the shielded common-mode voltage exactly at the midpoint of the bias supply, which is where the amplifier's CMR performs best. As single-supply amplifiers (5V) are becoming a more popular choice for sensor amplifiers, the method of connecting the shield to the 0V position is currently connected to the lower supply rail of the amplifier, which is usually a common mode with reduced CMR performance. Voltage. Connecting the shield to the common-mode voltage of the midpoint supply voltage value will cause the amplifier to operate at optimum CMR performance.
Another solution to improve the shield drive is to use the VA+ and VA- pins of the ISL2853x and ISL2863x to sense the common-mode voltage and drive the shield to this voltage (see Figure 6). Use the VA+ and VA- pins to generate a low impedance reference source for the input common-mode voltage. Driving the shield to the input common-mode voltage reduces cable impedance mismatch and improves CMR performance for single-supply sensor applications. For further buffering of the shield drive circuit, additional unused op amps on the ISL2853x product can be used, eliminating the need to add an external amplifier.
Figure 6. Active Shield Application Circuit Diagram
in conclusion
Instrumentation amplifiers are an ideal choice for many sensor applications, but the right choice of amplifiers is as large as the different sensors being measured. The latest INA on the market already has many advantages. Users also need to trade off performance and price as always. If the application is for high-precision INA, the ISL2853x and ISL2863x are the ideal solution.
These amplifiers provide rail-to-rail inputs and outputs to ensure maximum dynamic range without saturating the input stage. They are zero-drift amplifiers that provide automatic offset voltage correction and noise reduction with very low offset voltage drift of 5nV/°C and low 1/F noise (corner frequency drops below 1Hz). The input has an RFI input filter for EMI-sensitive applications, and integrates precision matching resistors for the front-end gain stage and the differential second stage, providing very low gain error (±0.05%) and excellent CMR (138dB) ).
Precision performance makes these amplifiers ideal for analog sensor front-end, instrumentation, and data acquisition applications such as weighing devices, flow sensors, and shunt current sensing that require very low noise and high dynamic range.
About the author
Don LaFontaine is a Senior Principal Application Engineer at Intersil's Precision Products Line, based in Palm Cove, Florida. His work in the engineering field focuses on precision analog products. He holds a BSEE from the University of South Florida.
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