Application of automatic zeroing amplifier in strain tester

Introduction

Strain gauge sensors are widely used. They adopt a bridge circuit structure to improve output sensitivity. However, the output of a micro-strain bridge is only about 2 mV. Even under full load, the maximum output of the strain gauge is only tens of mV. This requires the pre-measurement amplifier circuit to have high gain, high precision, low noise, low drift, etc. Features. Generally, integrated operational amplifiers use direct coupling or resistance-capacitance coupling based on the parameter compensation principle. Their initial offset parameters are not equal to zero. Instead, they use zero-adjusting potentiometers or precision correction techniques to compensate for the offset parameters. This allows the direct coupling amplifier to amplify the signal while also amplifying the temperature drift. Although the resistor-capacitive coupling amplifier can suppress the temperature drift, it cannot be used to amplify weak DC signals or slowly changing signals. It will treat this signal as a temperature drift. Suppress drift. The precision instrumentation amplifier AD8230 using automatic zero stabilization technology can well solve the problem of suppressing temperature drift while amplifying weak DC signals to meet the design requirements of precision strain testers.

The working principle and characteristics of AD8230

AD8230 is a zero-stable precision instrumentation amplifier produced by ADI using dynamic zero calibration technology and using ultra-small SOIC technology. Compared with the industry standard AD62x series instrumentation amplifiers, the AD8230 has many key performance improvements: it has a high input impedance of 109W, which can effectively suppress errors caused by asymmetry in the impedance of the signal source and the transmission network; it operates at -40℃~+125℃ Within the operating temperature range, the input offset voltage is 10mV, the offset voltage temperature drift is only 50nV/°C, and the common mode rejection ratio is as high as 140dB, which can effectively suppress errors caused by common mode interference and improve the system signal-to-noise ratio and resistance to temperature effects. ;The input/output swing can reach the power limit (-VS~+VS) to adapt to a wider range of signal source levels; it has higher gain and a wider gain adjustment range (G=2~1000), which The typical gain error is ±0.01%, and the gain non-linear error is only 20ppm, effectively ensuring the measurement accuracy of the system.

The amplifier gain is set by two external resistors for temperature coefficient (TC) matching.

The AD8230 determines the zero output voltage by the reference terminal potential. It is particularly useful when the load and the system ground are not clearly connected to the same ground. It provides a method to introduce precise compensation to the output. The reference terminal can also provide a virtual ground voltage amplification bipolar. Signal. If the AD8230 outputs relative to ground, the reference terminal should be connected to ground. In order to minimize the ground loop impedance and achieve the best CMR, the reference terminal should be connected to a low-impedance contact. It is recommended to use a ground plane.

Auto-zero (or auto-zero stabilization) is a technology that dynamically offsets the offset voltage and offset voltage drift. It can reduce the offset voltage at the relative input end to the mV level and reduce the offset voltage drift to the nV/°C level. Another advantage of dynamic offset cancellation is that it reduces low-frequency noise, especially 1/f noise. The basic guiding idea of ​​the automatic zero-stabilizing operational amplifier is: if the output voltage (error voltage) when the two input terminals of the operational amplifier are short-circuited or a common-mode input signal is added, first register it with a capacitor (referred to as sampling), and then combine it with the operational amplifier Subtracting the output voltage during normal operation (referred to as zero calibration) can effectively reduce the offset voltage, offset current, and drift caused by temperature changes and power supply voltage fluctuations, and can also effectively suppress common-mode signals.

AD8230 has an auto-zero circuit structure, and its internal signal path consists of an active differential sample and hold stage (preamplifier) ​​and a differential amplifier stage (gain amplifier). Both amplifier stages can achieve auto-zeroing to minimize offset and drift, and the fully differential circuit structure enhances resistance to spurious noise. The basic principle of automatic zero stabilization is shown in Figure 1. Here, two consecutive clock phases A and B are used to describe its internal working sequence.

The circuit switches between two stages of cyclic operation through an electronic switch: in the first half cycle of the clock, the circuit is in the sampling stage, the sampling capacitor CSAMPLE is connected to the signal input terminal, the differential voltage VDIFF of the input signal is stored on CSAMPLE, and the common mode voltage is suppressed . During this period, the gain amplifier is disconnected from the preamplifier so that its output remains at the previously sampled input signal amplitude; during the second half of the clock cycle, the circuit is in the dynamic zeroing and amplification phase, and the differential signal collected on CSAMPLE is provided to The gain amplifier refreshes the voltage value stored on CHOLD and is amplified by the gain amplifier. When CSAMPLE is connected to the output of the preamplifier, the common-mode output voltage of the preamplifier is pulled down to the reference potential VREF. In this way, CSAMPLE and the preamplifier have the same common mode voltage.

Implementation of Precision Strain Tester

The strain tester is mainly composed of circuits such as bridge voltage generation, radio frequency interference (RFI) filtering, common mode suppression, signal amplification, low-pass filtering and buffer driving. Its structural block diagram is shown in Figure 2.

Figure 1 Basic principle of automatic zero stabilization


Figure 2 Structural block diagram of strain tester

In actual application environments, the increasing radio frequency interference may appear as a DC offset error that is difficult to eliminate after being rectified by the amplifier. At the same time, considering the long and weak signal transmission line, a differential low-pass filter is set up in front of the instrument amplifier. to remove as much RF energy from the inputs as possible, maintain AC signal balance between each input and ground, and maintain input impedance high enough within the measurement bandwidth to avoid degrading the loading of the input signal source. ability.

The working voltage of the strain sensor is supplied by the bridge voltage generating circuit, and its stability directly affects the measurement accuracy of the input signal. In order to minimize the measurement error and input signal drift, the bridge voltage circuit should use a precision reference voltage regulator chip with a low temperature coefficient, such as LM399, LM3999, etc. They use sub-surface buried technology and have the advantages of good long-term stability and low noise voltage. Their excellent constant temperature characteristics αT=(0.3~2)×10-6/℃ can effectively eliminate the impact of temperature changes on the reference voltage.

The system adds a common mode suppression circuit, which can further reduce system noise and DC zero drift error, and improve test accuracy; a low-pass filter is set at the output of the instrument amplifier to filter out high-frequency components and reduce low-frequency noise; a buffer drive circuit is added , increase the load capacity of the amplifier, and the effect is obvious when the amplifier is far away from the load. This system solves the problems of insufficient frequency band and low accuracy of previous strain gauges, and is a new type of precision testing instrument.

Application of AD8230 in strain testers

AD8230 is mainly used for common mode suppression, signal amplification, automatic zero stabilization and output buffering in precision strain testers.

Since the output voltage of the common mode suppression circuit

of the strain bridge is very weak, shielded cables are mostly used for signal transmission. When measuring at long distances, there is a distributed capacitance that cannot be ignored between the signal line and the cable shield. If the shield is directly connected to the ground, when the two input terminals have different ground capacitances, the common mode suppression capability of the system will be compromised. Decrease, affecting the measurement accuracy of the subsequent stage. Figure 3 adopts an active data protection measure. The shielding layer is properly driven and then connected to the equal potential point of the common-mode signal to improve AC CMR, thereby preventing leakage current and improving the signal-to-noise ratio.

Figure 3 Improved common mode suppression


circuit Figure 4 Output filtering and buffer circuit

The output filter and drive buffer circuit

AD8230 amplifier has a small driving load capacity and can only drive a load impedance above 10kW. If the load impedance is less than 10kW, a precision drive buffer should be added to the output end. Depending on the application frequency band, add a low-pass filter at the output to filter out high-frequency components. It is recommended to use UAF42AU. It integrates filtering and driving. By changing the connection between pins, it can flexibly realize low-pass, high-pass, band-pass or band-stop filtering. When the driving load is 2kW, the UAF42AU output swing is ±11.5V, which can meet the application requirements of the tester in various fields. The output buffer circuit is shown in Figure 4.

Conclusion

AD8230 can replace the instrumentation amplifier composed of discrete components. It has the characteristics of good linearity, high temperature stability, small size and high reliability. It can be used as a low-power medical instrumentation amplifier, thermocouple amplifier, bridge strain measurement amplifier and other applications. in sensor interfaces, industrial process control, and low-power data acquisition systems. The strain tester composed of it is widely used in material measurement scales, sensor instruments, etc. Practice has shown that the maximum dynamic measurement error of the tester is ≤1.53‰.