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AD620 Application: Signal Amplification System Design

  • Contents

I. Introduction

The amplification of weak signals has high requirements and high difficulty. The signal amplification is related to the requirements of stability and accuracy of signal amplification. Differential amplification technology has the characteristics of suppressing common mode signals and only amplifying differential mode signals with high gain, so it is applied to small signal amplification technology. The system design adopts the AD620 chip with differential amplification function to amplify the weak voltage signal of the strain sensor to achieve the high precision requirements of the system. This article uses virtual instrument technology to collect and analyze the amplified signal, and write the corresponding display interface. The measurement data is analyzed by the second-order interpolation method to verify the accuracy of the circuit.

AD620AD620

Catalog

I. Introduction

II. System Design

III. System Hardware Circuit Design

3.1 Pressure Measurement Circuit

3.2 Voltage Signal Amplifier Circuit

3.3 Reference Voltage Source Circuit and Voltage Zeroing Circuit

3.4 Voltage-current Conversion Circuit

IV. The Overall Software Design of the System

V. Quantitative Testing and Result Analysis

5.1 Data Processing Method

5.2 Data Processing Results

5.3 Error Analysis

VI. Conclusion

FAQ

Ordering & Quantity

II. System Design

The system is provided with two voltages of ±12 V and ±5 V from a DC stabilized source. When setting ±12 V power supply, the system voltage output full range is 5V, and the sensor withstands static pressure full range is 19.6N. When measuring within the full-scale range, the maximum absolute error of the static pressure signal is <9.8×10-3N, and the relative error is <0.02%. The output signal of the load cell provides two output modes: voltage output and current output after the amplifier circuit.

III. System Hardware Circuit Design

The overall design process of the system is shown in Figure 1. The system hardware circuit is mainly composed of LC7012 load cell, AD620 instrumentation amplifier, reference voltage source, voltage zeroing circuit, signal filtering and shaping circuit and voltage-current conversion circuit.

Figure 1 System hardware circuit overall design process

Figure 1 System hardware circuit overall design process

 

3.1 Pressure Measurement Circuit

 

Pressure measurement adopts LC7012 load cell, with full bridge measurement circuit. LC7012 load cell has the following two characteristics when subjected to pressure: (1) Under the same pressure, the strain of the sensor strain gauge and the output voltage of the bridge are constant and have nothing to do with the precise position of the pressure acting on the load end of the sensor. (2) The output voltage and pressure of the full bridge circuit composed of strain gauges are basically linear.

 

The 4 pieces of resistance strain gauges in the LC7012 load cell are attached to the strain zone of the double-hole beam. When there is static pressure, the double-hole beam produces quadrilateral deformation under the action of the pressure and the supporting force of the system chassis on the double-hole beam. The four strain gauges are connected to a full bridge circuit in a full bridge mode. Under the excitation of the bridge voltage, different weak voltage signals are output with different pressures, and the amplifier circuit amplifies the weak voltage signals sent by the bridge.

 

The full-bridge equal-arm bridge has simple structure, strong symmetry, high sensitivity, and good consistency of the parameters of each arm. The effects of various interferences can cancel each other, for example, it can suppress the effects of temperature changes and suppress the interference of lateral forces. It is easier to solve the problem of compensation of the load cell. The full-bridge measurement circuit enables the output of the weak voltage signal to eliminate errors caused by the circuit itself as much as possible, and provides the initial guarantee for the overall accuracy of the system.

 

3.2 Voltage Signal Amplifier Circuit

 

In order to improve the amplification accuracy of the weak voltage signal output by the bridge, the signal amplifying circuit selects the AD620 chip produced by ADI as the core element, and designs a special adjustable reference voltage source for it to meet the reference voltage requirements of different voltage sources. And the need to accurately amplify weak signals.

Figure 2 AD620 Pinout

Figure 2 AD620 Pinout

 

AD620 is a low-cost, high-precision instrumentation amplifier. It only needs an external resistor to set the gain. The gain range is 1 to 10 000 dB. And AD620 power consumption is low, the maximum operating current is 1.3 mA. AD620 has the characteristics of high precision (maximum linearity 40×10-6), low offset voltage (maximum 50μV) and low offset drift (maximum 0.6μV/℃), making it an ideal choice for precision data acquisition systems such as sensor interfaces. Figure 2 shows its pin arrangement.

 

AD620 monolithic structure and laser crystal adjustment allow circuit components to be closely matched and tracked, thus ensuring the inherent high performance of the circuit. AD620 is a three-op-amp integrated instrumentation amplifier structure, in order to protect the high precision of gain control, the input transistor provides differential bipolar input, and uses β process to obtain lower input bias current, through the feedback of the input stage internal op-amp , Keep the collector current of the input transistor constant, and add the input voltage to the external gain control resistor RG. AD620 internal gain resistance is adjusted to an absolute value of 24.7 kΩ, so an external resistance can be used to achieve precise programming of the gain.

 

The gain formula is

 

The voltage signal amplified by the AD620 can pass through a filtering and shaping circuit and be displayed in digital form with a digital tube through the analog-to-digital converter module. In order to fully utilize and demonstrate the functions of virtual instruments, the system uses LabVIEW to design the corresponding signal acquisition and processing program and display interface.

 

3.3 Reference Voltage Source Circuit and Voltage Zeroing Circuit

 

The reference voltage source circuit is mainly composed of a Zener diode LM285, a low-power dual operational amplifier chip LM258, a variable resistor and a number of fixed resistance resistors, as shown in the lower left part of Figure 3. This reference voltage source circuit can provide AD620 with 1.25 V or 2.5 V accurate reference voltage.

Figure 3 Voltage signal amplifier circuit

Figure 3 Voltage signal amplifier circuit

The voltage stabilizing diode LM285 provides the primary stable voltage, but the temperature drift of the diode is large, and the voltage stabilization value of different diodes in the same batch is not the same, so the corresponding auxiliary voltage stabilizing circuit must be designed for it. The operational amplifier LM258U1A amplifies the voltage from the Zener diode and feeds back the output voltage through the feedback resistor R2, making the output voltage more stable. Resistor R5 and potentiometer W1 divide the output voltage of the Zener diode.

 

Potentiometer W1 has two functions:

 

(1) Adjusting W1 can make the voltage follower composed of operational amplifier LM258U1B have different output voltages, and then provide different stable reference voltages to AD620.

 

(2) The potentiometer W1 also plays a role of zero adjustment on the amplifying circuit composed of AD620. The voltage follower is used because the voltage follower can increase the input impedance and reduce the output impedance, and the requirement of the power supply is that the circuit has a smaller output resistance.

 

AD620 itself has an internal zero adjustment function, but according to actual measurement, it is found that when the differential input is zero, the output is not zero, but about a few tenths of mV. Therefore, in order to improve the accuracy of the output, it is necessary to perform the AD620 External zero adjustment, by providing different reference voltages to the AD620 reference voltage pins, the output voltage of the instrumentation amplifier AD620 can be zero when the differential input is zero. The circuit just adjusts W1 to make the output terminal of the voltage follower have different voltage output, adjusts the reference voltage of AD620, thus plays the role of zero adjustment to AD620.

 

The instability of the reference voltage will directly affect the stability of the amplifier circuit composed of AD620, and lead to inaccuracy of the final output result. Therefore, the system does not directly use the relatively stable -12 V or -5 V provided by the DC stabilized source as the reference voltage.

 

3.4 Voltage-current Conversion Circuit

 

The voltage-current conversion circuit enables the system to output in the form of current. The AD620 is combined with an AD705 operational amplifier and two resistors (as shown in Figure 4) to form a quiet current source. AD705 provides a buffer for the reference pin. Ensure good common mode rejection (CMR) performance. The output voltage of AD620 appears on the resistance RL, the latter converts it into electric current output.

 

Figure 4 Schematic diagram of voltage-current conversion circuit

Figure 4 Schematic diagram of voltage-current conversion circuit

 

AD705 is a low-power, bipolar operational amplifier with a bipolar field effect transistor input stage. Therefore, it has the characteristics of high input impedance, low input offset voltage, small input bias current, and small input offset voltage drift. The input bias current has reached the pA level. It not only has many advantages of bipolar field effect transistors and bipolar operational amplifiers, but also overcomes the defect of large bias current drift in the full temperature range. In the full temperature range, the typical value of the bias current of AD705 only increases by 5 times, and the bias current of the general bipolar field effect transistor operational amplifier increases by 1,000 times. Compared with OP07, the temperature drift value is 1/2 of OP07, the maximum input bias current is only 1/5 of OP07, and the input offset voltage is only 1/20 of OP07. Because it is a bipolar field effect transistor input pole, the signal source impedance is much higher than OP07, while its DC accuracy remains unchanged.

 

IV. The Overall Software Design of the System

 

The system software is written in LabVIEW. LabVIEW is a graphical programming language, which is widely used in various fields as a standard for data acquisition and instrument control software. LabVIEW is a powerful and flexible software. Use it to easily build your own virtual instrument. In the case of one piece of hardware, different functions of different instruments can be realized by changing the software programming, which is convenient and fast.

 

Combined with the new development direction of the current testing field instruments, the final output analog voltage signal is collected by Advantech's USB4716 universal data acquisition module and transmitted to the computer. Use NI virtual instrument (LabVIEW) to design voltage signal acquisition control program and voltage data real-time display interface. Use LabVIEW software platform to analyze and process the digital voltage signal from USB4716. The part program of LabVIEW voltage signal acquisition control and display is shown in Figure 5.

 Figure 5 Voltage signal acquisition program

 Figure 5 Voltage signal acquisition program

 

V. Quantitative Testing and Result Analysis

 

5.1 Data Processing Method

 

Second-order interpolation (parabolic interpolation): select (x0, y0), (x1, y1), (x2, y2) corresponding interpolation equations from a set of data.

 

5.2 Data Processing Results

In order to obtain an accurate correspondence between pressure and voltage and facilitate subsequent analysis of absolute and relative errors, the experiment uses static measurement methods to measure a series of static pressure values, and quantitatively analyze the experimental results to determine the accuracy of the circuit. Commonly used waveform time domain and frequency domain analysis methods.

Table 1 Brightness/Contrast Comparison

Pressure/N

0

...

2.94

...

8.82

9.8

...

13.72

...

19.62

19.6

Voltage/V

0

...

0.75

...

2.247

2.498

...

3.498

...

4.75

5.001

Measure 20 static pressure values from small to large within the full scale range, and make the pressure increment △ the same. Let △=0.98 N, and use the second-order interpolation method to analyze the relationship between voltage and pressure. Select three representative points from Table 1: (x0, y0) = (0, 0); (x1, y1) = (2.498 V, 9.8 N); (x2, y2): (5.001 V, 19.6 N ). Bring in second-order interpolation

 

The relationship curve between the pressure on the sensor and the system output voltage is

 

Y=(-1.568×10-3)x2+3.927x (3)

 

5.3 Error Analysis

 

The absolute error reflects the deviation of the measured value from the true value, that is, the absolute value of the difference between the measured value and the true value. The absolute error can be defined as:

 

  Ε=|X-L| (4)

 

In the formula, ε is the absolute error; X is the measured value; L is the true value.

 

Relative error is the ratio of absolute error to the measured value or the average value of multiple measurements, and the result is usually expressed as a percentage, so it is also called percentage error.

 

Absolute error can indicate the reliability of a measurement result, while relative error can compare the reliability of different measurement results. When measuring with the same tool, the larger the measured value, the smaller the relative error of the measurement result.

 

The absolute error and relative error of the strain gauge pressure sensor test system are shown in Figure 6 and Figure 7. The two figures respectively show the absolute error and relative error curves of two other data processing methods: linear interpolation and average selection method. It can be seen from Figure 6 and Figure 7 that the calculation accuracy of the second-order interpolation method is higher than the other two methods, which also proves that the choice of the data processing method is correct.

 

Figure 6 Absolute error curve

Figure 6 Absolute error curve

 

Figure 7 Relative error curve

Figure 7 Relative error curve

 

VI. Conclusion

 

Known from the relative error and absolute error graph that, the measurement result error of the circuit in the range of 0~4.9 N is relatively large, but it still meets the system design requirements. After analyzing the sensor and the experimental measurement circuit, it is believed that the reason for the larger error comes from the rigidity of the cantilever beam material of the sensor and the flexible influence of the viscous material that fixes the strain gauge. Because the accuracy of the weak voltage signal output by the bridge is affected, the error is also amplified after passing through the amplifying circuit, resulting in a larger error in the experimental result when the measured value is small. In summary, the pressure signal amplification system satisfies the design requirements of absolute full-scale error <9.8×10-3N and relative error.


FAQ

  • What is AD620?

AD620 is a low-cost, high-precision instrumentation amplifier. It only requires an external resistor to set the gain. The gain range is 1 to 10,000.

  • Can I change AD620 to AD623 when making MCU products?

Both AD620 and AD623 are single instrumentation amplifiers, and the pin arrangement is exactly the same.

The main difference is: AD620 must use positive and negative power supplies, AD623 can be a positive and negative power supply or a single power supply.

If the original board is AD620, you can replace it with 623; if the original board is AD623, you may not be able to replace it with 620 (it depends on whether the power supply of the original board circuit is dual power supply or single power supply).

After replacing AD620 and AD623 in single-chip products, the program can work normally without modification.

  • What is the difference between AD620BR and AD620AN?

Their packages are different.

  • What is the output resistance of AD620? How to adjust it?

AD620 is a kind of low power consumption instrument amplifier, its output resistance is about 10K, this is the inherent characteristic of this chip, generally it is difficult to adjust.

If you have requirements for output resistance, you can generally use an external circuit to solve it.

  • Is AD620 a positive phase amplification or a reverse phase amplification?

AD620 is an instrument amplifier, the output voltage is [(Vin+)-(Vin-)]*gain.

If the desired signal is (Vin+)-(Vin-), the gain is positive, which is equivalent to positive amplification.

Conversely, if the desired signal is (Vin-)-(Vin+), the gain is equivalent to negative, which is equivalent to reverse amplification.

  • What is an instrumentation amplifier?

Instrumentation amplifier, an improvement of the differential amplifier, has an input buffer, does not require input impedance matching, so that the amplifier is suitable for measurement and electronic instruments

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