原标题：ADI CN0301 通用LVDT信号调理解决方案

　　ADI公司的AD698是完整的单片线性可变差动变压器(LVDT)信号调理子系统, 结合LVDT使用，能够以较高精度和可重复性将传感器机械位置转换为单极性或双极性直流电压.工作频率20Hz到20kHz,单极或双极输出,线性度 0.05%,输出电压611V,可以用于线性位移测量.本文介绍了AD698产品亮点和主要特性,几种典型应用电路以及CN0301通用LVDT信号调理电路,材料清单和PCB元件布局图.

　　The AD698 is a complete, monolithic Linear Variable Differential Transformer (LVDT) signal conditioning subsystem. It is used in conjunction with LVDTs to convert transducer mechanical position to a unipolar or bipolar dc voltage with a high degree of accuracy and repeatability. All circuit functions are included on the chip. With the addition of a few external passive components to set frequency and gain, the AD698 converts the raw LVDT output to a scaled dc signal. The device will operate with half-bridge LVDTs, LVDTs connected in the series opposed configuration (4-wire), and RVDTs.

　　The AD698 contains a low distortion sine wave oscillator to drive the LVDT primary. Two synchronous demodulation channels of the AD698 are used to detect primary and second-ary amplitude. The part divides the output of the secondary by the amplitude of the primary and multiplies by a scale factor.

　　This eliminates scale factor errors due to drift in the amplitude of the primary drive, improving temperature performance and stability.

　　The AD698 uses a unique ratiometric architecture to eliminate several of the disadvantages associated with traditional approaches to LVDT interfacing. The benefits of this new circuit are: no adjustments are necessary; temperature stability is improved; and transducer interchangeability is improved.

　　AD698产品亮点:

　　1. The AD698 offers a single chip solution to LVDT signal conditioning problems. All active circuits are on the mono- lithic chip with only passive components required to com-plete the conversion from mechanical position to dc voltage.

　　2. The AD698 can be used with many different types of posi-tion sensors. The circuit is optimized for use with any LVDT, including half-bridge and series opposed, (4 wire) configurations. The AD698 accommodates a wide range of input and output voltages and frequencies.

　　3. The 20 Hz to 20 kHz excitation frequency is determined by a single external capacitor. The AD698 provides up to 24 volts rms to differentially drive the LVDT primary, and the AD698 meets its specifications with input levels as low as 100 millivolts rms.

　　4. Changes in oscillator amplitude with temperature will not af- fect overall circuit performance. The AD698 computes the ratio of the secondary voltage to the primary voltage to deter-mine position and direction. No adjustments are required.

　　5. Multiple LVDTs can be driven by a single AD698 either in series or parallel as long as power dissipation limits are not exceeded. The excitation output is thermally protected.

　　6. The AD698 may be used as a loop integrator in the design of simple electromechanical servo loops.

　　7. The sum of the transducer secondary voltages do not need to be constant.

　　AD698主要特性:

　　Single Chip Solution, Contains Internal Oscillator and

　　Voltage Reference

　　No Adjustments Required

　　Interfaces to Half-Bridge, 4-Wire LVDT

　　DC Output Proportional to Position

　　20 Hz to 20 kHz Frequency Range

　　Unipolar or Bipolar Output

　　Will Also Decode AC Bridge Signals

　　Outstanding Performance

　　Linearity: 0.05%

　　Output Voltage: 611 V

　　Gain Drift: 20 ppm/8C (typ)

　　Offset Drift: 5 ppm/8C (typ)

　　图1. AD698框图

　　图2.半桥 LVDT和双电源的内部连接图

　　图3. 串联相对配置(4线)LVDT和双电源的内部连接图

　　图4.单电源的内部连接图

　　图5.AC桥应用AD698内部连接图

　　通用LVDT信号调理电路CN0301

　　The circuit shown in Figure 1 is a complete adjustment-free linear variable differential transformer (LVDT) signal conditioning circuit. This circuit can accurately measure linear displacement (position).

　　The LVDT is a highly reliable sensor because the magnetic core can move without friction and does not touch the inside of the tube. Therefore, LVDTs are suitable for flight control feedback systems, position feedback in servomechanisms, automated measurement in machine tools, and many other industrial and scientific electromechanical applications where long term reliability is important.

　　This circuit uses the AD698 LVDT signal conditioner that contains a sine wave oscillator and a power amplifier to generate the excitation signals that drive the primary side of the LVDT. The AD698 also converts the secondary output into a dc voltage. The AD8615 rail-to-rail amplifier buffers the output of the AD698 and drives a low power 12-bit successive approximation analog-to-digital converter (ADC). The system has a dynamic range of 82 dB and a system bandwidth of 250 Hz, making it ideal for precision industrial position and gauging applications.

　　The signal conditioning circuitry of the system consumes only 15 mA of current from the ±15 V supply and 3 mA from the +5 V supply.

　　This circuit note discusses basic LVDT theory of operation and the design steps used to optimize the circuit shown in Figure 1 for a chosen bandwidth, including noise analysis and component selection considerations.

　　图6. CN0301通用LVDT信号调理电路

　　图7. EVAL-CN0301-SDPZ板和EVAL-SDP-CB1Z (SDP-B)板连接图