Adopt ARM high resolution piezoelectric ceramic D/A circuit design

　　According to the demand of piezoelectric ceramic micro-displacement device for driving power supply, a scheme of piezoelectric driving power supply system was designed. The scheme first introduced the digital circuit part and analog circuit part in the power supply system, and analyzed and improved the accuracy and stability of the driving power supply. Finally, the performance of the driving power supply was experimentally verified. The experimental results show that the output voltage noise of the power supply of this design scheme is lower than 0.43 mV, the maximum nonlinear error of the output is lower than 0.024%, and the resolution can reach 1.44 mV, which can meet the needs of static positioning control in high-resolution micro-displacement positioning system.

　　The piezoelectric ceramic actuator (PZT) is the core of the micro-displacement platform. Its main principle is to use the inverse piezoelectric effect of piezoelectric ceramics to produce deformation, thereby driving the actuator to undergo micro-displacement. Piezoelectric ceramic actuators have the advantages of high resolution, fast response frequency, large thrust and small size. They have been widely used in aerospace, robotics, micro-electromechanical systems, precision machining and bioengineering. However, the application of piezoelectric ceramic actuators is inseparable from piezoelectric ceramic drive power supplies with good performance. To achieve nano-level positioning applications, the output voltage of the piezoelectric ceramic drive power supply needs to be continuously adjustable within a certain range, and the resolution of the same voltage needs to reach the millivolt level. Therefore, piezoelectric ceramic drive power supply technology has become a key technology in piezoelectric micro-displacement platforms.

　　D/A Circuit Design

　　Since the piezoelectric drive power supply requires an output voltage range of 0~100 V and a resolution of millivolts, the resolution of the D/A must reach sub-millivolts. This design uses AD5781 as the D/A device. AD5781 is an SPI interface 18-bit high-precision converter with an output voltage range of -10~10 V, providing ±0.5 LSB INL, ±0.5 LSB DNL and 7.5 nV/Hz noise spectrum density. In addition, AD5781 also has extremely low temperature drift (0.05 ppm/℃). Therefore, this D/A converter chip is particularly suitable for the acquisition and control of precision analog data. The D/A circuit design is shown in Figure 2.

　　In the hardware circuit design, due to the precision architecture adopted by AD5781, it is required to force detection to buffer its voltage reference input to ensure the specified linearity. Therefore, the amplifier selected for buffering the reference input should have low noise, low temperature drift and low input bias current characteristics. Here, AD8676 is selected. AD8676 is an ultra-precision, 36 V, 2.8 nV/ Hz dual-channel operational amplifier with 0.6 μV/℃ low offset drift and 2 nA input bias current, so it can provide a precision voltage reference for AD5781. The CLR and LDAC pin levels of AD5781 are pulled down through pull-down resistors to set AD5781 to DAC binary register encoding format and configure the output to be updated on the rising edge of SYNC. In the software design on the ARM side, in addition to correctly configuring the relevant registers of AD5781, the clock phase, clock polarity and communication mode of SPI should also be correctly configured.

　　Linear amplifier circuit design

　　From an engineering perspective, the presence of interference sources will change the stability of the system, causing the system to oscillate. Therefore, the method to ensure that the control system has a certain degree of anti-interference is to make the system have a certain stability margin, that is, phase margin. Due to the existence of stray capacitance in the actual circuit, the capacitance to ground at the reverse input end of the amplifier has a greater impact on the stability of the system. As shown in Figure 6, C5 and C6 are used to compensate for the stray capacitance at the reverse end. From the perspective of system function, it constitutes lead correction, increases the open-loop cutoff frequency of the open-loop system, and increases the system bandwidth to improve the response speed. PA78 has two pairs of phase compensation pins, and the zero poles inside the amplifier are compensated through an external RC network. It can be seen from the data sheet of PA78 that the zero poles inside PA78 are located in the high frequency band. According to the requirements of the control system's anti-noise ability, the RC network is configured to make the amplitude characteristic curve of the high frequency band decay rapidly, thereby improving the system's anti-interference ability. In the figure, R4, C1 and R5, C2 constitute an RC compensation network.

　　In addition, the function of C3 in the circuit is to prevent interference caused by vibration on the falling edge of the output signal; R10 acts as a bias resistor, injecting the power supply current into the output stage of the amplifier to improve the driving ability of PA78. The parameters of the PI controller are set to KP=10, KI=0.02 respectively; the lead correction compensation capacitors are 12 pF and 220 pF respectively; the RC compensation network is R=10 kΩ, C=22 pF. The amplitude-frequency characteristic and phase-frequency characteristic curves are simulated using the Spice model of the linear amplifier circuit as shown in the figure. It can be observed from the figure that the bandwidth of the amplifier system can reach 100 kHz, thus ensuring the good dynamic characteristics of the system, and the phase margin of 6 makes the system have high stability.