DDS+MCU realizes the operational amplifier parameter measurement system

introduction

In order to help users accurately grasp the various parameters of the operational amplifier in their hands, this paper provides a measurement system using a programmable DDS chip and MCU , which can automatically measure the five basic parameters of the integrated operational amplifier, display the measurement results on a small LCD screen, and print the measurement results as needed. Compared with the existing expensive testers such as BJ3195, this measurement system has streamlined functions, intelligent operation, and a friendly human-machine interface.

Overall system design

The system block diagram is shown in Figure 1. The system uses the SPCE061 microcontroller as the control core and adopts a master-slave structure. The slave microcontroller is responsible for peripheral functions such as LCD display, printing, and voice prompts. The master microcontroller is responsible for receiving the input information from the infrared keyboard. According to the current user input, it sets the parameter test part and the automatic range switching part to the appropriate state, then reads the measurement results, and notifies the slave to display or print the measurement results. The DDS sweep signal source of the system can be set to output any frequency within 4MHz and any frequency segment with any step of sine signal through the infrared keyboard. In order to improve the measurement accuracy, the system is equipped with a set of standard op amp parameter measurement circuits to perform initial calibration on the system.

Measurement function circuit structure

SPCE061 Introduction

SPCE061 is a 16-bit MCU launched by Sunplus Technology Co., Ltd., with a maximum operating frequency of 49MHz, built-in 32KB ROM and 2KB RAM, and has an infrared communication interface and an asynchronous full-duplex serial interface. In addition, SPCE061 provides a very convenient development platform and audio codec tools, making the SPCE series of microcontrollers not only powerful in control functions and short in development cycle, but also easy to implement master-slave architecture.

Measuring the main circuit

The circuit for measuring the parameters of the operational amplifier is shown in Figure 2. Two amplification links are introduced into the transfer function of the circuit system, so there are two or more poles. According to the Nyquist stability criterion, for a closed-loop feedback system, if there are poles distributed in the right half plane of the frequency domain, self-oscillation will occur during the deep negative feedback test, resulting in failure to test normally. Therefore, this system improves the circuit, adds a 560pF capacitor in parallel with RF in the feedback loop, compensates the phase of the signal, changes the amplitude-frequency characteristics of the entire feedback channel, and increases its phase margin. After testing, the working stability of the closed-loop circuit is greatly improved.

In Figure 2, S1, S2, S3, and S4 are all relays, which are controlled by SPCE061 to be turned on and off, thereby realizing the automatic measurement of VIO, IIO, AVD, KCMR, and BWG, among which BWG is switched by the relay to another sweeper for separate measurement.

According to formulas (1), (2), (3), and (4), VIO, IIO, AVD, and KCMR can be calculated:

Programmable amplifier circuit

Since the measured parameters are all mV voltage, the output signal of the closed loop of the auxiliary operational amplifier should be measured in two levels. During automatic measurement, the switching of these two levels is controlled by the MCU, so a programmable amplifier circuit needs to be designed. This design uses the instrument amplifier AD620, and changes its feedback resistance through S5 and S6 to control the gain. Since the instrument amplifier has a differential input and the input is a low-frequency signal of 5Hz, in order to suppress power frequency interference, a second-order active filter circuit is used for filtering at the input stage of AD620. Considering its passband flatness, a second-order Butterworth low-pass filter is used, and the cut-off frequency is set to 20Hz.

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Unity-gain bandwidth measurement circuit

Input a constant amplitude AC sinusoidal signal at the input end, change the signal frequency, and the frequency corresponding to the voltage drop of 3dB at the circuit output end is the unit gain bandwidth. In order to improve the measurement efficiency, this design isolates the unit gain bandwidth measurement circuit from other parameter test circuits and uses relays for switching control. The unit gain bandwidth is closely related to the input signal amplitude. When the input signal is large, the unit bandwidth becomes narrower and the measurement result error is large. In the system, a broadband op amp is used to attenuate the input signal, and then the test op amp is passed through the test op amp, and then the broadband op amp is used to amplify the output signal of the test op amp to improve the measurement accuracy. The broadband op amp is selected as AD811, and its unit gain bandwidth is 140MHz.

DDS frequency sweep signal source

AD9851 is a digital frequency synthesis chip, its maximum operating frequency is 180MHz, the maximum output frequency of AD9851 is 40% of the system clock, the spurious frequency is small, it has a 40-bit control word, of which 5 bits are phase control, 1 bit is 6 times the reference clock multiplier switch control, and 32 bits are frequency control. When an external 20MHz clock source is connected, the system clock Fsysclk = 120MHz after the 6-times frequency is turned on, and the frequency control word is Fcw, then the output frequency is given by the formula

Since the peak-to-peak value of the AD9851 output signal is 1V, and it is more accurate to use a sinusoidal signal with an effective value of 2V when measuring BWG, it must be amplified 5.656 times. The maximum output frequency of the swept signal source is designed to be 4MHz, so the gain-bandwidth product GBW of the inverting amplifier is required to be ≥5.656×4MHz=22.624MHz. The system uses a high-speed operational amplifier AD817 with a GBW of 50MHz.

Software Algorithms and Procedures Software Algorithms for Unity-Gain-Bandwidth Measurements

The system is designed to have a sweep range of 100KHz to 3.5MHz, a frequency resolution of 1KHz, and a total automatic measurement time of ≤10s. Therefore, from 100KHz to 3.5MHz, at least (3500-100)/1=3400 times should be scanned, and the maximum time used each time is: 10/3400=0.0029s, and within this 0.0029s, the frequency setting and reading of the A/D conversion results must be completed. The high-precision A/D conversion time is generally longer, and the time-consuming setting of the sweep frequency makes the traditional full-band step sweep more time-consuming.

System delay. In view of the characteristics of unity gain bandwidth, this design adopts a binary search algorithm to continuously narrow the frequency sweep range and step-scan in a smaller frequency band. Only a few dozen frequency points need to be captured to meet the measurement time requirement of ≤10s at a resolution of 1 KHz.

System Error Overview

System tests show that the measurement accuracy of VIO, IIO, and AVD mainly depends on the accuracy of the integrated operational amplifier input resistance and feedback resistance. Ensuring the balance of the external equivalent resistance of the two input ports of the operational amplifier can reduce the measurement error. The measurement error of KCMR is mainly due to the external electromagnetic interference, power supply ripple, power frequency interference, transmission network asymmetry, and the series mode interference caused by the non-uniform ground potential. The measurement error of KCMR can be reduced by single-point grounding, low-pass filtering, power supply filtering, and selecting high-precision resistors.

Conclusion

The test results show that the system can intelligently measure the five parameters of the integrated operational amplifier, with sensitive switching, small delay, more accurate measurement accuracy than previous measurement methods, and higher cost performance. The design method provided in this paper also has a certain reference value for the design of conventional measuring instruments.