Calibration data and analysis of an AC resistor voltage divider

Author： GOZ Electric Time：2024-11-17 09:35:37 Read：11

1 Design of voltage divider resistor

1MHZ high-accuracy broadband AC voltage divider is based on double-rate design. Each voltage divider has a fixed voltage divider ratio. There are 10 in total. The rated input voltages are 2V, 4V, 8V, 16V, 32V, 64V, 120V, 240V, 480V, and 600V from small to large. The output voltage is 0.8V, ensuring that the high-range voltage is twice the low-range voltage. The input impedance is between (0.5 and 150) kΩ, and the working current is 4mA. In order to ensure that the voltage divider ratio has good long-term stability, the resistor components selected for the broadband AC resistor divider focus on several main technical indicators such as accuracy, long-term stability, frequency response characteristics, temperature coefficient, and power coefficient.

After the resistor type is determined, the parameters of the required resistor are determined according to the working voltage, resistance value, power, temperature coefficient, and voltage coefficient. Due to the use of sampling to measure the voltage divider ratio, the AC resistor voltage divider must consider the stability of the voltage divider and the voltage divider ratio error, as well as the phase angle error of the voltage divider. At the same time, considering that the voltage divider phase angle error can be calibrated by the half-voltage load recursion method, the structure of the voltage divider adopts the most basic principle of resistor series voltage division, which improves reliability while reducing the influence of temperature and switch leakage current. In order to achieve a good shielding effect, the article designs a fully coaxial resistor voltage divider. The current flows into the resistor symmetrically located with the axis and returns through the external cylindrical conductor. The lower arm resistor adopts a disc-shaped coaxial structure design. According to Maxwell's electromagnetic field theory, the electromagnetic field under 5 or multiples of 5 can be approximated as a circle. The upper arm resistor and the lower arm resistor of the resistor voltage divider in this article are both in 5-wire parallel form. After testing, the coaxial resistor voltage divider has smaller distributed parameters and has better ratio error and phase angle error than the existing AC resistor voltage divider products. At a high frequency of 1MHZ, the frequency upper limit of the AC resistor divider frequency of 100kHz is broken, and at a frequency of 1MHz, it still maintains high technical indicators.

2 Calibration results and data analysis

The proportional error and phase angle error of the AC resistor divider are calibrated using a self-calibration sampling and analysis system. The system consists of a multi-function standard source FlUKE5270 (including amplifier), an NI high-precision digital acquisition controller, and a computer. Since the output impedance of the broadband resistor divider is large (generally there are hundreds of ohms of resistance and tens of picofarads to hundreds of picofarads of capacitance), if a broadband AC voltage measurement device is used directly for measurement, a large error will be introduced. Therefore, during the test, a 1:1 buffer amplifier is placed at the end of the voltage divider for impedance transformation. In the test, the buffer amplifier is regarded as part of the resistor divider, and the overall error calibration of the resistor divider part, the capacitor box, and the buffer amplifier is performed.

The sampling and analysis system is calibrated and tested using an AC/DC conversion standard device. Then, the AC voltage signal output by the 5270A/5725A multifunctional standard source is divided by the wide-band resistor voltage divider to be measured and then enters the sampling and analysis system. The measured value obtained after sampling and analysis is the output value of the voltage divider. The calibration of the initial voltage divider 2V/0.8V relies on the high-precision digital sampling system to have good linearity at the full-scale point and the half-scale point. The input value of the resistor voltage divider is obtained by tracing the AC/DC thermoelectric conversion device to the DC voltage source. The entire system adopts the half-voltage load recursion method. First, the zero point of the device is calibrated. The multifunctional standard source outputs a voltage of 0.8V. Without passing through the voltage divider, it is directly connected to the CH0 and CH1 channels of the high-precision digital sampling controller for operation and testing. The proportional error and phase angle error obtained by the system test at this time are recorded, and the zero point error of the system is corrected. Then continue to use the high-precision digital sampling controller to calibrate the 2V/0.8V voltage divider from the same 0.8V signal, and then use 2V/0.8V to calibrate 4V/0.8V at half-load voltage. After determining 4V/0.8V, calibrate 8V/0.8V at half-load voltage, and then recursively to 600V/0.8V. In this process, CH0 channel is always used as the reference and CH1 channel is the measured channel. The calibration software is programmed in Labview language, and the actual proportional value of the voltage divider is obtained by comparing the input and output voltages of the voltage divider. The calculation process is based on the non-integer cycle compensation algorithm. The calibration process uses the natural standard of 1:1 voltage ratio and 0° phase angle under the same signal to ensure the accuracy and stability of the system test.