Circuit design of isolated acquisition board

Board Crystal 3.2*2.5mm 3225 24M (24.000MHZ) 12PF 10PPM 20PPM 30PPM

Probe domestic double-head probe 038-BB-5.7L The shape of the two ends of the needle is outside the pointed needle

Overview:
Aiming at the noise interference problem in current and voltage acquisition, the importance of accurate sampling and measurement of current and voltage in the whole control system is analyzed. The working principle of HCNR201 is introduced. The hardware circuit of the isolated acquisition board is designed. The isolation of the analog voltage signal is realized by the special setting of its working current. The experimental results show that the design is correct and usable, with high acquisition accuracy and linearity, and has broad application prospects in the occasions with high requirements for stability and linearity.

0 Introduction In the power system, it is often necessary to collect and measure signals such as current and voltage. The whole system is easy to generate electromagnetic interference due to the compact installation. Sometimes the instability of the power supply system exacerbates the deterioration of the electromagnetic environment. Data acquisition is performed under strong interference, and various strong interference signals enter the acquisition and measurement system along with the measured signal. On the one hand, these strong interference signals are superimposed on the useful measured signals, which will affect the measurement accuracy. On the other hand, high-voltage, high-current interference signals are injected into the processor circuit (such as CPU), which may cause system instability. If it is heavy, it will cause damage to these devices.

In order to reduce the influence of environmental noise on the acquisition circuit, improve system stability and ensure accurate measurement results, the circuit under test and the test circuit are often electrically isolated. Hall current and voltage sensors can be used to achieve current and voltage. Isolated, but high-performance current and voltage sensors are costly and bulky and difficult to lay out. Optocoupler isolation is an ideal choice. Ordinary optocouplers have nonlinear current transfer characteristics, which is not a problem for the transmission of digital quantities and switching quantities, but the isolation of analog quantities does not guarantee transmission accuracy. HCRN201 is a high linearity analog optocoupler manufactured by HP. The photoelectric isolation circuit realized by it can realize the stability and linearity of analog signals. This paper introduces the working principle of HCNR201. The device is designed for the isolation circuit of current and voltage signals. The selection of parameters is discussed and the test results are analyzed.

1 Linear analog optocoupler HCNR201 structure and working principle

Figure 1 Internal structure of HCNR201

The internal structure of HCNR201 is shown in Figure 1. Including a high-performance AlCaAs type LED (LED in Figure 1), two extremely similar photodiodes (PD1 and PD2 in Figure 1), when the current Ip flows through the LED, the light will be emitted at PD1. The photocurrents IPD1 and IPD2 which are proportional to the luminous intensity of the LED are induced in PD2, wherein IF, IPD1 and IPD2 satisfy the following relationship:
IPD1=K1IF (1)
IPD2=K2IF (2)
K=IPD1/IPD1 (3)
In the formula, K1 and K2 are the current transmission ratios of the input and output photodiodes, respectively, and their typical values ​​are about 0.5%. Since IF is generally between 1 and 20 mA, IPD1 and IPD2 are generally below 50 μA; K is defined as transmission gain. When an HCNR201 is manufactured, K is a fixed value, and the K value of HCNR201 is about 1. ±0.05. In addition, the accuracy of the mounting position of PD1 and PD2 and the advanced package design of the components ensure the high linearity and gain stability of the HCNR201.
In use, the 3rd and 4th pin outputs can be connected to the control loop together with the 1st and 2nd pin lead-in terminals. The 3rd and 4th pin photodiodes PD1 serve as feedback, which can generate the output current. It is fed back to the LEDs of pins 1 and 2 to feedback control the input signal.

(Please read the PDF for details)