Several typical program controlled voltage sources

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Several typical program-controlled voltage sources and their EMC design

1 Introduction

in some electronic instruments and equipment, adjustable voltage source or current source with excellent performance is often needed, which was mostly completed by operational amplifier in the past. Now, with the improvement of the accuracy of a/D and D/A converters integrating recycled materials into buildings, a/D and D/a conversion have been widely adopted. The city has built one provincial academician workstation, one provincial scientific and technological innovation service platform, one Provincial Intellectual Property Demonstration Park, one provincial Anhui aluminum product quality supervision and inspection center, three provincial engineering technology research centers The four converters in the provincial enterprise technology center constitute a precise program-controlled voltage source or current source. As for which type of converter to choose, whether the output is voltage or current should be determined according to the actual situation. In addition, the system where the converter is located is full of various forms of noise. If there is noise, it will inevitably produce measurement error, and if there is error, it will inevitably reduce the measurement accuracy. Electromagnetic interference is the main factor affecting the measurement accuracy. Therefore, effective measures should be taken to eliminate 323-2002 beryllium bronze plate and strip when designing the system with converter

2 typical program controlled voltage source

the working principle of program controlled voltage source is described by taking D/a converter as an example. When the output of D/a converter is voltage output, it itself is a better precision program control voltage source. The accuracy of the controlled voltage depends on the relative accuracy of the D/a converter. Figure 1 is a basic programmed voltage source circuit

Figure 1 NC programmable voltage source

it should be pointed out here that amplifier A2 can not be used when the output driving capacity is sufficient. According to the basic principle of D/a converter, the circuit should meet the following transfer function:

uo = (ur/RR) rfai? 2-I (1)

where: AI -- input digital quantity of the ith bit, I = 1,2,..., N, The value of AI is "1" or "0;

ur - reference voltage;

RR - reference input resistance;

RF - feedback resistance of amplifier.

for offset binary code, its transfer function is:

uo = (ur/RR) RF (AI? 2-i-1/2) (2)

slave formula (1), (2) It can be seen that the output voltage is a function of the reference current IR = VR/RR, the feedback resistance RF and the input digital quantity. Change the reference current IR and feedback resistance RF, that is, change the maximum output amplitude, and change the input digital code to control the value of the output voltage. That is, programming is realized by changing the input digital quantity of D/a converter

if each output of a group of power supplies required has an integer multiple or power relationship, n multiplicative D/A converters can be directly used to form a program-controlled series of power supplies, as shown in Figure 2. In the figure, each power supply is iterated and multiplied, and the relationship is:

U1 = - URD

Figure 2 CNC series power supply

U2 = - u1d = urd2


UN = - un-1d = (- 1) nurdn

where: D - input digital quantity

Figure 3 is a practical program-controlled voltage source, which is applied in the interface circuit of computer-aided test equipment

Figure 3 program controlled voltage source

the circuit can output 0 ~ 20V voltage. There are 1024 step-by-step program-controlled voltage sources, each step is 20mV, the zener diode is used as the reference voltage, the transistor V is used for voltage and current amplification, and the digital input comes from the output bus of the computer

the whole circuit adjustment procedure is: first set the digital input to "1" and adjust w to make VO = 20V, and then set the digital input to all "0, adjust the operational amplifier to make VO = 0.

3 electromagnetic compatibility design of program-controlled voltage source

selecting the appropriate a/D and D/a converter is only the starting point of the design work. It is often necessary to comprehensively consider the specific system where any converter is located, especially when there are power supply voltage fluctuations and other noises in the system, it is necessary to ensure the normal operation of the converter. This is because in the circuit, the noise is easy to drown one or more bits Digital signal, which is particularly harmful in high-speed and high-precision systems. Therefore, effective and reasonable EMC design is essential

the following criteria should be followed in EMC Design:

1) cut off the channels of electromagnetic radiation entering the converter to produce mutual interference and reduce spatial coupling

2) to prevent the interference generated by external equipment, the influence of various pulse modulation signals (radar, radio wave, TV signal), changes of electromagnetic field and other factors, shielding protection must be taken for devices and components sensitive to electromagnetic effect, such as shell shielding, cable filtering and internal cable shielding

3) reasonably design circuits, correctly select devices and circuits, accurately calculate various parameters of devices and circuits, formulate measures to identify and isolate critical circuits and adopt technical schemes to suppress interference, and try to select high working signal level to meet the actual load capacity of devices and circuits, be careful "Interface design.

4) adopt high stability regulated power supply to improve the sensitivity of power supply voltage, reduce the linear error, gain error and adjustment error caused by power supply fluctuation, and ensure the stability and reliability of accuracy.

5) correct grounding and circuit layout, considering the characteristics of interference in different frequency bands and the types of circuits, the grounding points can be selected:

- floating grounding;

- one point grounding;

- multi-point grounding Point grounding

reasonable circuit layout should be:

-- correctly arrange the position and direction of components

-- connecting wires with different purposes and different levels, such as input line and output line, weak wire and strong wire, should be far away from each other, let alone parallel, high-frequency wire should be as short as possible, transmission line should be shielded, and grounding wire should be short and thick

-- for a complex working system (including working circuits with different frequencies, as well as different subsystems of micro electricity, weak electricity and strong electricity), it is necessary to make a reasonable layout. In EMC equipment, the ground wires of different circuits shall be set separately for small signal, large signal, relay and power circuits, and the ground wires together with the casing shall be grounded in three or four sets

in short, EMC design is diverse, and one thing must be clear, that is, it should be targeted and effective

4 Conclusion

A/D and D/A converters are not only in the electromagnetic environment generated by electronic equipment in their own system, but also in the electromagnetic radiation environment generated by nature and man-made outside the system. Therefore, the electromagnetic environment has a potential adverse impact on the performance of the converter, which is easy to cause its performance degradation and even loss of function, which requires designers not only to pay great attention to the problem of electromagnetic compatibility, Moreover, effective measures should be taken to eliminate it

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