Program-controlled voltage source and its electromagnetic compatibility design

1 Introduction

In some electronic instruments and equipment, an adjustable voltage source or current source with excellent performance is often required. In the past, operational amplifiers were mostly used to complete this. Now, with the improvement of the accuracy of integrated A/D and D/A converters, A/D and D/A converters have been widely used to form precise program-controlled voltage sources or current sources. As for the type of converter to be selected, whether the output is voltage or current, it should be determined according to the actual situation. In addition, the system where the converter is located is full of various forms of noise. Noise will inevitably produce measurement errors, and errors will inevitably reduce measurement accuracy. Electromagnetic interference is the main factor affecting measurement accuracy. Therefore, when designing a system containing a converter, effective measures should be taken to eliminate it.

2 Typical program-controlled voltage source

The following uses the D/A converter as an example to explain the working principle of the program-controlled voltage source. When the output of the D/A converter is a voltage output, it is itself a good precision program-controlled voltage source. The accuracy of the controlled voltage depends on the relative accuracy of the D/A converter. Figure 1 is a basic program-controlled voltage source circuit.

Figure 1 Digitally controlled programmable voltage source

It should be pointed out here that when the output drive capability is sufficient, amplifier A2 can be omitted. According to the basic principle of D/A converter, the circuit should satisfy the following transfer function:

UO=（UR/RR）RFai·2－i (1)

Where: ai——the input digital value of the i-th position, i=1,2,…,n, the value of ai is "1" or "0";

UR——reference voltage;

RR——reference input resistance;

RF – The feedback resistor of the amplifier.

For offset binary code, the transfer function is:

UO=(UR/RR)RF(ai·2－i－1/2) (2)

From equations (1) and (2), we can see that the output voltage is a function of the reference current IR = VR/Rr, the feedback resistor Rf, and the input digital quantity. Changing the reference current IR and the feedback resistor Rf, that is, changing the maximum output amplitude, and changing the input digital code can control the value of the output voltage. In other words, programming is achieved by changing the input digital quantity of the D/A converter.

If the outputs of a group of power supplies required all have integer multiples or power relationships, n multiplication-type D/A converters can be used directly to form a programmable series power supply, as shown in Figure 2. In the figure, the power supplies are iteratively multiplied, and the relationship is:

U1=－URD

Figure 2 CNC series power supply

U2=－U1D=URD2

…

Un=－Un－1D=(－1)nURDn

Where: D is the input digital quantity. Figure 3 is a practical program-controlled voltage source, which is used in the interface circuit of computer-aided test equipment.

Figure 3 Program-controlled voltage source

This circuit can output 0~20V voltage. There are 1024 steps of programmable voltage source, each step is 20mV. The voltage regulator diode is used as reference voltage. The transistor V1-3 is used for voltage and current amplification. The digital input comes from the output bus of the computer.

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

3 Electromagnetic compatibility design of programmable voltage source

Choosing the right A/D or D/A converter is only the starting point of the design work. The specific system in which any converter is located often needs to be considered comprehensively, especially when there are power supply voltage fluctuations and other noises in the system, the normal operation of the converter must be ensured. This is because in the circuit, noise can easily drown out one or more digital signals, which is particularly harmful in high-speed and high-precision systems. Therefore, effective and reasonable electromagnetic compatibility design is essential.

The following guidelines should be followed when designing for electromagnetic compatibility:

1) Cut off the channels through which electromagnetic radiation enters the converter and causes mutual interference, thus reducing spatial coupling.

2) To prevent interference from external devices, various pulse modulation signals (radar, radio waves, TV signals), changes in electromagnetic fields and other factors, shielding protection must be taken for devices and components that are sensitive to electromagnetic effects, such as: shell shielding, cable filtering and internal cable shielding.

3) Design the circuit reasonably, select devices and circuits correctly, accurately calculate the various parameters of devices and circuits, formulate measures to identify and isolate critical circuits and adopt technical solutions to suppress interference, try to choose a high working signal level that meets the actual load capacity of devices and circuits, and pay attention to the "interface" design.

4) Use a highly stable voltage-stabilized power supply to improve the power supply voltage sensitivity, reduce linear errors, gain errors, and adjustment errors caused by power supply fluctuations, and ensure stable and reliable 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 as follows:

——Floating ground;

——One point grounding;

——Multiple grounding points.

A reasonable circuit layout should be:

——Correctly arrange the position and direction of components;

——Connecting wires of different purposes and different levels, such as input wires and output wires, and weak wires and strong wires should be kept away from each other and not parallel. High-frequency wires should be as short as possible, transmission lines should be shielded, and ground wires should be short and thick.

——For a complex working system (including working circuits of different frequencies, as well as different subsystems of micro-electricity, weak current and strong current), a reasonable layout is required. In EMC equipment, the ground wires of different circuits should be set separately for small signals, large signals, relays and power circuits, and the ground wires of the casing should be grounded in three or four sets.

In short, electromagnetic compatibility design is diverse, and one thing must be clear, that is, it must be targeted and effective.

4 Conclusion

A/D and D/A converters are located in the electromagnetic environment generated by the electronic equipment in their own system, and in the electromagnetic radiation environment generated by nature and man-made outside the system. Therefore, the electromagnetic environment has a potential adverse effect on the performance of the converter, which can easily cause its performance degradation or even loss of function. This requires designers to not only attach great importance to electromagnetic compatibility issues, but also take effective measures to eliminate them.