Common ground interference problem in measurement circuit

introduction

In the practice of electronic product development, many interference problems are often encountered. These problems will have a serious impact on the reliability and performance of the product. At the same time, it requires a lot of time and resources to investigate and correct, and become the product of mass production. bottleneck. In the design of test and measurement products, it is a relatively common type of interference.

Common ground interference

Common ground interference, also known as common ground impedance coupling interference, is caused by the common ground or ground plane for various signals in the circuit. The ground wire is used as a common signal return path or reference plane. In theory, there should be no impedance, but the actual situation is not the case. The voltage drop on the ground wire caused by the return of different signals will become mutual interference. If the signal is sampled , The conditioning circuit, or the unreasonable PCB design, will have a significant impact on the sensitive and weak small signals. As shown in Figure 1, the return current i of other circuits produces an error voltage on the signal loop.

Figure 1. Real grounding system

In terms of the common ground interference not only affects the accuracy of the measurement, but even fails due to the inability to stabilize the reading. The following is an actual product development.

Use RMS chip to measure the true effective value of AC voltage

When measuring the AC voltage input signal through the RMS dedicated chip AD636, a malfunction occurred: the effective value reading of the chip output drifted slowly from the moment the circuit was powered on, and the speed of change was fast and slow, irregular. Since this is a precision measurement circuit, it has high requirements for measurement accuracy, and such measurement readings cannot meet the product specifications and accuracy requirements. So it must be improved.

Analyze the design of the circuit, Figure 2 is a schematic diagram of the chip wiring. Marked as Iout, RL, COM are pins 10, 1, and 2 respectively. Iout is the RMS output current pin, RL is the load resistance pin, and the COM pin is the common pin. The working principle of the chip is to output a signal proportional to the effective value of the input in the form of a current source, and convert the current into a voltage through an internally connected RL load resistor. Refer to the block diagram of the chip to explain that the Iout pin is exactly this output voltage, and the signal is connected to the subsequent circuit for further processing. In summary, Vac in and COM form the input measurement signal, COM is the input reference ground, Iout and RL form the output signal, and RL is the output reference ground. According to the recommended method of using the chip, connect the RL and COM pins together, and connect them to the reference ground GND of the circuit as shown by the solid line in the figure.

There is nothing wrong with this wiring method from theoretical analysis. However, in actual use, serious problems have occurred. What's the reason? Check the PCB design carefully, and found that due to the limitations of the physical layout of the components on the board, the input signal Vac in is connected to the AD636 chip through a long trace, and its reference ground is connected to the ground plane GND nearby, away from the sampling The reference ground of the signal is quite far away. In other words, the return path of the input signal is shared with other circuits on the board, and the common ground interference will be superimposed on the input signal, resulting in malfunction.

The measure to solve this fault is to disconnect the nearest connection between COM and RL, and take a ground wire to this pin separately. As shown by the dotted line in the figure. Although the trace is finally connected to the GND ground plane, the return path of the input signal has been changed at this time and no longer shares the ground plane return flow with other circuits, so there is no influence of common ground interference. Practice has shown that after such a simple rectification, the fault has completely disappeared and the readings are very stable.

figure 2

in conclusion

In the precision measurement circuit, the signal loop under test is separated from other functional loops to avoid interference from other circuits, and the signal can be measured more accurately. It is manifested in PCB design, that is, not only the signal line should be far away from the interference source, but also the return path, which is the signal ground line, is affected by common ground interference. With the development of electronic technology, test and measurement equipment is becoming more and more complex and precise, and interference problems are becoming more and more prominent. Only by comprehensively analyzing various interferences and taking effective preventive measures can we develop stable and reliable high-quality products.

About Dingyang

SIGLENT is a company specializing in general electronic test and measurement instruments and related solutions.

Since the launch of digital oscilloscope products in 2005, Dingyang Technology has been a fast-developing digital oscilloscope manufacturer for 10 years. After years of development, Dingyang's products have been extended to general test and measurement instruments such as digital oscilloscopes, handheld oscilloscopes, function/arbitrary waveform generators, spectrum analyzers, desktop multimeters, and DC power supplies. In 2007, Dingyang established a strategic partnership with LeCroy, a high-end oscilloscope. In 2011, Dingyang developed into a digital oscilloscope manufacturer with sales volume in China. In 2014, Dingyang released the SDS3000 series, China's first smart oscilloscope, leading the trend of transition from functional oscilloscopes to smart oscilloscopes for "one hand" type laboratory oscilloscope for research and development. At present, Dingyang has established branches in Cleveland, USA and Hamburg, Germany, and its products are exported to more than 70 countries. SIGLENT is gradually becoming a brand of test and measuring instruments.

About Dingyang Hardware Design and Testing Think Tank

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