TU Chemnitz - January 2019 Mohammad Hossein Tarokh - tuc.tarokh@gmail

1. Mixed Signal Circuit Layout TU Chemnitz - January 2019 Mohammad Hossein Tarokh - tuc.tarokh@gmail.com 1

2. Introduction to mixed-signal circuits What are mixed-signal circuits? Where are mixed-signal circuits used? Why are mixed-signal circuits important? How are they implemented in practice? 2

3. What are mixed-signal circuits? • Mixed-Signal Circuits • logic gates, MCUs, MPUs, or FPGAs • circuits such as amplifiers and ADCs • In general where embedded systems and Analog signals meet. Basically almost everywhere! 3

4. Why are mixed-signal circuits important? • Industrial Automation and Control – DCSs, PLCs, Smart Relays,… • Smart Sensors – Temperature, Pressure, Strain, … • Instruments and Data acquisition – data loggers, oscilloscopes, RLC meters, … • Multi media – Signal Acquisition, Sound card of PCs,… • Medical equipment – ECG,… 4

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7. How are they implemented in practice? 7

8. Mixed-signal PCB problems usually involve either one of two situations: • 1stinvolves digital logic circuits that interfere with sensitive low-level analog circuits [often audio or radio frequency (RF)] • 2ndinvolves high-power motor and relay driver (noisy analog) circuits that interfere with both digital and analog circuits. 8

9. Some basic Circuit layout definitions PCB (Printed Circuit Board) – Technology behind building electronic circuits. Trace – copper connection of components like wire in electrical systems. Power/Ground Plane – copper or grid areas on PCB that are used to distribute power and ground conductors in the circuit to make them as ideal as possible. Placement – How the components are placed on the board. Pad – the place where component pins are solder to the board . Routing – the process of connecting components pins together to form the circuits. Footprint – A footprint or land pattern is the arrangement of pads of a component Microstrip – a trace above a ground/power plane. Stripline – a trace between 2 ground/power planes. Via – holes in the PCB that connects different layers. Stack up – PCB layers spacing and material and characteristics. 9

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11. Always keep in mind two basic principlesof electromagnetic compatibility (EMC). The currents should be returned to their source as locally and compactly as possible, that is, through the smallest possible loop area. If the current is not returned locally and compactly, it creates a loop antenna. second is that a system should have only one reference plane.If a system has two reference planes, it creates a dipole antenna. Both are undesirable results! 11

12. The key to determining the optimum mixed-signal board layout is understanding how and where the ground return currents flow. Most designers only think about where the signal current flows (obviously on the signal trace) and ignore the path taken by the return current. 12

13. SPLIT GROUND PLANES 1stbasic problem • The high-speed digital logic might interfere with the low-level analog circuits. • We want to make sure that the digital ground currents do not flow in the analog ground plane: • hence, the often-heard recommendation to split the ground plane into analog and digital sections. 13

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15. if the digital signal traces are routed properly, the digital-ground currents have no desire to flow through the analog portion of the ground plane and corrupt the analog signal. 15

16. Therefore, it is not necessary to separate the analog and digital ground planes. 16

17. How to deal with an ADC/DAC analog and digital grounds? It is recommended to connect both the AGND and the DGND pins of theADC/DAC converter to the analog ground plane 17

18. 18 An acceptable layout of a mixed signal PCB

19. A proper placement of mixed-signal PCB with multiple A/D converters and a single ground plane 19

20. When should split ground planes be used? • medical equipment • Some industrial process control equipment where the outputs are connected to noisy, high-power electromechanical equipment 20

21. An example of wrong high-speed signal routing which violates the split planes and causes radiated emission due to large loop; although, no digital return current goes in to analog section. 21

22. Important points of split-ground planes • Overlapping the planes must not occur :will increase the interlayer capacitance, which will decrease the high-frequency isolation, and isolation is the reason for splitting the planes in the first place • Routing traces across the split :digital traces crossing over the split will have increased crosstalk to adjacent traces, and it can increase the radiated emission by 20dB or more. 22

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24. The Chain-Saw Test On a split-plane board, you should be able to cut through the board at the split and not cut any traces or planes. If that is the case, the split plane board was most likely properly laid out and routed. 24

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27. What about multi-board systems? Another way to isolate the digital ground from the analog ground is to place the digital circuits on one PCB and the analog circuits on another PCB. Which board should then have ADC/DAC mounted? 27

28. The A/D or D/A converters should be mounted on the analog board 28

29. What if we have high resolution ADCs? The use of a single solid ground plane, properly partitioned and routed (as discussed) is usually adequate for most low-to-moderate resolution A/D converters (8, 10, 12, 14, or even 16 bit) 29

30. However, in case of high-resolution measurements, even a small current of only 0.1% or even 0.01% of total digital return current may cause a problem if it flows through analog ground plane. 30

31. What is the solution then? 31

32. One possible solution is : Isolated analog and ground planes approach In this approach the ground plane is divided in to separate analog and digital regions, all still solidly connected to the digital ground plane under each ADC/DAC ground planes. This way the digital ground currents can not flow in analog portion of ground. It means additional noise isolation without splitting the planes. Note: still in this approach no trace on any layer can pass over the isolating slots. 32

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34. Mixed Signal support circuitry In high-resolution converters, it is also a good idea to connect the digital output to an intermediate buffer register, which is located adjacent to the converter, to isolate it from the noisy digital data bus. The buffer serves to minimize loading (minimizing the required output current) on the converter’s digital outputs and prevents the system data bus from coupling noise back into the converter’s analog input through the converter’s stray internal capacitance. In addition, series output buffer resistors (100Ω to 500Ω) could also be used in addition to, or in place of, the buffer to minimize the loading on the digital drivers, which reduces the transient currents in the converter's output 34

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36. 2nd basic problem – The IPC Problem Industrial Process equipment, presents somewhat different mixed signal problem. Here it is often the case where the noisy analog circuits that consist of motors, relays, and solenoid drivers interfere with digital or low-level analog circuits. Motors, relays, and solenoid drivers are low-frequency signals, therefore the return currents from them takes the path of least resistance. All previous principles are the same but with a slightly different application. 36

37. Following are some methods of dealing with the IPC problem: • Use a return trace, not a plane, for the noisy analog signal (works well). Segment the ground plane and use a single bridge for traces that must cross over. • Route noisy analog traces such that the return path of least resistance (direct path) does not pass through the digital or analog portion of the board. • Split ground planes and use opto-isolators, transformers, or magneto-resistive isolators for signals that must pass over the split in the ground plane. 37

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41. Thank you for your time and consideration 41