Circuit board layout and wiring precautions for analog / digital mixed signals

Excellent simulation / Precautions for circuit board layout and wiring of digital mixed signals

1. brief introduction

To understand when using a resolution equal to or higher than 12 position ADC Problems that may occur , You need to determine ADC How small the voltage value can be handled . The voltage range is 2 V Of 8 position ADC The minimum voltage value that can be detected is 2 V/256 = 0.008 V, namely 8 mV about . Even though 8 mV It looks small , Let's compare this value with a higher resolution ADC Compare , surface 1 It shows that the input range of the pair is ±1 V And resolution is 8 To 20 Each of bits ADC Make a comparison .

surface 1. ADC The resolution of the

 
When the resolution is 20 When a ,ADC Capable of handling at least 2 μV The voltage of . Slightly increase the gain , You can handle less than 1 μV The voltage of . in addition , Contains a narrow input range ( high ADC gain ) Low resolution ADC The system can also handle voltage values in the microvolt range .

Use low resolution ADC when ,1 mV The following offset and noise sources are meaningless . But use 12 A to 20 position ADC when , This value will play an important role . Designers who are not used to sensitive analog circuits will easily ignore these deviations . At present, electronic products are becoming smaller and smaller , Therefore, a small circuit board geometry alone can cause many problems .

2. The wiring resistance is really important

When PCB Shrink the hours , The routing width will be narrower , The distance is closer . In the current electronic products, the wiring width and the gap between the wiring generally do not exceed 6 Mils
(0.006 Inch ). Even if you specify a size of 6 Mil's routing , This value can still be easily reduced to 4 or 5 Mils . that , Why do we need to pay attention to the phenomenon of smaller routing ? When the routing becomes narrow , The wiring resistance will increase . The formula 1 The standard formula for calculating the wiring resistance is provided :

The formula 1

PCB The thickness of the upper wiring is 1 Ounce copper , The length is 1 Inch , Width is 8 Mils , Its resistance will be 0.062 ohm . surface 2 The impedance values calculated for several routing lengths and widths are shown .

surface 2 Wiring resistance

 
As shown in the table 2 As shown in , All impedances are much lower than 1 ohm . This doesn't seem to affect the circuit that much , But the specific situation depends on the routing in the circuit The position on the board . If it is the wiring of high impedance amplifier input , Is no problem , But in other cases , Will have an impact . Use the table again and walk for each Line combinations pass through 5 mA Current . although 5 mA The current is not large , And the wiring resistance is less than 1 Ω, But when using high-resolution ADC when , Combined offset Will become very significant , As shown in the table 3 Shown .

surface 3. Line voltage offset

 
It's in the table , If inflow routing ( Its width is 6 Mils , The length is 2 Inch ) The current of is 5 mA, The voltage will be 820 μV, namely 0.82 mV Left Right . In the table 1 in , Please note that , Used in the system ADC The resolution is lower than 12 When a , The voltage is not significant . The units shown in green are at least affected 16 position ADC The condition of half least significant bit . The Yellow cells indicate that... Is in use 12 Bit or higher ADC Conditions that result in the same deviation . At this time , hypothesis 12 Bit and 16 position ADC The input range of is 2 Volt (+/- 1 Volt ).

An example application ( Where the magnitude of the offset causes a significant deviation ) Is the use of thermocouples to measure temperature . If you use K Type thermocouple , The output voltage will be 40 μV/°C about . that ,410 μV The offset is equivalent to more than 10°C The deviation of . If the same routing is over etched , Reduce its width to 4 Mils , deviation Will increase 50%. With this example , You can see each... In the evaluation signal path PCB The importance of routing . although 12 position ADC Not the worst case , However, such as fruit ADC Add in front 16 Times the gain , The corresponding voltage resolution is equivalent to 16 position ADC.

3. Shared return path

Design with mixed signal or high precision ADC Of the circuit board , It is necessary to recognize that the current is PCB The specific location in . A few cents on the line Ann (mA) The current can cause serious problems .

When digital devices or high current analog devices share sensitive analog signals When returning the path , The wiring resistance will affect the circuit . This situation Condition , The unit of high current is no longer the ampere (A), It's milliamperes (mA). In the previous example , Thermocouples and 5 mA The load is... In total Share a return path . Even if the load is reduced to 0.5 mA, partial The difference is still 1 °C. therefore , A few hundred μA The influence of current is also relatively large .

chart 1 An example is shown , Analog grounding and digital grounding The earth shares a return current path , Sensor and LED Share another A return current path . These two shared paths may cause the system to System offset or gain deviation .

chart 1. The impedance of the signal return path

 
When in this example ADC When measuring the output voltage of the sensor , it It also measures the voltage on the wiring resistance . Common ground and sensor Current and LED The longer the wiring length between the positions where the current is combined Big , The more serious the voltage excursion may occur . The severity of the deviation Depends on the accuracy required by the system 、 The voltage gain of the sensor and The magnitude of the offset voltage . chart 2 It is shown that PCB Layout An example .

chart 2. Sample layout of shared return path

 
Simulate (VSSA) Like all the signals you are measuring , Play an important role .PSoC Of VSSA The pin and the system are located at the same place The routing length and its impedance must be as small as possible . Even a few hundred Microamps (μA) The current component of shares the path , When measuring several Millivolt signal , It can also lead to many problems . Use single ended measurement when , The offset here can be considered as the measurement offset . In the figure 3 in ,LED The current of and the supply current share a path , Dan Chuan The sensor uses its own path . The internal bandgap reference circuit is also connected Receive VSSA. Cause and LED Any... Consumed by sharing the return path Voltage will make ADC The reference voltage fluctuates , Voltage drop The size is I*R. Reference voltage and VSSA The offset between them will lead to ADC Gain deviation .

chart 3. Simulate the current in the ground path

 
Digital grounding (VSSD)、 Analog grounding (VSSA)、 Sensing Utensils and LED After providing a separate grounding path , There will be no shared returns Back path ( Refer to the figure 4). The sensor 、ADC And reference electricity All circuits are connected to the same analog ground , therefore LED Electricity in Flow changes have little effect on the output of the sensor . another Also pay attention to , In this diagram , Sensor and VSSA In the same Connected to analog ground at position . The geographical location of the grounding connection can be So it is a point , Or a very low impedance layer .

chart 4. Good grounding connection

 
By making the difference ADC Connect to the sensor , Sensors can be eliminated Return common mode voltage bias caused by sharing a path with high current move ; See figure 1. Normal voltage refers to the sensor Vss And sensing Normal offset of the output of the . However , The differential connection of the sensor is not Can reduce VSSA Deviation when sharing the grounding path ( chart 3). See figure 5.

chart 5. Difference ADC And a separate return path

 
chart 6 An example of improved routing is shown , Including separate Return path 、 Separate analog and digital power supplies , And the sensor Differential connection .

chart 6. Example layout of a separate return path

 

3.1 Consider potential problems carefully

When the sensors share the return path or modulate the load ( Such as PWM drive Of LED) share VSSA Pin time , May not be immediately detected Bad . If the debug load is the same as ADC Full synchronization , The resulting deviation can be It can be big , It could be small . If no... Is generated during synchronization Measurable deviation , that , When starting development and testing , I can't Any questions now . But if, in this case, you modify ADC sampling Rate or PWM frequency , The deviation or noise will change significantly . Such changes are difficult to test , Because in many applications , negative The on-board modulation may vary according to different environments or software . because this , A circuit board design can sometimes work properly , occasionally You can't work . therefore , Even if the design works properly , still Good design rules need to be followed .

4. Wiring of analog and digital signals

Ideally , Analog and digital signals will be located opposite the circuit board Supralateral , But it doesn't usually happen . Many designs require Analog and digital signals are located in the same area . Unfortunately , stay High impedance analog signal and digital signal are simultaneously operated in a region May cause unexpected crosstalk , The crosstalk brings too much to the analog signal noise .

4.1 What is crosstalk ?

Crosstalk refers to when there is no direct connection , One signal to another The phenomenon of No . A number with fast rise and fall times It is most common for a word signal to affect a high impedance analog signal path Crosstalk phenomenon of . Digital signals are also affected by crosstalk . High speed data Word signals are easy to affect other digital signals . String between signals The disturbance type is : Conduction 、 Capacitive or sensual . In all cases , By increasing the distance between the signals and shortening the parallel between them length , It can reduce signal crosstalk .

The effect of conducted crosstalk is generally small . Only the impedance of each signal is too high ( exceed 10 MΩ) when , This kind of crosstalk will cause problems . When PCB Soil appears on the surface 、 oil 、 Salt or other liquid foreign matter , Increasing the Between each route PCB The conductivity of the material , High... Usually occurs Conducted crosstalk . Crosstalk caused by impedance drop will affect circuit operation Do have an adverse effect . In some cases , The solder mask can protect guard PCB. But there will always be bare areas , Such as PCB Upper device welding The location of . If these materials are found in the environment in which the product is used , Various measures must be taken to make PCB Isolate from these materials . If you can't make PCB Isolate from foreign matter , Can be in PCB Use exterior paint on , But this method will increase the cost .

When a route is directly above another route in another layer , Capacitive coupling will occur . A capacitance is formed between the copper wires . These copper The more lines overlap , The higher the capacitance formed by their coupling . adopt Reduce the overlap area between signals to reduce the capacitance , Thus reducing coupling close . In some cases , Especially on double-layer circuit boards , almost The cross between sensitive analog signal and fast digital signal cannot be eliminated condition . At this time , These signals need to be in the form of 90 o The angle of , To the best of Reduce the capacitance formed between them .

If more than two layers of multilayer circuit boards are used , Please ensure that there is a power layer between the two intersecting signals , To minimize coupling . Please note It means , chart 7 The capacitance in the is formed between two wires , It is proportional to the overlap .

chart 7. Capacitive coupling of parallel routing

 
If multilayer circuit board is used , Please make sure that the analog and digital cables are routed to 90° The angles intersect . This greatly reduces the overlap area , So as to reduce the cost of Capacitive coupling between signals . chart 8 An example is shown .

chart 8. Capacitive coupling of vertical routing

 
chart 9 It is shown that PCB An example of a layout , Among them, analog routing ( Red ) Must be connected to the digital wiring ( Blue ) cross . Please note that , Between analog and digital cabling is 90°.

chart 9. Digital routing is based on 90o Cross with analog routing

 
Each line running on the same or adjacent layer may be magnetically coupled close . This situation is called inductive coupling . Inductive coupling consists of three mechanisms Characteristics cause . These characteristics are : Separation between each route 、 Two The distance between parallel routes 、 Distance between wiring and its nearest power layer leave . Distance between signals and between signals and ground plane Distance is the most important factor , As formula 2 Sum graph 10 Shown .

The formula 2

 

chart 10. Space of inductive coupling

 
As you can see , Distance between wiring and ground plane ( That is to say Height ) It's an important factor . By shortening the distance , Maybe The value of height squared reduces crosstalk . If you need to run adjacent numbers Word and analog routing , that , Bringing them close to the ground plane would be The best way to reduce crosstalk .

4.2 3W principle

3 W The rules specify the logic routing ( Center to center ) Between The distance must be three times the routing width . for example , If PCB Go up The width of the line is 0.008 Inch , Between two adjacent routing centers The distance will be 0.024 Inch (0.008 Inch x 3), Its edge The distance to 0.016 Inch (0.008 Inch x 2). This will make Each route is in the middle of another route 70% Magnetic flux boundary range Outside . In order to be able to locate 98% Outside the magnetic flux boundary , Two The distance between adjacent routes must be... Of the route width 10 times . these The condition depends on the impedance of each line and the rising time of each signal between . See figure 11.

chart 11. 3 W Examples of rules

 
Reduce ( Adjacent running on the same side of the board ) Between signals Another method of coupling is to place a guard between these signals Line , And ground it . This reduces the capacitance between signals coupling . See figure 12.

chart 12. Use protective wire

 
In a multilayer circuit board , The distance between the layers is different . for example , At a thickness of 0.062 " 4 Layer circuit board , With the second layer and The distance between the third layer , The distance between the first floor and the second floor Closer . therefore , Walk both analog and digital signals in the same area when , Please distribute the routing to non adjacent layers , It can be expanded as much as possible Increase the distance between them .

5. Multi power domain

In sensitive analog systems , It is necessary to separate the analog power supply from the digital power supply . It is generally recommended to use independent external analog and digital voltage regulators . If the cost of additional voltage regulators is too high , And the digital part of your design does not include high-speed or High current switching function , A single voltage regulator can be used . Just as you have a separate regulator , It should be noted that analog and digital power supplies should always be isolated in design road . Analog power supply (VDDA、VSSA) And digital power supply (VDDD、VSSD) Provide independent power supply and grounding signals . Please shorten these two power supplies as much as possible ( Analog and digital power supply ) The connection distance from the circuit board power supply . The output impedance of the circuit board power supply is generally low , So through the above connection , Digital power supplies are hardly Will affect the analog power supply .

6. Ground plane

Ground plane is always useful in mixed signal design , But for a given design , The cost of additional layers is high . Even in a double-layer circuit board , You can also add some layers in the sensitive simulation section . Whether you use a ground plane or not , It is necessary to ensure the shortest connection between the return path and the power supply . Please note that , If the impedance of the ground plane power circuit is not low enough , Or over dispersing the layer , You can't use this layer to improve your design . On a double-layer circuit board , Don't just rely on the final ground level fill , This may lead to a high impedance ground path . If you don't check carefully , It is difficult to find such defects . A good design habit is , First lay out the grounding path through wiring , Then fill the ground plane .

If you can use separate analog and digital ground planes in your design , So in almost all cases , They need to be connected at a single point . This single point requires To be located between the power supply and SoC Between devices .

When only one single voltage regulator is used , Only if the analog and digital components are isolated from each other , The ground plane may not be divided .

7. Bypass capacitance

7.1 Capacitor selection

surface 4. Summary of bypass capacitor connections

 

chart 15. Examples of power connections Schematic diagram

 

There are two kinds of capacitors for power supply stability : Bypass capacitor and large capacity capacitor . Sometimes large capacity capacitors are also called energy storage capacitors . The bypass capacitance must be located in the bank Near the power supply pin . The use of bypass capacitors can eliminate high-frequency noise and provide current for instantaneous conversion . The range of these capacitors is 0.001 μF To 0.1 μF. NPO、X5R And X7R Equal dielectric capacitor is an excellent bypass capacitor . The range of these capacitors is hundreds of picofarms (pF) To a few Micromethods (μF).

The energy storage capacitor is usually located near the voltage regulator . If the circuit board is large ( More than a few square inches ), And there are some active devices everywhere , that , These electricity The capacity will be distributed throughout the circuit board . The energy storage capacitor can supply power for a long time , And can filter out low-frequency noise . On the circuit board with high current signal or power supply in , The value range of storage capacitance is 1 μF To 100 μF, Or greater .X5R、 Tantalum and some surface mount electrolytic capacitors are suitable for this purpose .

The bypass capacitance is generally only 0.01 μF or 0.1 μF. Some simple calculation operations are recommended , To get the best energy storage capacitance . If the value is too high , said The energy storage capacitance exceeds the capacitance you need . If the value is too low , It will cause excessive power ripple and noise . Please use the following formula :

Capacitance calculation formula

 

8. All capacitors are not equal

When selecting a capacitor for various applications ( Even a simple bypass capacitor ) when , Its specification is very important . Voltage and temperature coefficients are two of the most neglected capacitance specifications , But it can greatly affect the device capacitance under the normal operating environment .

Devices are getting smaller and smaller , There is a trade-off between performance and size . The nominal value is 1 uF, Withstand voltage is 6.3 V The capacitance of , At a voltage of 5 V when , The capacitance value can Less than 0.1 uF. therefore , You need to pay attention to the voltage coefficient . in addition , It is not assumed that the voltage coefficients of all device series are the same . Voltage coefficient and temperature coefficient Can vary greatly from package to package . And 0603 Package compared to ,0805 The package has good voltage coefficient , But sometimes the other way around . So read the data manual . If the temperature and voltage coefficient of the capacitor are not provided in the data book , Please consider using capacitors from other manufacturers .

9. Mixed signal PCB Rule summary for

When designing a mixed signal circuit board , The following rules must be followed :

1. Consider separate analog and digital power supplies .
2. Know all return paths .
3. Although it's expensive , But if possible , Please use a four layer circuit board .
4. Do not route analog signals in parallel with clocks or fast digital signals .
5. If the analog and digital signals must cross , Please make sure that these signals are in the form of 90 o The intersection , So as to minimize the coupling capacitance .
6. The power layer should appear in the corresponding area of its signal line . for example , Run only analog signals on the analog power layer .
7. Place the bypass capacitor between and IC As close as possible . in addition , Also make sure that the bypass connection of the power signal is low impedance .
8. If you can , Please use independent analog and digital signals and independent digital and analog components on the circuit board . Appoint PCB Of “ simulation ” and “ Numbers ” Area .
9. Avoid long wiring for high impedance input signals , Otherwise, it will couple noise into the signal link like an antenna .
10. Maximize the width of the power cable to reduce the impedance .
11. Place the analog signal at the position closest to the ground layer , To minimize inductive crosstalk .
12. When connecting the power signals between layers , Please use large or multiple vias , Important to reduce impedance .
13. Reduce the digital rise and fall time of the digital signal as much as possible .
14. Use protective wire to isolate analog and digital signals from each other .

10. PCB Tools for layout and automatic routing

PCB The layout tools are 20 Year history . By using these tools, the signals can be grouped , It also provides the following information for the routing length and the distance between each routing Different rules . So as to avoid errors . Automatic routing is becoming more and more powerful , And has many common tools . These tools follow the requirements for manual routing The same rule . skilled PCB Layout designers can use these rules to improve the performance of automatic routing . Although these tools are very powerful , But it still needs special attention It means the wiring mode of analog and digital signals . It is specially recommended that you manually move the sensitive parts on the circuit board first , Then the rest of the unimportant parts can be removed through automatic wiring branch . Either way , Be sure to check the final wiring .

Placing the devices in the best position is very helpful for manual wiring and automatic wiring . After the device placement and circuit board layout are arranged , It's easy to use To verify whether there is a problem with the shared return path . Print the board layout and draw the most direct path between the power supply and each component . For analog components And digital components use two different colors . If the two colors cross , Need to re evaluate your design . Please refer to the picture 19.

chart 19. stay PCB Draw the return path on the layout

 
The article comes from Lao Wu's blog , You can pay attention to Lao Wu's blog , first-class EDA The engineer . Original reference