Analog and Digital Grounding...

Discussion in 'General Electronics Chat' started by mwalden824, Apr 1, 2014.

  1. mwalden824

    Thread Starter Member

    Mar 6, 2011
    I'm working on a Brushless DC Electronic Speed controller, and I have successfully implemented an open loop method by controlling the commutation rate manually. Obviously this won't work well for varying loads (which the propeller is just one load but winds and other factors I can't think of right now will easily cause it to fall out of open loop).

    So I was attempting to do the Zero Cross Back-EMF detection method on a breadboard but there is so much noise most likely from various sources such as the fast switching digital signals from the PWM and other signals of the MCU coupling into the analog signaling. The wires on the breadboards can also act like little antenna and pick up no telling what EMI, and the breadboard with its various parasitic effects. I need a pretty decent Back-EMF signal to be able to do this method. So I am designing a PCB in Proteus Ares.

    Now from what I understand, please correct me if I am wrong at any of this, is that you need two separate ground planes and they need to connect at single point somewhere where the analog ground plane kind of traps the analog current and the digital ground plane traps the digital current but with a small trace between the two so that they are pretty much still at the same potential.

    Now my question is, do I need two separate power supplies? The way I have it now is a terminal block with 3 input; Positive from the 14.8 volt lipo battery, negative, and a rather low frequency analog signal (0 to 2.7 volt) for the control of the speed that will go to the ADC of the MCU. And another terminal block, 3 out, one for each phase for the BLDC motor.

    Now I have an adjustable linear regulator that is taking the 14.8 V down to 3.3V for the MCU. Am I right to assume I should put this ground pin on the regulator on the digital ground plane since it will power all the digital components and the battery will basically be the power supply for everything really but will supply the analog components directly without any regulator....? Or am I wrong here..?

    This is my first PCB I am designing and ordering. I can upload the Gerber files if needed or pictures of them.

    Mainly I am just asking about different methods used to separate the analog and digital signals so there isn't interference when the two different components are placed on the same board.....?

    I currently only have a 2 layer through hole board mostly designed because I was planning on buying the equipment and chemicals to etch my own boards but I decided to redesign it with 4 layers using a ground plane and power plane and SMT components. This way I can make the board a lot smaller and possibly put a large enough heat sink to cover the whole board for good heat dissipation keeping all the components cooler so I could run higher current, so faster and more RPM and maybe even overclock the MCU and ADC.

    Should I put the ground and power planes in the middle layers or on the top and bottom with the signaling in the middle layers....? I've heard good things about both ways.

    Well sorry if this thread is too long, but I appreciate any help and advice any of you can give me. If you have ever seen my other posts you know I have been working on this for a long time. It is because I don't have enough time to work on it much so I pick it back up whenever I can. But slowly and surely I will eventually get this done.

    Thanks again,
    Michael Walden

    P.S. I know some of your are going to say use Google, but I just want some opinions from you guys. Some of you guys have been here a long time and I have much respect for the opinions here. I have read app notes on the subject but could not find anything particular to BLDC ESC PCB design of ground planes, if it even matters because I am sure mixed signal PCB design with mentioning of analog and digital ground planes would suffice.
  2. tindel

    Well-Known Member

    Sep 16, 2012
    mwalden824 likes this.
  3. mwalden824

    Thread Starter Member

    Mar 6, 2011
    Yeah, I'm just getting started into PCB design. Been reading a lot about it lately, but Awesome! ... and thanks for the link.

    Michael Walden
  4. MrChips


    Oct 2, 2009
    It takes a lot of know-how and experience in order to design a mixed signal PCB correctly. Your concerns are well founded for if not done properly it will cause havoc.

    The first thing you should do is examine all your power requirements, voltage and current. How much voltage and current does each of the subsystems require, motor and motor controller, analog and MCU?

    Going from 14.8V down to 3.3V with a linear regulator is not a good idea if the MCU is going to require a bit of current. If you're consuming 100mA then that is about 1.5W wasted as heat in the regulator. Consider using a step-down switching regulator instead of a linear regulator.

    Going to a 4-layer PCB will help but do you need the extra trouble and expense? I would consider a 2-layer layout first to see if that is feasible.

    Yes, you definitely want separate ground planes.

    I would lay out the PCB in separate areas, motor controller, analog and MCU. Try to physically locate the sensitive analog sections away from the motor section and any switching supply area.

    Put a continuous ground plane on the bottom side, continuous except split the ground planes into three areas. Then you connect the ground planes together where the power connector is mounted on the board. Don't worry about having a thin connection. Just join them at the power connector pin.

    Depending on the power requirement of the motor you will need a hefty 1000-4700μF aluminum electrolytic capacitor where the power enters the board.

    Next, layout your supply lines giving ample width where extra current is required.

    Use ferrite core bead inductors to feed the analog supplies.
    Use SMD filter capacitors across all power supply pins and GND as close to the IC chips as you are able. Follow manufacturer's recommendations for capacitor values. Typically, you will use lots of 0.1μF ceramic SMD caps at the power supply pins and 10μF tantalum electrolytic caps also on the analog ICs.

    A good rule is to think worst case scenario and design for it. It pays off in the end.

    Hope this helps.
    mwalden824 likes this.
  5. mwalden824

    Thread Starter Member

    Mar 6, 2011
    Wow, thank you MrChips! That does help my understanding a very great deal. Between your advice and tindel's link, I should be able to have enough information to make a decent PCB, hopefully.

    You know I wasn't even thinking about the power dissipation in that linear regulator with such a large drop. I probably would have eventually caught that mistake because the MCU (ATxmega32A4U) is rated at 200 mA max per Vcc pin which has 3 and an AVcc pin which I assume is another 200 mA since the datasheet doesn't specify. I think the Power for linear regulators is calculated by (Vout-Vin)*Iout if I'm not mistaken. So 4x200mA=800mA and (14.8-3.3)*0.8 =9.2 W which is way over the limit for the device. Not to mention, like you said, it's wasted power even if it wasn't over the limit.

    With the voltage regulator, I was just worried about using a switch-mode voltage regulator because of the extra noise it would generate, but as long as I follow your advice and the advice from the link above I shouldn't have a problem.

    My worst case requirements are:
    Motors - 30 A max (which means I definitely need good heat sink for the MOSFETs) and the driver chips.
    MCU - 800 mA max at 3.3 V, 32 MHz clock.
    Drivers - 1 A

    So I was thinking of using the mc34063 switch mode power supply chip now. It has multiple configurations (step-up, step-down, and invert) and I already have many of them. Unless you can recommend a better solution? It switches at 100 kHz so I will have to worry about the placement of that on the PCB as well. I guess just keep it in the digital section since it will be powering only the MCU.

    Well thanks again,
    Michael Walden