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Routing Sensitive analog signals

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  • Routing Sensitive analog signals

    Hi guys,
    I have one project with me.described below.
    Project Title:3 Phase BLDC motor controller
    Summary:Basically project is to control 3 Phase BLDC motor around 10amp per phase.project contain Main controller with which motor driver IC is connected.Driver IC is connected with half bridge motor driver power section.

    Problem : To measure current consumption there are shunt resistors are connected to each phase of power section.so in physical layout distance between main controller and shunt resistors is very long so, how to route this very long distant and sensitive analog signal.what is strategy to do routing of this kind of signals.

    I won't be able to share schematic as it's confidential.

  • #2
    Can't you measure the current at the battery input perhaps? It might be less noisy. Also the circuit should be quite compact, why do you have such a long distance for those traces (how long)? I wouldn't be that worried about the length of the traces but about the switching noise affecting the measurements. I've designed a BLDC controller board before, but I didn't need to measure the current.

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    • #3
      It's industrial board.This board require to measure current per phase,voltage per phase,Rpm of motor using hall sensor,and send this to control room via zigbee.For vibration measurement they have used accelerometer mpu9150.For speed control they have used pwms.
      I have designed this board such that distance between main controller and power section is long so if I route that "current sens" analog tracks from power section to main controller it might be affected by lots of noise and thats why I asked this question

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      • #4
        how long is long in your case..?

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        • #5
          Is this current feedback connected directly back to the dedicated motor controller chip or you just could be processing the current information by additional microcontroller? If it is just for reading currents, you could use an AD converter placed as close as possible to each resistor and read these AD converters with a microcontroller.

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          • #6
            In this diagram I kept rc low pass filter very near to stm controller but distance from shunt to differential amplifier is somewhat longer and that is sensitive analog signal which I was talking about.In addition I have splitted the ground plane for high and low voltage DC Supply.

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            • #7
              Hmm, I am not expert for analogue stuff. However, I am not sure if I would be doing it this way. The output from the amplifier may be correct value of the difference on resistor, however, you will be measuring this output relatively to GND of microcotnroller (which may be noisy and different comparing to GND under amplifier) and you may not get accurate results even if the tracks between resistor and amplifier are fine.

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              • #8
                Then whats the way to make it accurate.
                I have asked same thing to one of my senior engineer and he told me to add via shield onto track between amolifier and microcontroller.He hasn't given any comment related to the GND

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                • #9
                  If you for example use a cheep and simple AD converter close to the resistor, then the signals from AD to Microcontroller are digital signals and you do not need to worry about noise on sensitive analogue signals. That is what I would maybe try in this situation.

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                  • #10
                    Thanks robert and everyone for help.

                    For now what i did is described here.As there are three phases of motor and three shunts for each phases.Now there are two wires going into the differential inputs of AD8418(DIFFERENTIAL AMPLIFIER WITH Gain 20v/v).For routing i kept all the amplifiers nearer to the microcontroller and provided seperate ground track connected to the Main system supply GND .Output of amplier is connected to the RC Low pass filter and i have also kept RC low pass filter nearer to the adc pins of microcontroller.Reason for seprate ground track is to not use common noisy digital ground plane which may introduce potential changes in GND.Another thing i did is i kept all digital signals far away from sensitive analog signal.

                    LET ME KNOW THIS IS GOOD WAY TO DO IT OR NOT?

                    Robert, I can not use seprate ad converter because i have to use inbuilt adc of stm microcontroller.

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                    • #11
                      Please, let me know how it worked out.

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                      • #12
                        To gauge current utilization there are shunt resistors are associated with each period of intensity section.so in physical design remove between principle controller and shunt resistors is long thus, how to course this long far off and delicate simple signal






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