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DRV8305: Current sense filtering

Part Number: DRV8305

Helllo,

I want to add current sense filtering to DRV8305N and have the following questions:

1. Should I add filters both at the input and the output for the DRV8305 like in the below picture or maybe only at input or only output?

2. For the input filter should I use two resistors and one capacitor (resistors Ri1 and Ri2 on SN and SP) or only one resistor (Ri1) as SP input is on the ground?

3. If i should use two input resistors then the time constatnt for that filter is 2*RC or RC or RC/2 ?

4. What is suggested Ci for the input filter? Sth around 100nF? I suppose I should choose firstly suggested input capacitance and then calculate resistors values based on required cut-off frequency, right?

5. Input and output filters should have the same cut-off frequency or maybe input should be according to electrical speed (20x higher) and output set to 2x below sampling of ADC? What is the best solution for most accurate current sensing? 

Adam

  • Hi Adam, 

    Thanks for posting your question to the E2E motor drivers forum!
    A few of our team members are OOO currently for the holidays, but will be returning later this week to address your questions/concerns. 

    Please anticipate a response from them around that timeframe. Thanks 

    Best Regards, 
    Andrew

  • Hi Adam,

    I want to add current sense filtering to DRV8305N and have the following questions:

    1. Should I add filters both at the input and the output for the DRV8305 like in the below picture or maybe only at input or only output?

    Are you using the current sense for overcurrent protection, or for motor torque control? In the latter case the proposed filter will add more delay to the feedback current, and this might cause instability to the closed loop control  -- leading to oscillation to the motor. 

    Have you tried to use the Blanking delay to filter out the switching noise when the gate outputs are switching?

    I would think if additional filter is needed it  is easier done at the outputs of the amplifiers. 

    Brian

  • Thanks Brian.

    I am using it to torque control.

    What do you mean by Blanking delay?

    Adam

  • I am using it to torque control.

    OK, so you don't want to add filter to the current sense path as this will delay the current feedback loop, leading to oscillation of the closed loop control. Why do you want more filter on the current amp?

    What do you mean by Blanking delay?

    When the driver gate drive outputs switching, this can cause spikes on the measured current and so you want to filter or blank out this short moment by program a proper value to the Blanking register. Search Blanking in the datasheet for more info on this.

    Brian

  • Hey Adam,

     As Brian mentioned adding filter at the input could delay the feedback. 

    Are you expecting a lot of noise? 

    Best,

    Akshay

  • Thanks,

    not much noise but some for sure, but you are right, filtering it will add delay in feedback.

    So I will not add filter close to electrical frequency of motor...

    but..

    should I add anti-aliasing filter on ADC input 2x sampling freq? Or this as well will delay feedback?

  • Anti-aliasing is always necessary at the input of the ADC, otherwise the sampling rate would need to be infinite. Just make sure the anti-aliasing cutoff frequency is beyond the frequency you are trying to measure, typically 5 to 10 times. The use of an RC filter is also important for the stability of the amplifier and for proper charging of the sample and hold capacitor of the ADC. If using the proper cutoff frequency, the anti-aliasing filter will not add a delay to the signal of interest.