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MSP430F5529: RC value for low-pass filtering

John Pan
Intellectual 990 points
Part Number: MSP430F5529
Other Parts Discussed in Thread: DRV8343S-Q1EVM,

Hi,

Tommy directed me to this forum regarding a question on RC values for MSP430F5529 MCU low-pass filtering used in DRV8343S-Q1EVM board. The current sensing pins, ISEN_A, ISEN_B, ISEN_C use R=56 ohms and C=2200pF, which leads to very fast time constant, about 120 ns. I check the datasheet of MSP430F5529, minimum sampling time is 1000 ns in Section 5.36 in page 42 of the datasheet. Assuming 1% error, 5 times RC equals to 600 ns, which is smaller than the minimum sampling time of 1000 ns. I am wondering whether the RC values are correct in this EVM board design. Please advise.

Another question is which frequency the TIDA-DRV8343S-Q1EVM design try to filter? For 45KHz PWM, it seems to me that above 45KHz frequency should be filtered. If this is the case, RC should be about 1/(5*f_sw) = 4.5 us. From this perspective, both R and C should be larger. My understanding is that C value should be larger than C_SAMPLE inside the MCU so that C_SAMPLE can be fully charged. Does the MCU has a maximum RC value limit? If yes, what is it?

I will appreciate your insight in RC filter design for ADC.

Thanks,

John

  • 0
    TI__Mastermind 26420 points

    Hey John,

    I'm slightly confused by the question in your first paragraph.  1000 ns is the fastest sampling speed the MSP430F5529 can perform, but it can also have much longer sample periods.  This can be controlled in software when configuring the ADC.  To determine the minimum sampling period needed, the formula is given in note 5 of the table you pointed to, 5.36 of the datasheet.  The input resistance and ADC capacitance is provided in table 5.35. 

    I'm not familiar with the motor drive EVM, but it seems based on the values you provide above it looks like the roll-off frequency is like ~1MHz, so it's just filtering high-frequency noise. 

    Thanks,

    JD    

  • 0 in reply to JD Crutchfield
    Intellectual 990 points

    Hi JD,

    Thank you very much for the information. I am sorry for not asking the question clearly.

    Based on the note 5 of the table 5.36 in the datasheet, t_sample = ln(2^(12+1))*(Rs+Ri)*Ci+800 and it should be greater than 1000 ns as specified in Table 5.36. Based on the Ri and Ci values listed in Table 5.35 and 5.36, let's assume Ri=1000 ohms and Ci=20pF, minimum (Rs+Ri) calculated is 1,111 ohms. Thus, minimum Rs is 111 if Ri=1000 ohms and minimum Rs is 911 ohms if a typical Ri=200 ohms as specified in table 5.35 is used. However, Rs of 56 ohms was used in the design reference. Could you please comment on it?

    I am not sure how the Cs=2200 pF was selected. Assuming the roll-off frequency of 1Mhz (considering the MCU internal oscillator of 4MHz) and also assuming 10 Tau are used, the RC time constant should be less than 100 ns. Thus, Cs should be less than 1700 pF if Rs = 56 ohms selected in the design, and Cs should be less than 100 pF if Ri = 911 ohms as calculated in the last paragraph. Could you please comment on it.

    I have limited knowledge on ADC. Your support will be greatly appreciated.

    Thanks,

    John

  • 0 in reply to John Pan
    TI__Mastermind 26420 points

    Hey John,

    This is only if you are trying to run the ADC at it's absolutely fastest sample time, which is not usually necessary.  You can configure this sample period in software, the 1000ns Is just an absolute fastest the MSP430 can sample.  

    The way you should typically use the formula is use worst-case RI and CI, then use the effective Rs and Cs from the hardware and this will tell you your minimum sample time.  Once you have this value, you just need to make sure the MSP430 keeps the sample window open longer than this to charge the ADC's internal cap.  

    So, you don't have to design to be <1000ns, you should just design for whatever you are sensing, then adjust the ADC in software to read the value correctly.  

    Section 28.2.5 in the Family User's guide examples the different Sample and Conversion Timing controls available.

    So, basically as long as the EVM software isn't using the absolute shortest sample window, then this circuit seems fine from the ADC sampling point of view.  

    Thanks,

    JD

  • 0 in reply to JD Crutchfield
    Intellectual 990 points

    Hi JD,

    Thank you very much for taking your valuable time in answering my questions. That is very helpful.

    Best regards,

    John

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