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MSP430FR2633: Maximum Capacitance on All External Electrodes on all Captivate Blocks

AI31
Expert 1600 points
Part Number: MSP430FR2633
Other Parts Discussed in Thread: TPD1E10B06

Hello, 

In MSP430 datasheet, Table 5-23 states a maximum value for all electrodes capacitance:

Questions:

1- I have 16 electrodes each connected to a pin, is this 300pF the sum of all of them or for each block? (i.e. each electrode of the 16 will have a max value of 300pF/16 = 18pF or 300 each)?

2- The frequency of the captivate output signal is 4-16MHz, is there a way to decrease it further or not?

Thanks

  • 0
    TI__Guru 71216 points

    Hi Ahmed,

    Let me check on the 300pF spec.

    The conversion frequency range can be selected from 16MHz down to 250KHz.  Typical frequency used is 2 or 4MHz.

    Here is the selection in the Captivate Design Center:

  • 0 in reply to Dennis Lehman
    TI__Guru 71216 points

    Hello Ahmed,

    I confirmed it is 300pF total across all pins and blocks.  That means in your case with 16 channels you are limited to just about 18pF per channel (pin).

  • 0 in reply to Dennis Lehman
    Expert 1600 points

    Hi Dennis, 

    This sounds like a very limited budget, so each electrode should be less than 18pF.

    More Questions:

    1- What if I'm only scanning one electrode at a time with the Firmware? will it be 300pF per electrode then?

    2- Could you please explain from where this limit came?

    3- I'm adding TPD1E10B06DPYT ESD protection diode which have 12pF IO capacitance, should I subtract this from the remaining 18pF to have only 6pF?!

    4- Does lowering the Captivate frequency to 250KHz help increase this limit (as the datasheet says "at 4MHz")?

    5- Does increasing the REG capacitor from 1uF to a higher value relaxes this 300pF budget?

    6- In case my sense electrode had higher capacitance and I couldn't push it smaller, can I add a series chip capacitor between the 470Ohm resistor from the MCU pin and the Electrode to get a total capacitance less 300pF?

    7- Finally, what is the minimum capacitance change I should have to be able to detect it with the Captivate circuit?

    Thanks

  • 0 in reply to AI31
    TI__Guru 71216 points

    Hi Ahmed,

    I asked our analog designer to confirm and he is indicating otherwise.

    This spec is assuming a device with 4 measurement blocks, which the FR2633 does and that you are scanning in parallel.

    At 4MHz, the maxium is 75pF / channel or 4 x 75 = 300pF.  I 'll need to see about adding some clarification to the datasheet.

    Ahmed Ibrahim31 said:
    1- What if I'm only scanning one electrode at a time with the Firmware? will it be 300pF per electrode then?

    If scanning sequentially ( 1 channel at a time), then yes it would be 300pF/electrode.

    Ahmed Ibrahim31 said:
    2- Could you please explain from where this limit came?

    This is a limitation with the internal 1.5v LDO in conjuction with the 1uF external REF capacitor.  In order to keep the die size small, the combination of LDO design and 1uF limit the total charge capacity to 300pF.

    3- I'm adding TPD1E10B06DPYT ESD protection diode which have 12pF IO capacitance, should I subtract this from the remaining 18pF to have only 6pF?!

    That's quite high.  TI offers several other TVS devices that have <1pF.  For example TPD1E04U04.

    Ahmed Ibrahim31 said:
    4- Does lowering the Captivate frequency to 250KHz help increase this limit (as the datasheet says "at 4MHz")?

    Probably, but not sure.  We don't have any test data at that frequency.  I would recommend trying to operate with a 1,2 or 4 MHz conversion frequency.

    Ahmed Ibrahim31 said:
    5- Does increasing the REG capacitor from 1uF to a higher value relaxes this 300pF budget?

    You would think so, but no.  I'm being told the internal LDO was designed to work with a 1uF capacitance and this capacitance should not be varied.

    Ahmed Ibrahim31 said:
    6- In case my sense electrode had higher capacitance and I couldn't push it smaller, can I add a series chip capacitor between the 470Ohm resistor from the MCU pin and the Electrode to get a total capacitance less 300pF?

    No.  Adding extra capacitance won't help.  In fact is will slow the rise time forcing you to run with lower conversion clock frequency.  Not that is necessarily bad, the conversions will take longer and if this for a battery powered application it will impact your current draw.

    Ahmed Ibrahim31 said:
    7- Finally, what is the minimum capacitance change I should have to be able to detect it with the Captivate circuit?

    This is not spec'd, but in theory it is on the order of a few fF (femto Farads).

  • 0 in reply to Dennis Lehman
    Expert 1600 points

    Hi Dennis, 

    Thanks for the detailed answer, it was very helpful.

    Just two things

    - for number 3: TPD1E10B06 was recommended in Application Report SLAA842B–December 2018–Revised August 2019 Capacitive Touch Design Flow for MSP430™ MCUs With CapTIvate™ Technology, 
     Will I have to change the resistors in case I switched to the TPD1E04U04 you recommended?

    - for number 6, I want to make sure I communicated it clearly: I meant a capacitor in series not in parallel like this

    so that if Celectrode was a bit higher than the limit, the total capacitance seen by the MCU is less

  • 0 in reply to AI31
    TI__Guru 71216 points

    Hi Ahmed,

    I'm checking with the author to see why he suggested the TPD1E10B06. It will work, but since you are starting with such large parasitic capacitances, using the TPD1E04U04  will help reduce those parasitics.

    Regarding #6, sorry I missed that. Hmmm.... I don't know what to tell you.  I have never seen that done so you may have to some experimentation to verify that will work.

    Ok, you have peaked my curiosity.  Why do you have or are expecting such large parasitic capacitances?  What is this application?

  • 0 in reply to Dennis Lehman
    TI__Guru 71216 points

    Hi Ahmed,

    The author indicated that, at the time that was a general purpose TVS that is suitable for most HMI applications where the expected input capacitance on an electrode is ~ 30pF or less.

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