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TMS320F28335: recommended external RC circuit value for AD input

Aki Li
Genius 9885 points
Part Number: TMS320F28335


For the hardware design for AC input circuit, as said in the Hardware Design Guidelines SPRAAS1D, Rin <100 ohm, Cin: 20～30pF. If used for high-frequency signal sampling, smaller RC value should be used, right? So what is the suggested Rin in this case? Is there any risk to use to smaller Rin? Thank you!

  • 0
    TI__Guru* 80880 points

    Aki,

    Yes, you can change the passives on the amp output to suit the expected frequency range of your input.  In the case of the recommendation this a bit of a generalization based on the value of the sampling cap Csh, and that most amps require some series resistance in order to drive capacitive loads.  My recommendation would be to refer to the datasheet of the amplifier that is in your system for any stability restrictions.

    I would recommend using http://www.ti.com/tool/TINA-TI to simulate the circuit over your expected operating inputs and note the response.  If you are using a TI op-amp the model should be in the FW package already, and you can use the input model of the C2000 ADC shown above to represent the sampling network.  For the input to not limit the ADC resolution we should charge the sampling cap Csh to at least a 13 bit accuracy(one greater than the 12-bit ADC on chip), 1/2^13 or within 0.012% of ideal.

    Best,

    Matthew

  • 0 in reply to MatthewPate
    TI__Genius 9885 points
    Matthew,

    Thank you for the detailed explanation!