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TMS320F280049C: TMS320F280049 ADC Input Impedance

ilgin sarican
Prodigy 225 points
Part Number: TMS320F280049C
Other Parts Discussed in Thread: TMS320F280049, UCD3138

Hi,

What is the input impedance of ADC pins of TMS320F280049?

For example, In the datasheet of UCD3138 R(EA) is stated as 0.5Mega ohm.

 

Do I have to use an op amp to buffer the signal (high voltage DC sensing)?

  • 0
    TI__Mastermind 36526 points

    Hi,

    Please see this app note: https://www.ti.com/lit/pdf/spract6

    Also, we have ADC input models in our device datasheets.

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    Prodigy 225 points

    Thanks for your reply.

    In Design Guide:TIDM-2008, section 2.2.2,  bus voltage is sensed by resistor divider network.

    This sense circuitry has high impedance because of voltage divider with large R values.

    Should an Op-Amp as buffer used? 

    Similarly, section 2.2.1,  input AC  voltage is sensed by resistor divider network, but application did not use opamp as a buffer.

  • 0 in reply to ilgin sarican
    TI__Mastermind 36526 points

    Hi,

    I will need to ask a coworker, but I assume that it would be a good idea to use a buffer for this signal, or you may need a very long sampling window.

    Best Regards,

    Ben Collier

  • 0 in reply to Benjamin Collier
    Prodigy 225 points

    Hi again,

    I got this answer: "Yes, using an op-amp will give better signal band width. But in this case the frequency of AC voltage sensed is  60Hz and DC bus voltage which was assumed reasonably stable. So, the voltage will track close enough to the input already. Hence the need of an op-amp is not mandatory unless we need better signal band width"

  • +1 in reply to ilgin sarican
    TI__Mastermind 36526 points

    Hi Ilgin,

    This makes sense. In the app note that I linked, we deal with a case where an input signal might go from 0V to 3V in sequential samples. In this case, it is very important that the input signal is able to charge the sample and hold capacitor quickly. In your case, since the AC signal is moving slowly and the sample rate is probably much higher than the 60Hz, there will not be too much voltage difference between subsequent samples. In this case it is not necessary for the sample and hold capacitor to charge as quickly.

    Best Regards,

    Ben Collier