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TMS320F280049C: ADC with VDAC reference in application used

Johnny Chen
Expert 5260 points

Part Number: TMS320F280049C

Hi Champ,

I am asking for my customer.

The ADCINB3 is also used to supply DAC, so that this pin parasitic capacitor is much larger that other ADC pins.

For my understanding, it is recommended not to use this pin for sampling fast switching signal.

The source impedance of the ADC input Rs and Cs is chosen by sampling signal in different application, it is easier to clarify with the bandwidth.

The pin used in customer's application is about 20k Hz bandwidth, I think it isn't a problem, right ?

Is the pin(ADCINB3) be able to clarify used within the bandwidth suggested?

Thank you.

  • 0
    TI__Expert 5260 points

    Hi Champ,

    Is there any comment or suggestion ?

    Thanks for any help.

  • +1 in reply to Johnny Chen
    TI__Mastermind 43360 points

    Hi Johnny,

    In an ideal situation where Rs and Cs are negligible, then you can assume that any signal will pass through B3 without any drop at the ADC input node.  For the 20KHz signal, there will be a parallel impedance caused by the ~130pF B3 pin capacitance (see DS).  This would be 1/(2*pi*20KHz*130pF), or Xc of 61.2Kohms from B3 input to ground.  With negligible Rs and Cs (both at 0), the 20KHz signal would still show up at the B3 input pad with no attenuation at all and this would not cause any issues.

    In practical application though, it will not be safe to assume that impedances of Rs and Cs are 0.  Signal sources and board parasitics will have some effect on Rs and Cs.  Say for instance that Rs is 2K, and Cs due to the application and board parasitics contribute another 100pF.  This would essentially form a low pass RC filter at B3 input where Rs = 1Kohm and Cs (will be the 100pF + pin capacitance of B3, 130pF) = 250pF.  With a simple voltage divider illustration, the impedance would consist of Rs and the total of Cs and Cp.  Signal attenuation on B3 would be X(Cs+Cp)/sqrt(((Rs)^2 + X(Cs+Cp)^2)).  This would be an attenuation of 31.83K/sqrt(2Kohm^2 + 31.83Kohm^2) or 0.998.  If the amplitude of the 20Khz signal is 2V, the voltage at the internal B3 pad will be 1.996V.  This might not seem a lot but this will be enough to cause an error of 7LSBs on a 12-bit ADC assuming a VREF of 2.5V.

    Hope the above examples would stress the importance of knowing the series impedances associated with the ADC input.

    Best regards,

    Joseph