• State Resolved
  • Locked Locked
  • Replies 2 replies
  • Subscribers 82 subscribers
  • Views 544 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

TMS320F28377D: ADC pulling input lower Question

Michael David
Intellectual 460 points

Part Number: TMS320F28377D

Hello,

I am using the 12 bit ADCs at a sampling time of 15kHz to monitor voltages through a resistor divider. It seems that the ADC sampling is pulling the input signal lower. Using same method for other ADC inputs do not show this issue. I have a 10V source divider through a 33.2k and 10k resistors and a 0.1uf filter cap. The input to the ADC is expecting 2.3V but for some reason its being pulled down to about 2V. Is there something inside the ADC that I am missing or that could be causing this?  

  • 0
    TI__Guru 60865 points

    Hi Michael,

    You effective input impedance is (33.2k || 10k ) = 7.7k.  This is very high for driving an ADC input without an op-amp.

    Each sample the sampling capacitor needs to be re-charged.  At your sample rate of 15kHz, this could result in an in-rush current of up to 14.5pF*15kHz*2.3V = 500nA, which could cause a voltage drop up to about 7.7k*500nA = ~4mV.  If the ADC was actually sampling at ~1MSPS you'd expect something closer to the 300mV drop you are seeing. 

    ADC inputs A0, A1, and B1 have a parasitic pull-down if not being used as a DAC output.  This would result in a current of about 2.3V/50k =  46uA and 46uA*7700 = 350mV, which could also be in-line with what you are seeing.

  • 0
    Intellectual 460 points

    Looks like the 50k pull down is the likely cause. I did not even see this in the datasheet. Thank you for clearing this up.