• State
  • Locked Locked
  • Replies 1 reply
  • Subscribers 30 subscribers
  • Views 290 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.

LM4F110H5QR - A/D Equivalent Input Impedance Review

TI_Fan
Prodigy 40 points

Hello!   I have a customer that has a question on correctly terminating the A/D inputs on their LM4F110H5QR.

According to Table 22-23 on Page 1104 of the LM4F110H5QR data sheet, the total equivalent a/d input resistance is 2.5k Ohms:

With this is mind, they designed the A/D input nets to have a maximum voltage of 18.229 Volts.  This input voltage is applied through a 11.31k Ohm resistor connected to each A/D input pin.  Their thoughts were the 11.31k Ohm resistor would form a divider with the 2.5k Ohm input resistance (RADC) to yield a maximum of 3.3 Volts at the a/d input.  

Is their approach in calculating the total A/D input resistance - to set a maximum A/D input voltage of 3.3V correct?  If not, then can you please advise what the resistor values should be?

Thanks!

Mark A.

  • 0
    TI__Intellectual 2360 points

    Hello Mark,

    The max source resistance specified in the data sheet is 500 ohms (Electrical Specifications: ADC).  Source resistance can affect the sampling accuracy so should remain within the specified limits.  If they want to divide the voltage they could add the divider and then buffer it ot the ADC.

    Regards,

    Craig