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TM4C123BH6ZRB: What does Rs and Radc mean for application?

yihe
Guru 58100 points

Part Number: TM4C123BH6ZRB

Hello

In the data sheet, it mentions that the Rs is Max at 500ohms and Radc is max at 2.5Kohms.

What does these two parameters mean? how do they impact customer's design to get a better ADC accuray?

Thanks

Yihe

  • 0
    TI__Guru*** 128295 points

    Hello Yihe,

    These values go along with Figure 13-7. ADC Input Equivalency in the device datasheet.

    As for how to use this information to improve ADC accuracy, I would suggest they read through the follow application note about ADC source impedance: www.ti.com/.../spna061.pdf

  • 0 in reply to Ralph Jacobi
    TI__Guru 58100 points

    Hi 

    Thanks for the application notes. it does not explain the function of the Radc. if customer has a external voltage divider, will the Radc impact the voltage divider?

    Regards

    Yihe

  • 0 in reply to yihe
    Genius 3945 points

    Radc is the converter's intrinsic input impedance. It is what the external driver sees.

    Yes, Radc affects the external voltage divider.

    If the customer needs a voltage divider in front of the converter to, for example, reduce the range of an input signal that would otherwise exceed the ADC input, then buffer the divider with an op-amp. The converter then sees the low source impedance it prefers.

  • +1 in reply to Andy Peters
    TI__Guru 72500 points

    Just to add to Andy's post, the Radc resistance is between the pin and the sample capacitor. It will affect the time for the sample capacitor to charge or discharge along with your external source resistance. Once the sample capacitor is at the same voltage as the external pin, it will not affect the voltage divider (input leakage is small). 

    In short, you can ignore Radc when you calculate the resistor divider to determine the voltage range, but you must consider Radc when you calculate your minimum sample time. Adding the external capacitor will really help with the sample time as explained in the application note. It does act as a low pass filter so that needs to be taken into account.