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ADS8598H: Increase input voltage range

Jordan IMBERT
Prodigy 10 points
Part Number: ADS8598H

Hello,

I'have design a board with the ADS8598H following this application note to increase the input range to +-25V : http://www.ti.com/lit/an/sbaa244/sbaa244.pdf, but I have a problems with this note:

According to the note, I chose 2 resistors of 1.5Mohm so :

Vin = Vinadc.(Rin+Rext)/Rin = 10*(1.5+1)/1 = 25V

But at 12.5V the output code become negative so the practical range is +-12.5V and I don't understand why. 

Moreover, when I compute the input voltage with this equation:

Voutadc = code * 2 * FSR / (2^n)

I have a factor two compare of the real equation. 

Is the an problem with this note or I done something wrong ? 

Thank you

 

 

  • 0
    TI__Mastermind 35570 points

    Hi Jordan,

    Welcome to E2E forum.

    Since the author for the app note is travelling now, I have forwarded your question to her, please allow a delay for few days, thanks for your patience.

    Best regards

    Dale

  • 0
    TI__Mastermind 24940 points

    Hello Jordan,

    The resisiter value of 1.5M is correct to extend the useable voltage range to Vin = +/-25V from Vin_ADC=+/-10V.

    Are you making these measurements on the device EVM, or on your designed PCB? I am trying to see how are you measering the output code, through your own processor/code or through the GUI.

    At Vin =12.5, you should be measuring 5V at the ADC input. This would then correspond to a code of 0001 0000 0000 0000 0000 (binary)  or 65536. Would you please verify and share what you are expericiencing?

    The equation you listed is correct for Vout_adc, remember that the FSR should be 25, thus 2 *25 = 50, due to the bipolar input.

    Keep in mind that this device measures the input in the following method, a negative input is denoted with leading 1 in binary format.

    Regards

    Cynthia