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ADS1299：dBFS

Katsuhiko Aisu
Prodigy 10 points
Part Number: ADS1299

Thank you for your help
Let me ask you a question on the specsheet (SBAS499C–JULY2012–REVISEDJANUARY2017)

There is a description of the following contents

　7.5 Electrical Characteristics
　　　Gain error UNIT->% of FS
　　　SNR TEST CONDITIONS-> VIN = –2 dBFs

Do the FS and dBFs mentioned here fit in with the following ideas?

　FS or 0dBFs is the Full-Scale range (= 2 * Vref / Gain) in Equation 5.

---------------

FS also appears in Equation8., But this seems to be half of the Full-Scale range.
I'm confused.

Thank you for your cooperation.

  • 0
    TI__Mastermind 19815 points

    Hi Katsuhiko san,

      FS stands for Full-Scale of the differential input, that is VINxP - VINxN as stated in the section 9.3.1.3.2.

      dBFs is a unit/way/method to describe the relative magnitude to the FS(Full-Scale). So, -2 dBFs is roughly equal to 80% of the FS as -2dB = 0.794 in linear(ratio). The VIN = -2dBFs is just to show that electrical characteristic was tested under/in the condition that the differential input was around 80% of the FS given the Vref and Gain settings stated above the table, i.e. Vref=4.5V and Gain = 12.

      Yes, equation 5 applies to the Full-Scale(FS). It means as soon as user/designer decides and sets/selects the Vref and the Gain setting, the Full-Scale range is limited by 2*Vref/Gain. In other words, once user/designer supplies the Vref and sets the Gain setting, the Differential input, i.e. VINxP - VINxN, should/can not exceed 2*Vref/Gain.

    Thanks.