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AFE5808A LSB and Amplifier

Yuning Jin
Prodigy 185 points
Other Parts Discussed in Thread: AFE5808AEVM

Dear sir/madam:

I am using AFE5808AEVM. In the datasheet the full scale is 2Vpp. And the resolution is 14bits so code resolution is 2/(2^14), right? But when I chose 10mVpp input and LNA set to 12dB, PGA set to 24dB, VCA se to -36dB so there is no gain. But finally I got 200codes Vpp(=20mV accroding to my calculation) that doesnt match the resolution. I am just curious what's going on Maybe my computation is wrong.

And when I set the gain to 6dB and 12dB is pretty good I got about 400codes vpp(6dB is double) and 800 codesvpp (for 12dB which makes sense), but when I set the gain to 42dB and 54dB, I got 1036codes vpp and 3625codes vpp the results seem to be much smaller than the gain I set?

Thank you so much,

Jin

  • 0
    TI__Intellectual 2635 points
    Jin,
     
    I have found what I believe is your issue:  you are not actually feeding 10mVpp to the AFE input.  Is there a way that you can confirm this?   The AFE 50ohm activate termination does not work with LNA Gain = 12dB ( please see DS page 32).
     
    Therefore, unless you have an external 50ohm resistor on the board, the AFE input is high impedance with LNA=12dB.  Assuming that your signal source has a 50ohm source resistance, the signal into a high impedance will be twice the nominal value that you set.  In other words,  when you set 10mVpp on the source, it will look like 20mVpp at the AFE input.  Please see if you can verify this.
     
    With LNA = +12dB, VCA = -36dB, PGA = +24dB,  there is zero gain. Also, The LSB resolution is 0.122mV.  Therefore, the input to the ADC should also be 20mVpp, when should theoretically result in (20/0.122) =164codes.  This still doesn’t match your 200 codes, however. 
     
    Upon repeating your experiment in the lab here, we were able to match your results, ≈196.72codes. We also notice, however, that the gain is not exactly 0dB, but more like 1.75dB.  We are investigating this gain error.
     
    Hope it helps
     
    Fabio
    Applications Engineer
    TI Healthtech/Imaging