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AFE5808A: Common-Mode Voltage of LNA

Jejomar Ildefonso
Expert 8690 points
Part Number: AFE5808A

Hi Team,

The datasheet specifies the common voltage of the AFE5808A low noise amplifier to be 2.4V. However, our customer measured 2.25V on one channel using the AFE5808AEVM. Is this measured value from the expected characteristics?

Can we also confirm if the the common mode voltage at the inputs is expected to vary from channel to channel, or chip to chip?

Thanks in advance!


Kind Regards,

Jejomar

  • 0
    TI__Genius 12025 points

    Hi,

    Thank customers for using AFE5808A device.

    Since from the customer's measurement, they did not say more details where they tried to measure

    the common voltage at ACT pin, INP pin, or INM pin? Which pin they measured 2.25V?

    From the AFE5808A datasheet page 10, it mentions Input Common-mode Voltage (at INP and INM pins) has the Typical 2.4V value.

    Is it the same as the customers mention, right?

    If correct, the datasheet shows only the typical value,

    therefore we cannot confirm the value from the customer.

    However, we can make sure is we suggest the customer please use Figure 59 on page 30.

    Please using AFE5808A LNA with DC Offset Correction Circuit

    please turn on the register 52[12]  to be 0 (please also refer to Datasheet 8.6.3 on page 51).

    This can help for offset the common-mode voltage error at the LNA output.

    Thank you very much for helping the customer using AFE5808A.

    Thank you!

  • 0
    Prodigy 50 points

    We soldered thin wires on the chip side of the input capacitors for one channel (channel 8) & I measured the voltage with respect to one of the ground test points on the evaluation board.  I measured 2.247V on the positive input and 2.250V on the negative input.  I did not measure the voltage on the ACT input.  I'm 99% sure that these measurements were made with the default configuration of the AFE5808A, so 52[12]=0 (LNA DC offset correction circuit is active).

    My question is whether the difference between my measured value of ~2.25V and the nominal value of 2.4V is within the expected range, and if this is expected to be different channel to channel, or whether the difference is likely to be due to seen only between different wafers.

  • 0 in reply to David Christian
    TI__Genius 12025 points

    Thank you for more details.

    Since there is no Max or Min limitation of INP and INM pins from the datasheet,

    another question is:

    beside your measurement is coming from Channel 8, right?

    How about other Channel's INP and INM pin values?

    could you please let us know?

    so we need to ask our design team and let you know soon.

    Thank you very much!

    Best regards,

    Chen

  • 0 in reply to Chen Kung
    Prodigy 50 points

    I only measured the common mode voltage for one channel on the evaluation board.  We have also fabricated a board using AFE5808A and are starting to debug that board.  On our board I measured the common mode voltage at INP for a single channel and got ~2.1V (even slightly lower than I measured on the evaluation board).

  • +1 in reply to David Christian
    TI__Genius 12025 points

    Hi,

    Thank you for your more measurements.

    Here is our design team's reply for your question:

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

    If they are measuring the common mode without any overloaded signal applied to the device then it is expected. Input common mode voltage can change from wafer to wafer. Channel to channel variation will be small.

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

    Thank you!

    Best regards,

    Chen