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AFE5809: Noise level much too high

FENG SUN57683
Intellectual 895 points
Part Number: AFE5809

hi

I have made a custom board that use AFE5809 as ADC. I can read the sampled data through FPGA and show the wave form in CHIPSCOPE (an xilinx IP core). 

I found the noise is much too high, it seems like the lowest 6 bits are just noise. I have tested the LVDS capture codes using 5809's ramp patten and it is OK.

there is no other pre-amp stage before the 5809 input. the noise at the input pins are about 10mVpp (more like self noise of the oscilloscope)

noise shown in chipscope

noise in MATLAB

waveform when a 40mVpp, 1MHz sine wave is added to the input.

  • 0
    Intellectual 895 points

    turns out the MSB of a bus in the chipscope is at the bottom. after I reverse the bus bit order, I can get noise data as follow:(sample clock is 30MHz )

    and the noise spectrum:

    which is quite different from figure 12. LPF Response of the 5809 datasheet, i mean the high frequency attenuates much faster here. 

    below is the waveform when a 1MHz, 3mVpp sine wave is added to the input.

  • 0 in reply to FENG SUN57683
    TI__Genius 12025 points

    Hi Feng,

    How are you?

    Thank you for using TI AFE5809 device.

    For your high noise issue:

    Could you please double check the input set up shown as below:

    Please make sure your input signal is not floating

    (here is one example of the input channel's setting.)

    before capturing any data from that channel.

    Thank you very much!

    Best regards,

    Chen

  • 0 in reply to Chen Kung
    TI__Genius 12025 points

    Hi Feng,

    How are you?

    For the AFE5809 device register settings for your noise

    and sine wave input test observation,

    we were setting the following registers.

    including:

    Software Reset, Set Act_Term_Resistor=50ohms, Enable Active Term Resistors

    Choose Format=Offset Binary

    Choose Bit Order=MSB First

    LVDS Rate=14x

    This register setting is working and settings are:

    Address    Data
    =======  ====
    36             44
    16             1
    4               18
    3B             100
    1                0
    39              0
    2                0
    3                0
    3                0
    3                0
    36              44
    1                0
    2                0
    1                0
    1                0
    33              0
    2                0
    3                0
    37              0
    38              0
    35              0
    3                0
    3                0
    4               18
    34             100
    4               10
    4               18
    4                8
    4               18
    3               2000
    4               18
    3                0
    4               18

    Please try this method.

    Thank you very much.

    Best regards,

    Chen