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DC bias on LM96530

Ying Li
Prodigy 100 points
Other Parts Discussed in Thread: AFE5803EVM, AFE5803

Hi TI's ananlog expert,

        We are using the  TX_SDK_V1 evaluation board. We found that when we turn on the LM96530 receiving channel, the output of the LM96530 will have a DC bias which will add low frequency noise to our echo signal. The following picture shows the channel 1 output of the LM96530 before and after  turning on the channel 1.Can you help us solve this problem.

  • 0
    TI__Mastermind 18275 points

    Phx,


    I am moving your post to the Imaging AFEs Forum.

  • 0
    TI__Intellectual 1560 points
    Hi,

    Usually, the output of the LM96530 is AC coupled to the receiver. Or you can put a 50 Ohm resistor from the output to ground.

    Regads,
    Charles
  • 0 in reply to Charles Guan
    Prodigy 100 points

    Hi,

    Thank you for your reply. We use the AFE5803EVM to receive the output of the TX_SDK_V1. So the output of the LM96530 is AC coupled to the receiver. LM96530 OUTPUT

    AFE5803 INPUT

    Later we change the RREF of the LM96530 from (RREF = 6.25 kΩ → IS = 8mA) to (RREF = 50 kΩ → IS = 1mA), the output DC offset increases from -300mV to -16mV.  How can the internal current source effect the output?

    However, the received signal still has a low frequency noise, which limits the use of  low frequency (0.2MHz - 1MHz) ultrasound transducer. I am wondering where the low frequency comes from. Is it still comes from the DC offset or anywhere else.

  • 0 in reply to Ying Li
    TI__Intellectual 1560 points
    Hi,

    Can you double check that the LM96530 is turn on by looking at the supply current for the LM96530?

    Regards,
    Charles
  • 0 in reply to Charles Guan
    Prodigy 100 points

    Hi,

    Charles, Thanks for your reply.

    I am sure the LM96530 is turned on for the +-5V power supply's current  is 0.075mA and dropped to 0 when turn off the 8 channel of the LM96530.

    Besides, only turn on the LM96530 can AFE5803 receive the echo signal.

    Regards,

    Phx

  • 0 in reply to Ying Li
    TI__Intellectual 1560 points

    Hi Phx,


    I think you mean 75mA right? How about changing the 2.5KOhm output resistor (from output to ground) to 500 Ohm and/or increase the Rref resistor by redcucing the Icc? Looks like the offset is coming from leakage current.

    Regards,

    Charles

  • 0 in reply to Charles Guan
    Prodigy 100 points

    Hi Charles,

    Thanks for your help.

    Sorry, I made a mistake. The the +-5V power supply's current  is 0.075A and dropped to 0.013A when increase the Rref resistor  from (RREF = 6.25 kΩ → IS = 8mA) to (RREF = 50 kΩ → IS = 1mA). 

    And I think the low frequency noise comes from LM96530. Because even I turn off the LM96530's 8 channel, the AFE5803 still can receive a low frequency noise. But when I disconnect the LM96530 and the AFE5803, its gone.

    Best,

    Phx

  • 0 in reply to Ying Li
    TI__Intellectual 1560 points

    Hi Phx,

    What signal is at the input of LM96530? It's around 100KHz too? If it's, there would be capacitive coupling from the input.

    Regards,

    Charles

  • 0 in reply to Charles Guan
    Prodigy 100 points

    Hi Charles,

    The jumper between the LM96551 and LM96530 is taken away. So there is no input signal of LM96530. Later we found that when all power off except VSUB, the low frequency noise still exist. Is it come from the power supply?

  • 0 in reply to Charles Guan
    Prodigy 100 points

    Hi Charles,

    Thank you very much. The noise comes from the power supply. The picture shows the power supply noise. And I think you are right. The DC offset comes from the leakage current. Now the DC offset is -16mV (Rref = 50K, Is= 1mA) and seems works fine. We will change the power supply later. Thanks again.

    Best,

    Phx