AD8039 noise issue

In this project, we used Linear's high speed ADC LTC2175, LT6409(SE~Difference) as the driver. AD8039 as the first level amplifier and follower. Signal input Range: ±0.5V~±5V (i.e. Gain: 1~10); Input Freq: DC~60 MHz; When the input signal of AD8039 connected to GND, the noise is about 20mv for ADC getting. But if put off AD8039, just connecting the input signal of LTC6409 to ground, the noise is 3.5mv, but seems AD8039 no owns so bad CMRR, would you like help give some suggestion on that. thanks in advance

  • Hi,

    I'm currently looking into this. I will try to replicate your set-up in the lab. I'll get back to you once i have the results. May we know what you application is? Could you tell us more about more about it? We might able to suggest a suitable part for your application.

    Thanks,

    Roel

  • Hi luiquishe,

    Could you share to us your schematics for it would be easier for us to determine your problem. What values of resistors do you use? For this also contributes to the output noise of your amplifier. Also, in addition, there are many sources of noise on the amplifier, such as

    • Inverting input current noise=Rf+Inoise(-)
    • Johnson noise=sqrt(4kTR) which the resistor contributes
    • input-referred voltage noise = Vnoise*(1+Rf/Rg) and etc.

    Regards,

    Roel

  • Hi Rvieja,

    Thanks for your reply, the application is Ultrasound Partial Discharge; signal come from ultrasonic sensor.

    Regards

    Liu

  • 0
    •  Analog Employees 
    on Aug 2, 2018 3:21 PM
    This question has been assumed as answered either offline via email or with a multi-part answer. This question has now been closed out. If you have an inquiry related to this topic please post a new question in the applicable product forum.

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