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AD8331 Evaluation Board Unstable

Category: Hardware
Product Number: AD8331 Evaluation Board

I am using the AD8331 evaluation board to detect ultrasound signals (1-5MHz) from an ultrasound transducer. The transducer is connected directly to the amplifier. The transducer is capacitive (100pF) and has a resistance of approximately 70Ohms at resonance. For this application, I need to maximise the signal from the transducer, so I am not using active termination. Instead, I set the input impedance of the circuit to be maximum (6KOhms) by desoldering RFB, CSH and CFB from the evaluation board (as per the data sheet and application notes). What I noticed is that the amplifier starts having some low frequency oscillations (I estimate it to be around 50KHz or so) when the transducer is connected to it. The oscillations are not present when I connect a resistive load (such as 50Ohms) to directly to the amplifier, suggesting that the oscillations are related to the capacitive nature of transducer. I noticed that when I choose values of RFB (6.8KOhms), CSH(none) and CFB(18nF) to achieve active termination input resistance RIN = 1KOhms, the magnitude of the oscillations reduce, but it is still present. Could anyone advice how to solve this instability issue?

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  • Can anyone help please? I don't tend to get answers to my question from ADI Engineers...

  • Hi MrEngineer1990,

    Good day. It seems that the oscillation is most likely noise coming from some source. To isolate, please try having a clean sine wave and plot the output. You can also check the cable used in the measurement of the output since this could also provide parasitic and will affect the signal being plotted. Can you also send an image of your eval board for reference?

    Regards,

    Gilbeys

  • Hi Gilbeys, Thanks for your reply and apologies for the late response. I have narrowed it down and it appears the noise is coming from the power supply. When I use an ultralow noise power supply, then I no longer see the oscillations. I would have though a standard linear bench top power supply would suffice since Analog claim that the evaluation board/chip is stable. . Would an analog engineer care to comment ? I will not always have access to the expensive ultra low noise power supply, so I need a solution please.

  • Hi  ,

    Good day. Sorry for missing this thread. You can implement a bypass capacitor in the power supply. This would minimize the effect of the noise coming from the power supply. As a rule of thumb, you can implement a 0.1uF parallel to a 10 uF bypass capacitor. 

    Regards,

    Gilbeys

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