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What op amp to use as buffer in ultrasound application

We are working on a 40 MHz board for an ultrasound application, and on the receive side, we are planning to use an ultrasound-specific LNA ( that has differential output and a Vcm of 2.5 V.

I am looking for an op amp to buffer the signal from the LNA off-board, and came across the AD8044 ( If we could use this with G = +1 V/V or +2 V/V, I think that would be ideal for our application.

However, I am having difficulties understanding (1) whether this op amp is actually what we are looking for in terms of compatibility and (2) if it is what we are looking for, how to connect it.

If we connect the positive/negative output of the LNA to the positive/negative input of the AD8044, how do we connect the feedback to set the gain? I think we would use something similar to what is described here ( with R1 = R2 = R3 = R4, but there are probably more trade-offs that I am not yet aware of.

Page 11 of the LNA datasheet has 100 nF cas at the output to filter out the DC component of the signal, but if we use the AD8044 with the single +5 V supply, we would leave these caps out and the input voltage for the AD8044 would be correctly biased (around 2.5 V). We could then put the 100 nF caps at the output of the buffer to DC filter the signal. Does this look correct?

Thank you for taking the time to read and consider these questions.

  • Hi J,

    Sorry for the delayed response.

    Looking at the "Differential Amplifier" you provided in the link, it is actually a difference amplifier (subtractor) and not a fully-differential amplifier. Are you trying to implement a Differential-to-Single-ended signal conversion from the LNA output? Could you please share your block diagram for further investigation and understanding?

    Have you checked AD8334 (LNA + VGA)? It may also cater your application. Also, ADI offers devices specific for Ultrasound Applications. Please see the link.



  • Hello Jino,

    Here is a high level block diagram.

    We currently have an ADC to use with this system. I plan to look more into ADI's ultrasound specific applications in future iterations of this setup, but I do not believe they are what we are currently looking for.

    Thank you for your response!

  • Hi J,

    Thanks for sharing your block diagram. Unfortunately, the Voltage Subtractor or the Difference Amplifier that you want to implement is not suited for this kind of application.

    This configuration will amplify the difference between two voltages but rejects the common-mode voltage. Hence by powering the AD8044s with 5V single supply, the output is clipped during negative cycle of the signal with or without the 100 nF capacitors in the outputs of the LNA. You can either power the AD8044s using dual supply (i.e. + 5V) to get a full cycle signal (still rejecting the 2.5V common-mode voltage) or we can find replacements to suit your application.


  • Hi J,

    I'm sorry I missed out another solution for your application. You could still use a difference amplifier with a single supply. What you need is to provide a reference voltage to lift the output  up by 2.5 V (in your case), hence not saturating the output when the differential input signal is negative. See below figure.

    In this case, just make sure that VREF has to be low-impedance to preserve the balance of the difference amp. If it's not available in your circuit, you could generate one using a resistor divider and an op-amp buffer. See below.

    You will then be able to achieve your desired output:



  • Thank you, Jino!

    So if we change the power supply to +/- 5 V (and probably add filter caps before and after the AD8044), we shouldn't have the clipping problems.

    Thank you for your time.