AD8376 Load Impedance

Hello,

My customer has the following question. Can you advise if the 50 Ohms load impedance would be sufficient for his application?

On page 15 of the data sheet, it says: "Whereas the gain of the AD8376 is load dependent, RL in parallel with R1 + R2 should equal the optimum 150 Ω target load impedance to provide the expected ac performance depicted in the data sheet.”

 I would like to achieve a relatively flat response from below 1MHz to about 450MHz. For a given parasitic load capacitance, I should get better high frequency response with a 50 ohm load than a 150 ohm load. The bias inductors also need more choking impedance for a 150 ohm load than a 50 ohm load, which potentially means more parasitic capacitance. Going to 150 ohms increases both the R and the C. In my application, I need to convert back down to 50 ohms anyway, which means a 3:1 transformer. This is most likely a flux-coupled transformer, which means more frequency response degradation. My question is can I terminate the AD8376 into 100 ohms or 50 ohms instead of 150 ohms if I am willing to live with the lower gain? I will have to increase the input signal level to the device to compensate, but I can do that in my system. Also, it does not seem like this will violate the maximum input signal boundary. I think it boils down to whether there is something magical about 150 ohms to the AD8376 or not. I need to end up at 50 ohms, so when you factor in the transformer it does not seem to matter whether I terminate the device in 150 ohms and then convert to 50 ohms or simply terminate the device in 50 ohms. The signal swing will be less at the output of the AD8376 which should actually help linearity. It seems like a win from both a frequency response and linearity perspective, but the data sheet doesn’t address lower impedance loads.

  • 0
    •  Analog Employees 
    on Oct 16, 2020 6:52 PM 1 month ago

    Hi,

    I reviewed the schematic provided from the customer.

    Assuming the customer is okay with the 150 ohms target load impedance, his schematic will not need the 150 Ohm to 50 Ohm impedance transformation, which was not included in the schematic.. so that is correct?

     But I am not sure why he is using Coil craft transformer Vs ETC1-1-13 Balun, which is a 1:3 impedance ratio. What is input impedance of the DAC that he is looking to drive?

     I can see that the  wideband requirement he has from 1 MHz – 450MHz will be better met with Coilcraft than ETC1-1-13.

     And from the Coilcraft datasheet, I am not quite sure about the connections for the center-tap on the transformer.

    Other than these concerns, everything else looks okay to me.

    Warm Regards,

    Rachana