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THS4511: THS4511 CM voltage

Part Number: THS4511


I found THS4511 is very suitable for my application in aspect of linearity. but I have a problem

as you see in the below pic, Vcm=open (or zero). the supply is +5V.

  • can I use this circuit if the source voltage is 1Vpp within zero dc offset? (I mean AC-coupled)

actually in such circuits we apply some required Vcm to a FDA but it's odd in THS4511 the Vcm=open meanwhile the circuit is AC-coupled!

  • even for a unity gain amplifier we can use RF=RG=348 with Rt=59 ohm instead of some values in Table1. why the designer didn't use mentioned values?


  • Hi,

    If the Vcm pin is left floating then it will bias to midsupply by default. In the case of a single +5V supply the internal Vcm voltage would be 2.5V. You should have no problem using the device that way with an AC coupled 1Vpp signal.

    The Vcm controls only the output common mode voltage, so it can still be used with an AC coupled input and a DC coupled output. If the circuit is entirely AC coupled (both input and output) then the Vcm pin should be left open.

    I'm not sure why the original 56.2 ohm value resistor was chosen, but the 59 ohm resistor with the 348 ohm RG should give you a very good 50 ohm match.


    Jacob Freet 
    High Speed Amplifiers