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THS4140: Adding a BIT voltage to a fully-differential THS4140

Prodigy 90 points

Replies: 4

Views: 59

Part Number: THS4140

Adding a programmable BIT voltage (like from a DAC) to a summing amplifier stage (In front of an ADC) is straightforward. It's just a second summing node from a voltage source through an impedance. The goal being a short BIT sequence to add, say, a 0.5V test voltage on top of a signal already there just to see if the gain from a sensor stage is operating properly.

However, when using a fully-differential stage like a THS4140, there is no straightforward way of doing this. There doesn't seem to be a clean way of doing this short of using analog switches to switch in/out a separate voltage source instead of the differential sensor. (There isn't a fully-differential "summing" opamp)

Has anyone seen anything clever that I might be missing? I hate to convert to a non-differential opamp just to be able to add BIT capability.


  • Hello Jeff,

    The same concept can be applied to an FDA as you can see below.

    In regular configuration:

    With an added offset:

    Please let me know if you have any questions.


    Hasan Babiker

  • In reply to Hasan Babiker31:

    Thanks for the response.

    Yes, this seems to work with a unity gain, but unless I have a problem with my simulation, the offset seems to shift my attenuation by a small percentage.

    We were attenuating by about 5%, and with a 0.25V addition on to a 1V input, the attenuation ends up wrong by about 0.5%. We might have something wrong in our sim though.

  • In reply to Jeff Mercure:

    Hello Jeff,

    Can you send me your simulation files?


    Hasan Babiker

  • In reply to Hasan Babiker31:

    Our bad on the simulation. This method simulates fine now.

    Thanks for your help