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LMH6552 low differential input resistance

Mohamad Nizar Kezzo
Prodigy 175 points
Other Parts Discussed in Thread: THS4511, LMH6552
 
My question is about the low differential impedance in the fully differential amplifier LMH6552. In the datasheet, page 5, it mentions that the input resistance is 15 Ohm compared to several MegaOhms in other amplifiers (THS4511 for example). Isn't it too low? How would that affect performance. The idea is that we are driving a voltage difference of about 1.5Vpp and this means that current passing is 100mA. Can the circuit take that?
  • 0
    TI__Mastermind 22825 points

    Hi,

    This application note briefly discusses the difference between the Voltage Feedback (VFB) and Current Feedback (CFB) type Fully Differential Amplifiers (FDA) in terms of their differential input impedances, on pages 3-4 . The LMH6552 is of the CFB type:

    "In a VFB FDA, the differential input impedance is very high (usually hundreds of KΩ to several MΩ), and ..."

    "A very different result arises when a CFB FDA, such as the LMH6552, is considered. Here, the differential input stage is essentially a current controlled current source, with ideally zero differential input impedance...."

    With the LMH6552 operating in closed loop, there will be a "virtual short" across the differential inputs (similar to the idea of "virtual ground" in the more traditional op amp terminology). That is, the feedback ensures that the two inputs are at identical potential (except transient behavior when the loop is trying to servo the two inputs together). Thus, there is no steady state voltage differential and subsequent current flow with either VFB or CFB type FDA's.

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 175 points
    Thank you very much, this was quite helpful!

    Best,

    Mohamad Nizar