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THS4551: Common-mode input high

David Marty
Intellectual 635 points
Part Number: THS4551
Other Parts Discussed in Thread: TINA-TI

Hi,

The THS4551 is used in the ADS8900EVM-PDK Evaluation Module.

In the §5.2 of the user's guide, the input signal of the STK can be differential 10Vpp.

The THS4551 datasheet says that the common-mode input high is typically (V+) - 1.1V.

The THS4551 is powered, in the STK, at 5.3V, so the common-mode input high is 4.2V.

4.2V (THS4551 common-mode input high) < 5Vpeak (ADS8900EVM-PDK input signal), why the THS4551 output signal isn't saturated?

Thanks

David

  • +1
    TI__Mastermind 25975 points

    Hello David,

      Good question, for the input common-mode specification this applies to the voltage seen right at the inputs of the amplifier. In terms of the user's guide, this specifies the input signal to the amplifier with a 5Vpp signal with a 2.5 DC offset at each input. The Tina-TI circuit and simulation is recreated below:

      If the amplifier sees a +5V signal at the input node of the amplifier, then that is correct, it will violate the specified input common-mode high voltage of 5.3 - 1.1 = 4.2V. But, the voltage at the input is lower. In general, A FDA has a signal input node and a device input node. It is easier to see in simulation:

       

        The above three images show the signals at their maximum and minimum input range with their respective device input node and output node voltages. The device input voltage is lower than 4.2V. This will increase with increasing gain. 

     THS4551_10Vpp.TSC

    Thank you,

    Sima