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THS4131 Operation Basics

Sathish Palani
Prodigy 50 points

Hi All,

I got some circuit from my client. I am new to differential opamp(beginner). I tried to simulate this circuit(attached simulation file). If i simulate this circuit by giving 3V- 10V input sine wave the output overlaps each other. If I give less than 1V-2.5V the circuit works properly. Can some one please explain me what is happening. I am assuming the output will be given to ADC in some other board. 

Note: I need to understand the working principle of this circuit. Can someone give the notes or whitepapers to know the basics.

 sathish_backup.TSC

  • 0
    TI__Genius 12631 points

    Hello Sathish,

       I am not sure under what exact conditions the outputs will "overlap" each other. I just tried the circuit you gave me with V1=2.41V, and Vg1 = 7Vpp sine wave on a 6.5V dc offset and the 2 outputs of the FDA are out of phase with each other.

      When you mean "overlap each other" do you mean that the output common-mode of the differential outputs are the same? If so, then that is because the amplifier will amplify the difference in the common-modes between the 2 input signals (V1 and Vg1). An easy way for you to do the analysis is to change Vg1 to another dc source(rather than sine source) and run some dc simulations

    So for example, if V1=2.5V and Vg1(dc source now) =0V, then the outputs will be at 1V and 4V respectively. This is because:

    Amplifier is in a gain of approx. = 1.2

    Input difference = 2.5V = So output = 2.5*1.2 = 3.0V.

    Now the output common-mode is being forced to 2.5V through V2, so the 2 outputs of the amplifier will have a differential output of 3.0V on a common-mode of 2.5V.....hence the outputs are at 2.5V+(3.0/2) = 4.0V and 2.5V-(3.0/2) = 1.0V.

    In order to understand how FDAs work please refer to this application note.

    -Samir