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Questioning the formula in the INA827 data sheet for maximum voltage swings

Other Parts Discussed in Thread: INA827, INA826

Hello, I am looking at the INA827 data sheet, "Inside the INA827" on page 18. The formulas for A1 and A2 output voltages are:

Vcm + Vbe + 0.125V ± Vd / 2 * G

Since A3 provides a gain of 5 using the outputs of A1 and A2, is G, the programmed gain, the correct factor?

Thank you,

 

Chris Hakim

SSL

  • PS I noticed that the A1 and A2 gain formula is the same in the INA826 data sheet at the same place (p. 20, "Inside the INA826"). It seems that instead of G, the factor should be G/5 for the INA827.

  • Hello,

    In the equations for A1 and A2 output for the INA827, G represents the gain of the input stage (for clarification we can call it G_IS), which is:

    G_IS=1+(2*RF/RG)=1+(2*8k/RG)

    For the INA826, the equation is the same except RF=24.7kohms.

    The difference between the INA826 and INA827 is the gain of the output stage difference amplifier, or G_OS.  For the INA826, G_OS=1V/V.  For the INA827, G_OS=5V/V.

  • But if I multiply the gain of the INA827 input stage by that of the output stage, as stated in the above answer, I get:

    G = (1 + 2.8k / Rg) × 5 = 5 + 140k / Rg.

    The formula on the INA827 data sheet is:

    5 + 80k / Rg.

    Which is correct?

  • Here is how I calculated the gain equation:

    G_INA=GOS*GIS
    G_INA=5*(1+16k/RG)
    G_INA=5+80k/RG

    I am unsure of the origin of 2.8k and 140k values in your equation. In my equation, the numerator is "2*8k" which equates to 16k. When you multiply 16k by 5 you obtain 80k.