Design Steps

1. Calculate the output voltage V_out for this circuit using the following equation:
   
   In this equation, V_d = V₂ – V₁ is the differential input voltage, V_ref is set by R₃ and R₄ to level shift the output, and it is assumed that R_1a = R_2a and R_1b = R_2b. Integrated instrumentation amplifiers normally fix R_f1, R_f2, R_1a, R_2a, R_1b, and R_2b, leaving only R_g to set the gain of the circuit. In this discrete implementation, the designer has the freedom to alter all of these resistors, but the transfer function can be simplified by using standard values, such as R_f1 = R_f2 = R_1a = R_1b = R_2a = R_2b = 10kΩ, and using only R_g to set the gain. In this case, R_g can be calculated using the following simplified equation:
   
2. Set V_ref. For this design, V_ref has been set as shown in the following equation so that a symmetric input voltage range of –50mV to +50mV results in an output voltage range of 0.2V to 5V.
   
   
   R₄ = 13.83kΩ ≈ 13.8kΩ (standard value)
   Note: The magnitudes of R₃ and R₄ were chosen such that R₃ || R₄ is close to 10kΩ so that the low-pass filter formed by R₃ || R₄ and C₂ is close to the common low-pass filter with R = 10kΩ and C = 100nF.
   
   
3. Choose R_g to set the required gain using the simplified transfer function.
   
   R_g = 425Ω ≈ 427Ω (standard value)
   This corresponds to a gain of: