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CMRR

CMRR. Rev 1.0 9/15/13. Common Mode Rejection Ratio. CMRR Specification. Definitions and Equations for CMRR CMRR(dB ) = 20 Log ( Δ Vosi / Δ Vcm ) (data sheet) CMRR(Linear-Gain) = 10 (CMRR(dB)/20) (solve for linear gain) CMRR(Linear-Gain) = Δ Vosi / Δ Vcm.

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CMRR

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  1. CMRR Rev 1.0 9/15/13

  2. Common Mode Rejection Ratio

  3. CMRR Specification Definitions and Equations for CMRR CMRR(dB) = 20 Log (ΔVosi / ΔVcm) (data sheet) CMRR(Linear-Gain) = 10(CMRR(dB)/20) (solve for linear gain) CMRR(Linear-Gain) = ΔVosi / ΔVcm

  4. Note: Vos is defined for this condition. Vcm = Vs/2 Vs/2 = GND in this case Note: Vos is affected by Vcm. Vcm = Vs/2 + 12V

  5. AC CMRR Example OPA170 Vcm = Vin Vcm = 0V Vcm = Vsupply / 2

  6. AC CMRR Example OPA170

  7. AC CMRR Example OPA170

  8. AC CMRR – Run Transient

  9. AC CMRR – Transient Results

  10. AC CMRR – Simulated vs. Calculated

  11. CMRR for Dif Amps And INAs

  12. Common mode rejection for Dif-Amps and INAs

  13. Dif-Amp • Assuming the resistors are ideal the Op-Amp CMRR is dominant • Above shows a dc sweep with ideal 10k resistors • Note: CMRR is always input referred, but in this case Gain = 1.

  14. Relating Dif Amp CMRR with Op-amp CMRR

  15. Same Circuit, Different Op-Amp

  16. Worst Case CMRR for 0.1% Resistors • The output shifts 40mV (worst case) for 0.1% resistor tolerance • The output shifted 4μV for ideal resistor (limited by CMRR of Op-Amp) • 10,000x difference between ideal and practical resistors.

  17. Monte Carlo Analysis CMRR 0.1% Resistors • Monte Carlo Analysis shows expected distribution of CMRR • The majority of the population has poor CMRR (i.e. CMRR < 70dB) • Better CMRR could be achieved with tighter tolerance (i.e. 0.01%) • Worse CMRR could be achieved with looser tolerance (i.e. 1%)

  18. ac CMRR for Ideal and Practical Resistors

  19. Monolithic IC vs. Discrete Resistors • Laser trim gets better accuracy then 0.1% resistors. • Resistor Drift Cancels • Tested system (i.e. CMRR, Gain, and other specifications assured) • Smaller area and lower overall cost.

  20. Classic 3 Amp INA

  21. Common mode Variation for 0.1% Discrete Resistors Note: CMRR is always referred to the input. The output shift is divided by gain, so CMRR normally improves at higher gains.

  22. Special Topology – PGA280/281

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