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Published byLorena Holten Modified about 1 year ago

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CMRR Rev 1.0 9/15/13

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

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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

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

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AC CMRR Example OPA170

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AC CMRR – Run Transient

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AC CMRR – Transient Results

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AC CMRR – Simulated vs. Calculated

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Common mode rejection for Dif-Amps and INAs

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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.

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Relating Dif Amp CMRR with Op-amp CMRR

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Same Circuit, Different Op-Amp

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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.

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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%)

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ac CMRR for Ideal and Practical Resistors

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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.

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Classic 3 Amp INA

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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.

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Special Topology – PGA280/281

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