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Operational Amplifier based Charge Amplifiers

Operational Amplifier based Charge Amplifiers. Thomas Kuehl Senior Applications Engineer – PA Linear. Another C – V approach. R1 for DC Input Bias Path. *. * C2 111pF non-standard, 110pF ok. f -3dB = 1 / (2 π R 1 C 2 ) Set : R 1 ≥ 10 • | X C2 | and f GEN ≥ 10 ( f -3dB )

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Operational Amplifier based Charge Amplifiers

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  1. Operational Amplifier basedCharge Amplifiers Thomas Kuehl Senior Applications Engineer – PA Linear

  2. Another C – V approach R1 for DC Input Bias Path * * C2 111pF non-standard, 110pF ok f -3dB = 1 / (2π R1 C2) Set : R1 ≥ 10 • | XC2 | and fGEN ≥ 10 (f -3dB) Then: AV = 1 + (XC2 / XC1) where XC = 1 / (2π f C)

  3. Charge amplifier implementation with capacitive humidity sensor fexcite = 1kHz, Set Filter to 1/10 fexcite100Hz A B Vabs D C Vabs A B C equation Vabs +1V -1V -1.65 Vabs = -A-2B +1V -1V 0V +0.65 Vabs = -A* +1V Op amp always in closed loop (D=0V) *No current flow in D2 Filter Pro used to design filter

  4. About 140mV Vout delta for a +/-20pF change The DC voltage is 2x the Vin RMS value The nominal DC level may be an issue Single input charge amp circuit Output voltage for +/-20pF capacitance change

  5. Differential capacitance sensor amplifier INA154 Diff Amp, G=1, BW=3.1MHz INA132 Diff Amp, G= 1, BW=300kHz, µPower Synchronous Demodulator: Full-Wave Rectifies difference between Sensor Charge Amplifier and Reference Charge Amplifier. Differential Charge Amplifier Removes DC Level & Improves Sensitivity

  6. Differential CapacitanceSense Amplifier Output Humidity Sensor Capacitance Min = 160pF (180pF-20pF)  Vout = -350mV Humidity Sensor Capacitance Nominal = 180pF (180pF-20pF)  Vout = 0V Humidity Sensor Capacitance Max = 200pF (180pF+20pF)  Vout = +350mV

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