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Operational Amplifier-Based Charge Amplifiers in Capative Humidity Sensor Circuits

Explore a C-V approach for DC input bias path with charge amplifier implementation for capacitive humidity sensors. Learn about differential capacitance sensor amplifiers and synchronous demodulators. Design and optimize circuits for maximum sensitivity and accuracy.

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Operational Amplifier-Based Charge Amplifiers in Capative Humidity Sensor Circuits

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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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