Operational Amplifier based Charge Amplifiers Thomas Kuehl Senior

Operational Amplifier based Charge Amplifiers Thomas Kuehl Senior Applications Engineer – PA Linear

Another C – V approach R 1 for DC Input Bias Path * * C 2 111 p. F non-standard, 110 p. F ok f -3 d. B = 1 / (2π R 1 C 2) Set : R 1 ≥ 10 • | XC 2 | and f. GEN ≥ 10 (f -3 d. B) Then: AV = 1 + (XC 2 / XC 1) where XC = 1 / (2π f C)

Charge amplifier implementation with capacitive humidity sensor fexcite = 1 k. Hz, Set Filter to 1/10 fexcite 100 Hz B D C Vabs A B C equation Vabs +1 V -1. 65 Vabs = -A-2 B +1 V -1 V 0 V +0. 65 Vabs = -A* +1 V Op amp always in closed loop (D=0 V) *No current flow in D 2 Vabs A Filter Pro used to design filter

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

Differential capacitance sensor amplifier INA 154 Diff Amp, G=1, BW=3. 1 MHz INA 132 Diff Amp, G= 1, BW=300 k. Hz, µPower Synchronous Demodulator: Full-Wave Rectifies difference between Sensor Charge Amplifier and Reference Charge Amplifier. Differential Charge Amplifier Removes DC Level & Improves Sensitivity

Differential Capacitance Sense Amplifier Output Humidity Sensor Capacitance Min = 160 p. F (180 p. F-20 p. F) Vout = -350 m. V Humidity Sensor Capacitance Nominal = 180 p. F (180 p. F-20 p. F) Vout = 0 V Humidity Sensor Capacitance Max = 200 p. F (180 p. F+20 p. F) Vout = +350 m. V