Operational Amplifier based Charge Amplifiers

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Operational Amplifier basedCharge Amplifiers

• Thomas Kuehl
• Senior Applications Engineer PA Linear

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Another C V approach
R1 for DC Input Bias Path

C2 111pF non-standard, 110pF ok
f -3dB 1 / (2p R1 C2) Set R1 10 XC2
and fGEN 10 (f -3dB) Then AV 1 (XC2
/ XC1) where XC 1 / (2p f C)
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Charge amplifier implementation
with capacitive humidity sensor
fexcite 1kHz, Set Filter to 1/10 fexcite?100Hz
A
B
Vabs
C
D
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 (D0V) No current flow in D2
Filter Pro used to design filter
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Single input charge amp circuit
Output voltage for /-20pF capacitance change

• 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

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Differential capacitance sensor amplifier
INA154 Diff Amp, G1, BW3.1MHz INA132 Diff Amp,
G 1, BW300kHz, µPower
Synchronous Demodulator Full-Wave Rectifies
difference between Sensor Charge Amplifier and
Reference Charge Amplifier.
Differential Charge Amplifier Removes DC Level
Improves Sensitivity
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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
(180pF20pF) ? Vout 350mV