Transcript 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
T 30
C2 111p
Gain (dB)
Charge Amplifier
*
R1 10M
Capacitive
Sensor
Charge Amplifier
C1 = 1nF, C2 = 111pF, R1 = 10M
20
-3dB @ 143Hz
10
0
1
10
100
V2 15
C1 1n
U1 OPA132
VG1
V1 15
Phase [deg]
+
RL 20k
Vo1
+
10k
100k
10k
100k
60
50
-
+
1k
Frequency (Hz)
40
30
20
10
0
-10
1
10
* C2 111pF non-standard, 110pF ok
100
1k
Frequency (Hz)
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)
Charge amplifier implementation
with capacitive humidity sensor
Charge Amplifier
100Hz, 4-th order, low-pass filter
Absolute Value Circuit
(full-wave rectifier)
fexcite = 1kHz, Set Filter to 1/10 fexcite100Hz
R6 10k
C2 180p
A
R2 10k
R3 10k
B
R4 5k
R5 10k
C6 1.5u
R1 10M
C4 470n
D2 1N4148
Sensor
VV2 15
C1 180p
D
-
D1 1N4148
V-
V-
C
-
Vabs
V-
V-
R7 2.05k
+
V1 15
+
V+
+
VG1
f = 1kHz
V = 1Vpk
+
+
R10 16.5k
R8 17.4k
+
+
+
-
R9 1.5k
+
+
+
C5 68n
C3 150n
Vout
V+
V+
V+
V+
U1 OPA132
U2 OPA132
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
U3 OPA132
U4 OPA132
Filter Pro used to design filter
U5 OPA132
Single input charge amp circuit
Output voltage for +/-20pF capacitance change
T
2.00
Voltage (V)
1.50
1.00
Vout[3]: 200p[F]
•
About 140mV Vout delta for a
+/-20pF change
Vout[2]: 180p[F]
•
The DC voltage is 2x the Vin
RMS value
•
The nominal DC level may be
an issue
Vout[1]: 160p[F]
500.00m
0.00
0
5m
10m
15m
20m
Time (s)
25m
30m
35m
Differential capacitance sensor
amplifier
INA154 Diff Amp, G=1, BW=3.1MHz
INA132 Diff Amp, G= 1, BW=300kHz, µPower
Charge Amplifiers
R1 10M
C2 180p
Difference Am plifier
Synchronous Dem odulator
100 Hz 4-Pole Butterworth LPF
V-
C1 180p
(difference amplifier connected
as switched G =2V/V amplifier)
-
+
+
V-
C5 470n
U1 OPA132
1kHz 10V p-p
Si ne Wave
-
V+
Sense
-
+
VG1
V+
+
+
U3 INA154
+
R5 2.05k
R4 10k
V+
+
Sensor
R2 10M
+
U5 INA132
+
T 1 2N7000
U4 T LC339/5_1
VC4 180p
VLP1
R6 17.4k
+
+
-
-
Sense
Ref
U2 OPA132
C3 180p
V-
V-
U6 OPA132
V+
C6 150n
Ref
V+
C7 1.5u
R3 2k
+
+
V-
V3 5
-
R7 1.5k
V+
(add hystereses
to comparator)
V1 15
V-
Vout
R8 16.5k
VLP1
+
+
V2 15
V+
Synchronous Demodulator:
Full-Wave Rectifies difference between Sensor Charge
Amplifier and Reference Charge Amplifier.
Differential Charge Amplifier Removes DC Level & Improves Sensitivity
C8 68n
V+
U7 OPA132
Differential Capacitance
Sense 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
T 400m
Vout[3]: 200p[F]
Vout (V)
200m
Vout[2]: 180p[F]
0
-200m
Vout[1]: 160p[F]
-400m
0
5m
10m
15m
Time (s)
20m
25m
30m