LTC6241 LINER [Linear Technology], LTC6241 Datasheet - Page 19

• Manufacturer Part Number: LTC6241
• Description: Dual/Quad 18MHz, Low Noise, Rail-to-Rail, CMOS Op Amps
• Manufacturer: LINER [Linear Technology]

APPLICATIO S I FOR ATIO
Low Noise Shock Sensor Amplifi ers
Figures 9 and 10 show the LTC6241 realizing two different
approaches to amplifying signals from a capacitive sensor.
The sensor in both cases is a 770pF piezoelectric shock
sensor accelerometer, which generates charge under
physical acceleration.
Figure 9 shows the classical “charge amplifi er” approach.
The LTC6241 is in the inverting confi guration so the sensor
looks into a virtual ground. All of the charge generated
by the sensor is forced across the feedback capacitor
by the op amp action. Because the feedback capacitor
is 100 times smaller than the sensor, it will be forced to
100 times what would have been the sensor’s open circuit
voltage. So the circuit gain is 100. The benefi t of this ap-
proach is that the signal gain of the circuit is independent
of any cable capacitance introduced between the sensor
and the amplifi er. Hence this circuit is favored for remote
SHOCK SENSOR
MURATA-ERIE
PKGS-00LD
Figure 9. Classical Inverting Charge Amplifi er
UNKNOWN C
CABLE HAS
770pF
U
CRHV2512AF1007G
(OR EQUIVALENT)
VISHAY-TECHNO
BIAS RESISTOR
U
+
–
LTC6241
1/2
7.7pF
1G
R
C
f
f
W
V
OUT
MAIN
GAIN-SETTING
ELEMENT IS A
CAPACITOR
= 110mV/g
U
6241 F09
accelerometers where the cable length may vary. Diffi culties
with the circuit are inaccuracy of the gain setting with the
small capacitor, and low frequency cutoff due to the bias
resistor working into the small feedback capacitor.
Figure 10 shows a non-inverting amplifi er approach. This
approach has many advantages. First of all, the gain is set
accurately with resistors rather than with a small capaci-
tor. Second, the low frequency cutoff is dictated by the
bias resistor working into the large 770pF sensor, rather
than into a small feedback capacitor, for lower frequency
response. Third, the non-inverting topology can be paral-
leled and summed (as shown) for scalable reductions in
voltage noise. The only drawback to this circuit is that the
parasitic capacitance at the input reduces the gain slightly.
This circuit is favored in cases where parasitic input
capacitances such as traces and cables will be relatively
small and invariant.
CRHV2512AF1007G
(OR EQUIVALENT)
VISHAY-TECHNO
BIAS RESISTOR
Figure 10. Low Noise Non-Inverting Shock Sensor Amplifi er
SHOCK SENSOR
MURATA-ERIE
1G
PKGS-00LD
770pF
LTC6241/LTC6242
100Ω
100Ω
+
LTC6241HV
–
+
LTC6241HV
–
V
V
1/2
1/2
S
S
10k
10k
+
–
6241 F10
V
V
BW = 0.2Hz to 10kHz
OUT
S
= ±1.4V to ±5.5V
1k
1k
= 110mV/g
V
OUT
19
62412f