Datasheet

LTC6240/LTC6241/LTC6242
24
624012fe
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
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 12 shows a noninverting 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 noninverting 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.
The noise measured over a 50 second interval, in Figure 10,
is 40nV in a 0.1Hz to 10Hz bandwidth.This low noise is at-
tributed to the input JFET’s die size and current density.
Figure 11. Classical Inverting Charge Amplifi er Figure 12. Low Noise Noninverting Shock Sensor Amplifi er
Figure 10. Noise in a 0.1Hz to 10Hz Bandwidth
Low Noise Shock Sensor Amplifi ers
Figures 11 and 12 show the amplifi ers realizing two dif-
ferent 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 11 shows the classical “charge amplifi er” approach.
The LTC6240 is in the inverting confi guration so the sensor
looks into a virtual ground. All of the charge generated
APPLICATIONS INFORMATION
5s/DIV
6241 F10
20nV/DIV
BIAS RESISTOR
VISHAY-TECHNO
CRHV2512AF1007G
(OR EQUIVALENT)
MAIN
GAIN-SETTING
ELEMENT IS A
CAPACITOR
SHOCK SENSOR
MURATA-ERIE
PKGS-00LD
770pF
CABLE HAS
UNKNOWN C
R
f
1G
6241 F11
V
OUT
= 110mV/g
–
+
LTC6240
C
f
7.7pF
BIAS RESISTOR
VISHAY-TECHNO
CRHV2512AF1007G
(OR EQUIVALENT)
1G
V
S
+
6241 F12
10k
1k
1k
100Ω
V
OUT
= 110mV/g
V
S
= ±1.4V to ±5.5V
BW = 0.2Hz to 10kHz
V
OUT
–
+
1/2
LTC6241HV
V
S
–
10k100Ω
–
+
1/2
LTC6241HV
SHOCK SENSOR
MURATA-ERIE
PKGS-00LD
770pF