This educational guide will deal with the following types of basic sensor
Charge Output Sensors - high output impedance, piezoelectric sensors (without
built-in electronics) which typically require external charge or voltage
amplifiers for signal conditioning.
Internally Amplified Sensors - low impedance, piezoelectric force, acceleration
and pressure type sensors with built-in integrated circuits. (ICP® is
registered trademark of PCB Group. which uniquely identifies PCB's
sensors which incorporate built-in electronics.)
CONVENTIONAL CHARGE OUTPUT SENSORS
Historically, nearly all dynamic measurement applications utilized piezoelectric
charge mode sensors. These sensors contain only a piezoelectric sensing element
(without built-in electronics) and have a high impedance output signal.
The main advantage of charge type sensors is their ability to operate under high
temperature environments. Certain sensors have the ability to withstand
temperatures exceeding 1000°F (538°C). However, the output generated by the
piezoelectric sensing crystals is extremely sensitive to corruption from
various environmental factors. Low noise cabling must be used to reduce radio
frequency interference (RFI) and electromagnetic interference (EMI.) The use of
tie wraps or tape reduces triboelectric (motion-induced) noise. A high
insulation resistance of the sensor and cabling should be maintained to avoid
drift and ensure repeatable results.
To properly analyze the signal from charge sensors, the high impedance output
must normally be converted to a low impedance voltage signal. This can be done
directly by the input of the readout device or by in-line voltage and charge
amplifiers. Each case will be considered separately.
Voltage Mode (and Voltage Amplified) Systems
Certain piezoelectric sensors exhibit exceptionally high values of internal
source capacitance and can be plugged directly into high impedance (>1 Megohm)
readout devices such as oscilloscopes and analyzers. Others with a low internal
source capacitance may require in-line signal conditioning such as a voltage
amplifier. See Figure 1.
Figure 1: Typical Voltage Mode Systems
A schematic representation of these voltage mode systems including sensor, cable
and input capacitance of voltage amplifier or readout device is shown below in
Figure 2. The insulation resistance (resistance between signal and ground) is
assumed to be large (>1012 ohms) and is therefore not shown in the
Figure 2: Voltage Mode System Schematic
The open circuit (i.e. cable disconnected) voltage sensitivity V1 (mV per psi,
lb or g) of the charge mode sensor can be represented mathematically by
V1 = q / C1 (Eq. 1)
where: q = basic charge sensitivity in pC per psi, lb or g
C1 = Internal sensor (crystal) capacitance in pF
(p = pico = 1 x 10-12 F = farad)
The overall system voltage sensitivity measured at the readout instrument (or
input stage of the voltage amplifier) is the reduced value shown in Equation 2.
V1 = q / (C1 + C2 + C3) (Eq. 2)
where: C2 = cable capacitance in pF
C3 = input capacitance of the voltage amplifier or readout instrument in pF
According to the law of electrostatics (Equations 1 and 2), sensing elements
with a low capacitance will have a high voltage sensitivity. This explains why
low capacitance quartz sensors are used predominantly in voltage systems.
This dependency of system voltage sensitivity upon the total system capacitance
severely restricts sensor output cable length. It explains why the voltage mode
sensitivity of high impedance type piezoelectric sensors is measured and
specified with a given cable capacitance. If the cable length and/or type is
changed, the system must be recalibrated. These formulas also show the
importance of keeping the sensor input cable/connector dry and clean. Any
change in the total capacitance or loss in insulation resistance due to
contamination can radically alter the system characteristics. Furthermore, the
high impedance output signal makes the use of low-noise coaxial cable mandatory
and precludes the use of such systems in moist or dirty environments unless
extensive measures are taken to seal cables and connectors.
From a performance aspect, voltage mode systems are capable of linear operation
at high frequencies. Certain instrumentation has an frequency limit exceeding 1
MHz making it useful for detecting shock waves with a fraction of a microsecond
rise time. However, care must be taken as large capacitive cable loads may act
as a filter and reduce this upper operating frequency range.
Unfortunately, many voltage amplified systems have a noise floor (resolution) on
the order of a magnitude higher than equivalent charge amplified systems. For
this reason, high resolution ICP and/or charge amplified sensors are typically
used for low amplitude dynamic measurements.
Charge Amplified Systems
A typical charge amplified measurement system is shown below in Figure 3.
Figure 3: Typical Charge Amplified System
A schematic representation of a charge amplified system including sensor, cable
and charge amplifier is shown below in Figure 4. Once again, the insulation
resistance (resistance between signal and ground) is assumed to be large (>1012
ohms) and is therefore not shown in the schematic.
Figure 4: Charge Amplified System Schematic
In this system, the output voltage is dependent only upon the ratio of the input
charge, q, to the feedback capacitor, Cf as shown in Equation 3. For this
reason, artificially polarized polycrystalline ceramics, which exhibit a high
charge output, are used in such systems.
Vout = q / Cf (Eq. 3)
There are serious limitations with the use of conventional charge amplified
systems, especially in field environments or when driving long cables between
the sensor and amplifier. First, the electrical noise at the output of a charge
amplifier is directly related to the ratio of total system capacitance (C1 + C2
+ C3) to the feedback capacitance (Cf). Because of this, cable length should be
limited as was the case in the voltage mode system. Secondly, because the
sensor output signal is of a high impedance type, special low-noise cable must
be used to reduce charge generated by cable motion (triboelectric effect) and
noise caused by excessive RFI and EMI.
Also, care must be exercised to avoid degradation of insulation resistance at
the input of the charge amplifier to avoid the potential for signal drift. This
often precludes the use of such systems in harsh or dirty environments unless
extensive measures are taken to seal all cables and connectors.
While many of the performance characteristics are advantageous as compared to
voltage mode systems, the per channel cost of charge amplified instrumentation
is typically very high. It is also impractical to use charge amplified systems
above 50 or 100 kHz as the feedback capacitor exhibits filtering
characteristics above this range.
ICP® is a term that uniquely identifies PCB's piezoelectric sensors with
built-in microelectronic amplifiers. (ICP is a registered trademark of PCB
Group) Powered by constant current signal conditioners, the result
is an easy-to-operate, low impedance, 2-wire system as shown in Figure 5.
Figure 5: Typical ICP® Sensor Systems
In addition to ease-of-use and simplicity of operation, ICP sensors offer many
advantages over traditional charge mode sensors, including:
Fixed voltage sensitivity independent of cable length or capacitance.
Low input impedance (<100 Ohms) allows signals to transmitted over long cables
through harsh environments with virtually no loss in signal quality.
Two wire system accommodates standard low cost coaxial or other two conductor
High quality, voltage output compatible with standard readout, recording or
Intrinsic sensor self-test feature by monitoring sensor output bias voltage.
Low per channel cost as sensors require only low cost constant current signals
Reduced system maintenance.
Direction operation into readout and data acquisition instruments which
incorporate power for use with PCB's ICP sensors.
Figure 6 schematically shows the electrical fundamentals of typical quartz and
ceramic ICP sensors. These sensors are comprised of basic piezoolectic
transduction mechanism (which has an output proportional to force, pressure or
acceleration depending on the sensor type) coupled to a highly, reliable
Figure 6: Basic Quartz and Ceramic ICP® Sensors
Two types of integrated circuits are generally used in ICP sensors: voltage and
charge amplifiers. Low capacitance quartz sensing elements exhibit a very high
voltage output (according to V = q/C) and are typically used with MOSFET
voltage amplifiers. Ceramic sensing elements which exhibit a very high charge
output are normally coupled to charge amplifiers.
The theory behind ICP quartz sensing technology will first be explained. The
process begins when a measureand, acting upon the piezoelectric sensing
element, produces a quantity of charge referred to as Δq. This charge
collects in the crystal capacitance, C, and forms a voltage according to the
law of electrostatics: ΔV = Δq/C. Because quartz exhibits a very low
capacitance, the result is a high voltage output suitable for use with voltage
amplifiers. The gain of the amplifier then determines the sensor sensitivity.
This ΔV instantaneously appears at the output of the voltage amplifier,
added to an approximate +10 VDC bias level. This bias level is constant and
results from the electrical properties of the amplifier itself. (Normally, the
bias level is removed by an external signal conditioner before analyzing any
data. This concept will be fully explained later.) Also, the impedance level at
the output of the sensor is less than 100 ohms. This makes it easy to drive
long cables through harsh environments with virtually no loss in signal
ICP sensors which utilize ceramic sensing elements generally operate in a
different manner. Instead of using the voltage generated across the crystal,
ceramic ICP sensors operate with charge amplifiers. In this case, the high
charge output from the ceramic crystal is the desirable characteristic.
The sensors electrical characteristics are analogous to those described
previously in charge mode systems where the voltage output is simply the charge
generated by the crystal divided by the value of the feedback capacitor. (The
gain of the amplifier (mV/pC) ultimately determines the final sensitivity of
the sensor.) In this case many of the limitations have been eliminated. That
is, all of the high impedance circuitry is protected within a rugged, hermetic
housing. Concerns or problems with contamination and low noise cabling are
A quick comparison of integrated circuit voltage and charge amplifiers is
Note that the schematics in Figure 6 also contain an additional resistor. In
both cases, the resistor is used to set the time constant of the RC
(resistor-capacitor) circuit. This will be further explained in Section 7.1.
In-line Charge and Voltage Amplifiers
Certain applications (such as high temperature testing) may require the
integrated circuits to be removed from the sensor. For this reason, a variety
of in-line charge and voltage amplifiers are available. Operation is identical
to that of the ICP sensor, except that the cable connecting the sensor to the
amplifier carries a high impedance signal. Special precautions, like those
discussed earlier in the charge and voltage mode sections, must be taken to
ensure reliable and repeatable data.
Powering ICP Systems
A typical sensing system including a quartz ICP sensor, ordinary two conductor
cable and basic constant current power supply is shown in Fig.7. All ICP
sensors require a constant current power source for proper operation. The
simplicity and the principle of 2-wire operation can be clearly seen.
Figure 7: Typical Sensing System
The signal conditioner consists of a well-regulated 18 to 30 VDC source (battery
or line-powered), a current-regulating diode (or equivalent constant current
circuit), and a capacitor for decoupling (removing the bias voltage) the
signal. The voltmeter, Vm, monitors the sensor bias voltage (normally 8 to 14
VDC) and is useful for checking sensor operation and detecting open or shorted
cables and connections.
The current-regulating device is used in place of a resistor for several
reasons. The very high dynamic resistance of the diode yields a source follower
gain which is extremely close to unity and independent of input voltage. Also,
the diode can be changed to supply higher currents for driving long cable
lengths. Constant current diodes, as shown in Figure 8, are used in all of
PCB's battery-powered signal conditioners. (The correct orientation of the
diode within the circuit is critical for proper operation.) Except for special
models, standard ICP sensors require a minimum of 2 mA for proper operation.
Figure 8: Constant Current Diode
Present technology limits this diode type to 4 mA maximum rating, however,
several diodes can be placed in parallel for higher current levels. All PCB
line-powered signal conditioners use higher capacity (up to 20 mA) constant
current circuits in place of the diodes, but the principle of operation is
Decoupling of the data signal occurs at the output stage of the signal
conditioner. The 10 to 30 µF capacitor shifts the signal level to essentially
eliminate the sensor bias voltage. The result is a drift-free AC mode of
operation. Optional DC coupled models eliminate the bias voltage by use of a DC
Effect of Excitation Voltage on the Dynamic Range of ICP Sensors
The specified excitation voltage for all standard ICP sensors and amplifiers is
generally within the range of 18 to 30 volts. The effect of this range is shown
in Figure 9.
Figure 9: Typical Voltage Mode Systems
To explain the chart, the following values will be assumed:
VB = Sensor Bias Voltage = 10 VDC
VS1 = Supply Voltage 1 = 24 VDC
VE1 = Excitation Voltage 1 = VS1 -1 = 23 VDC
VS2 = Supply Voltage 2 = 18 VDC
VE2 = Excitation Voltage 2 = VS2 -1 = 17 VDC
Maximum Sensor Amplifier Range = ±10 volts
Note that an approximate 1 volt drop across the current limiting diode (or
equivalent circuit) must be maintained for correct current regulation. This is
important as two 12 VDC batteries in series will have a supply voltage of 24
VDC, but will only have a 23 VDC usable sensor excitation level.
The solid curve represents the input to the internal electronics of a typical
ICP sensor, while the shaded curves represent the output signals for two
different supply voltages.
In the negative direction, the voltage swing is typically limited by a 2 VDC
lower limit. Below this level, the output becomes nonlinear (nonlinear portion
1 on graph). The output range in the negative direction can be calculated by:
Negative Range = VB-2 (Eq. 4)
This shows that the negative voltage swing is affected only by the sensor bias
voltage. For this case, the negative voltage range is 8 volts.
In the positive direction, the voltage swing is limited by the excitation
voltage. The output range in the positive direction can be calculated by:
Positive Range = (Vs - 1) - VB = VE - VB (Eq. 5)
For a supply voltage of 18 VDC, this results in a dynamic output range in the
positive direction of 7 volts. Input voltages beyond this point simply result
in a clipped waveform as shown.
For the supply voltage of 24 VDC, the theoretical output range in the positive
direction is 13 volts. However, the microelectronics in ICP sensors are seldom
capable of providing accurate results at this level. (The assumed maximum
voltage swing for this example is 10 volts.) Most are specified to ±3, ±5 or
±10 volts. Above the specified level, the amplifier is nonlinear (nonlinear
portion 2 on graph). For this example, the 24 VDC supply voltage extended the
usable sensor output range to +10/-8 volts.
Refer to the installation and/or outline drawing included in the sensor manual
for mounting preparation and installation technique. Select desired operating
mode (AC or DC coupling) and make sure that cable connectors are tight to
provide reliable ground returns. If solder connector adaptors are used, inspect
solder joints. If vibration is present, use cable tie downs appropriately
spaced to avoid cable fatigue. Although ICP instruments are low impedance
devices, in extreme environments it is advisable to used shielded cables and
protect cable connections with heat shrink tubing. Complete installation
instructions will be provided with each sensor.
If a PCB signal conditioner is being used, turn the power on and observe the
voltmeter (or LEDs) on the front panel.
Figure 10: Typical Fault Indicators
Typical indicators are marked as shown in Figure 10. The green area (or LED)
indicates the proper bias range for the ICP sensor and the correct cable
connections. A red color indicates a short condition in the sensor, cable, or
connections. Yellow means the excitation voltage is being monitored and is an
indication of an open circuit.
Apparent Output Drift (when AC Coupled)
AC coupled signal conditioners require sufficient time to charge the internal
coupling capacitor. This capacitor must charge through the input resistance of
the readout instrument and, if a DC readout is used, the output voltage will
appear to drift slowly until charging is complete. A 1 Megohm readout device
will require 5 x 1 meg x 10 µF or 50 seconds for essentially complete charging.
(Assumes stable operation after five time constants: 5 x Resistance x
Capacitance. See Section Transducer Discharge Time Constant Section)
HIGH FREQUENCY RESPONSE OF ICP SENSORS
ICP sensor systems ideally treat signals of interest proportionally. However, as
the frequency of the measureand increases, the system eventually becomes
nonlinear. This is due to the following factors:
Amplifier/Power Supply Limitations
Each of these factors must be considered when attempting to make high frequency
The mechanical structure within the sensor most often imposes a high frequency
limit on sensing systems. That is, the sensitivity begins to rise rapidly as
the natural frequency of the sensor is approached.
w = √(k/m) (Eq. 6) where: w = natural frequency k = stiffness of
sensing element m = seismic mass
This equation helps to explain why larger sensors, in general, have a low
Figure 11 below represents a frequency response curve for a typical ICP
Figure 11: Resonse of an ICP® Accelerometer
It can be seen that the sensitivity rises as the frequency increases. For most
applications, it is generally acceptable to use this sensor over a range where
the sensitivity deviates by less than +5%. This upper frequency limit occurs at
approximately 20% of the resonant frequency. Pressure and force sensors respond
in a similar manner.
Mounting also plays a significant role in obtaining accurate high frequency
measurements. Be certain to consult the installation procedures for proper
Amplifier/Power Supply Limitations
When testing at extremely high frequencies (>100 kHz) the type of sensing system
becomes important. In general, voltage amplified systems respond to frequencies
on the order of 1 MHz, while most charge amplified systems may respond only to
100 kHz. This is typically due to limitations of the type of amplifier as well
as capacitive filtering effects. For such cases, consult the equipment
specifications or call PCB for assistance.
Cable Considerations and Constant Current Level
Operation over long cables may affect frequency response and introduce noise and
distortion when an insufficient current is available to drive cable
Unlike charge mode systems, where the system noise is a function of cable
length, ICP sensors provide a high voltage, low impedance output well-suited
for driving long cables through harsh environments. While there is virtually no
increase in noise with ICP sensors, the capacitive loading of the cable may
distort or filter higher frequency signals depending on the supply current and
the output impedance of the sensor.
Generally, this signal distortion is not a problem with lower frequency testing
within a range up to 10000 Hz. However, for higher frequency vibration, shock
or transient testing over cables longer than 100 ft. (30 m.), the possibility
of signal distortion exists.
The maximum frequency that can be transmitted over a given cable length is a
function of both the cable capacitance and the ratio of the peak signal voltage
to the current available from the signal conditioner according to:
where, Fmax = maximum frequency (hertz)
C = cable capacitance (picofarads)
V = maximum peak output from sensor (volts)
Ic = constant current from signal conditioner (mA)
109 = scaling factor to equate units
Note that in this equation, 1 mA is subtracted from the total current supplied
to sensor (Ic). This is done to compensate for powering the internal
electronics. Some specialty sensor electronics may consume more or less
current. Contact the manufacturer to determine the correct supply current.
When driving long cables, Equation 7 shows that as the length of cable, peak
voltage output or maximum frequency of interest increases, a greater constant
current will be required to drive the signal.
The nomograph below (Figure 12) provides a simple, graphical method for
obtaining the expected maximum frequency capability of an ICP measurement
system. The maximum peak signal voltage amplitude, cable capacitance and
supplied constant current must be known or presumed.
Figure 12: Cable Driving Nomograph
For example, when running a 100 ft. (30,5 m.) cable with a capacitance of 30
pF/ft, the total capacitance is 3000 pF. This value can be found along the
diagonal cable capacitance lines. Assuming the sensor operates at a maximum
output range of 5 volts and the constant current signal conditioner is set at 2
mA, the ratio on the vertical axis can be calculated to equal 5. The
intersection of the total cable capacitance and this ratio result in a maximum
frequency of approximately 10.2 kHz.
The nomograph does not indicate whether the frequency amplitude response at a
point is flat, rising or falling. For precautionary reasons, it is good general
practice to increase the constant current (if possible) to the sensor (within
its maximum limit) so that the frequency determined from the nomograph is
approximately 1.5 to 2 times greater than the maximum frequency of interest.
Note that higher current levels will deplete battery-powered signal conditioners
at a faster rate. Also, any current not used by the cable goes directly to
power the internal electronics and will create heat. This may cause the sensor
to exceed its maximum temperature specification. For this reason, do not supply
excessive current over short cable runs or when testing at elevated
Experimentally Testing Long Cables
To determine the high frequency electrical characteristics involved with long
cable runs, two methods may be used.
The first method illustrated in Figure 13 involves connecting the output from a
standard signal generator into a unity gain, low-output impedance (<5 ohm)
instrumentation amplifier in series with the ICP sensor. The extremely low
output impedance is required to minimize the resistance change when the signal
generator/amplifier is removed from the system. The alternate test method also
shown in Figure 13 incorporates a standard signal generator and the integral
electronics from an ICP sensor. Sensor simulators are available which contain a
signal generator and electronics conveniently packaged together.
Figure 13: Testing Long Cables
In order to check the frequency/amplitude response with either of these systems,
set the signal generator to supply the maximum amplitude of the expected
measurement signal. Observe the ratio of the amplitude from the generator to
that shown on the scope. If this ratio is 1:1, the system is adequate for your
test. (If necessary, be certain to factor in any gain in the signal conditioner
or scope.) If the output signal is rising (1:1.3 for example), add series
resistance to attenuate the signal. Use of a variable 100 ohm resistor will
help set the correct resistance more conveniently. Note that this is the only
condition that requires the addition of resistance. If the signal is falling
(1:0.75 for example), the constant current level must be increased or the cable
It may be necessary to physically install the cable during cable testing to
reflect the actual conditions encountered during data acquisition. This will
compensate for potential inductive cable effects that are partially a function
of the geometry of the cable route.
LOW FREQUENCY RESPONSE OF ICP SENSORS
With ICP sensors, there are two factors which must be considered when acquiring
low frequency information. These are:
The discharge time constant characteristic of the sensor (a fixed value unique
to each sensor)
The time constant of the coupling circuit used in the signal conditioner. (If
DC coupling is used, only the above (1) need be considered.)
It is important that both factors be readily understood by the user to avoid
Transducer Discharge Time Constant
The discharge time constant is the more important of the low frequency limits
because it is the one over which the user has no control.
Consider the ICP sensors shown previously in Figure 6. While the sensing element
will vary widely in physical configuration for the various types (and ranges)
of pressure, force, and acceleration sensors, the basic theory of operation is
similar for all. The sensing element, when acted upon by a step function
measureand (pressure, force or acceleration) at t = t0, produces a
quantity of charge, Δq, linearly proportional to this mechanical input.
In quartz ICP sensors, this charge accumulates in the total capacitance, Ctotal,
which includes the capacitance of the sensing element plus amplifier input
capacitance, ranging capacitor and any additional stray capacitance. (Note: A
ranging capacitor, which would be in parallel with the resistor, is used to
reduce the voltage sensitivity and is not shown.) The result is a voltage
according to the law of electrostatics: ΔV=Δq/Ctotal. This voltage is
then amplified by a MOSFET voltage amplifier to determine the final sensitivity
of the sensor. From this equation, the smaller the capacitance the larger the
voltage sensitivity. While this is true, there is a practical limit where a
lower capacitance will not significantly increase the signal to noise ratio.
In ceramic ICP sensors, the charge from the crystal is typically used directly
by an integrated charge amplifier. In this case, only the feedback capacitor
(located between the input and output of the amplifier) determines the voltage
output, and consequently the sensitivity of the sensor.
While the principle of operation is slightly different for quartz and ceramic
sensors, the schematics (Figure 6) indicate that both types of sensors are
essentially resistor-capacitor (RC) circuits.
After a step input, the charge immediately begins dissipating through resistor
(R) and follows the basic RC discharge curve of equation:
q = Qe-t/RC (Eq. 8)
Where: q = instantaneous charge (pC)
Q = initial quantity of charge (pC)
R = bias (or feedback) resistor value (ohms)
C = total (or feedback) capacitance (pF)
t = any time after t0 (sec)
e = base of natural log (2.718)
This equation is graphically illustrated in Figure 14. Note that the output
voltage signal from an ICP sensor will not be zero-based as shown below, but
rather based on an 8 to 10 VDC amplifier bias.
Figure 14: Charactoristic Discharge Curve
The product of R times C is the discharge time constant (DTC) of the sensor (in
seconds) and is specified in the calibration information supplied with each ICP
sensor. Since the capacitance fixes the gain and is constant for a particular
sensor, the resistor is used to set the time constant. Typical values for a
discharge time constant range from less than one second to up to 2000 seconds.
Effect of DTC on Low Frequency Response
The discharge time constant of an ICP sensor establishes the low frequency
response analogous to the action of a first order high-pass RC fitter as shown
in Figure 15A. Figure 15B is a Bode plot of the low-frequency response.
Figure 15: Transfer Charactoristics of an ICP® Sensor
This filtering characteristic is useful for draining off low frequency signals
generated by thermal effects on the transduction mechanism. If allowed to pass,
this could cause drifting, or in severe cases, saturate the amplifier.
The theoretical lower corner or frequency (f0), is determined by the
following relationships, where DTC equals the sensor discharge time constant in
seconds. See Table 1.
3dB down: f0 = 0.16/(DTC) (Eq. 9)
10% down: f0=0.34/(DTC) (Eq. 10)
5% down: to 0.5 (DTC) (Eq. 11)
DTC (sec) Frequency (Hz)
-5% -10% -3dB
.1 5 3.4 1.6
.5 1 .68 .32
1 .5 .34 .16
5 .1 .07 .03
10 .05 .03 .016
[Table 1 Low Frequency Response Table]
Effect of DTC on Long Duration Time Waveforms
Often it is desirable to measure step functions or square waves of various
measureands lasting several per cent of the sensor time constant, especially
when statically calibrating pressure and force sensors.
The following is an important guide to this type of measurement: the amount of
output signal lost and the elapsed time as a percent of the DTC, have a
one-to-one correspondence up to approximately 10% of the DTC. Figure 16 shows
the output voltage vs. time with a square wave input. (For accurate readings,
DC couple the signal conditioner.)
Figure 16: Step Function Response
At time t = t0 a step measureand (psi or lbs.) is applied to the
sensor and allowed to remain for 1% of the DTC at which time it is abruptly
removed. The output voltage change DV, corresponding to this input is
immediately added to the sensor bias voltage and begins to discharge at t = t0.
When t = t0 + (.01 DTC), the signal level has decreased by 1% of
ΔV. This relationship is linear to only approximately 10% of the DTC. (i.e.
If the measureand is removed at t = 0.1 DTC, the output signal will have
discharged by approximately 10% ΔV.)
After 1 DTC, 63% of the signal will have discharged, After 5 DTC's, the output
signal has essentially discharged and only the sensor bias voltage level
Upon removal of the measureand the output signal will dip below the sensor bias
voltage by the same amount that it has discharged. Then, it will charge toward
the sensor bias voltage level until reaching a steady state.
For a minimum 1% measurement accuracy, the discharge time constant should be at
least 100 times the duration of a square wave event, 50 times the duration of a
ramp and 25 times the duration for a half sine wave. Longer time constants will
improve measurement accuracy.
Effect of Coupling on Low-Frequency Response
As previously mentioned, if the constant current signal conditioner (shown in
Figure 5) is DC coupled, the low frequency response of the system is determined
only by the sensor DTC. However, since many signal conditioners are AC coupled,
the total coupling DTC may be the limiting factor for low frequency
For example, Figure7 illustrated typical AC coupling through a 10 µF
coupling capacitor (built into many constant current signal conditioners.)
Assuming a 1 Megohm input impedance on the readout instrument (not shown), the
coupling time constant simply equals R x C or 10 seconds. (This also assumes a
sensor output impedance of <100 ohms.) As a general rule, keep the coupling
time constant at least 10 times larger than the sensor time constant.
When acquiring low frequency measurements, low input impedance tape recorders
and other instruments will reduce the coupling time constant significantly. For
such cases, use a signal conditioner which incorporates DC coupling or a
METHODS OF DC-COUPLING
To take full advantage of the sensor DTC, especially during static calibration,
it is often essential to DC couple the output signal. The simplest method is to
use a signal conditioner which incorporates a DC coupling switch. However,
standard signal conditioners may also be adapted for DC coupling by using a "T"
connector as in Figure 17.
Figure 17: Adapted DC Coupled Sensing System
The important thing to keep in mind is that the readout instrument must have a
zero offset capability to remove the sensor bias voltage. If the readout is
unable to remove all or a portion of the bias voltage, a "bucking" battery or
variable DC power supply placed in-line with the signal may be used to
accomplish this task.
For convenience, several of the constant current signal conditioners
manufactured by PCB incorporate level shifting circuits to allow DC-coupling
with zero volts output bias. Most of these units also features an AC coupling
mode for drift-free dynamic operation.
These precautionary measures should be followed to reduce the risk of damage or
failure in ICP sensors:
Do not apply more than 20 mA current through ICP sensors or in-line amplifiers.
Do not exceed 30 volts supply voltage.
Do not apply voltage without constant current protection. Constant current is
required for proper operation of ICP sensors.
Do not subject standard ICP sensors to temperatures above 250°F (121°C).
Consult a PCB Applicitions Engineer to discuss testing requirements in higher
Most ICP sensors have an all welded hermetic housing. However, due to certain
design parameters, certain models are epoxy sealed. In such cases, high
humidity or moist environments may contaminate the internal electronics. In
such cases, bake the sensors at 250°F (121°C) for 1 or 2 hours to evaporate any
Many ICP sensors are not shock protected. For this reason, care must be taken
to ensure the amplifier is not damaged due to high mechanical shocks. Do not
exceed the maximum shock limit indicated on the specification sheet.