Signal Conditioning Basics for ICP^® & Charge Output Sensors

Recent developments in state-of-the-art integrated circuit technology have made possible great advances in piezoelectric sensor instrumentation. The intent of this guide is to enhance the usefulness of today's advanced sensor concepts by acquainting the user with the advantages, limitations and basic theory of sensor signal conditioning. This educational guide will deal with the following types of basic sensor instrumentation: 1.) Charge Output Sensors - high output impedance, piezoelectric sensors (without built-in electronics) which typically require external charge or voltage amplifiers for signal conditioning. 2.) 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.)

Historically, nearly all dynamic measurement applications utilized piezoelectric charge output 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 output sensors is their ability to operate under high temperature environments. Certain sensors have the ability to withstand temperatures exceeding +1000 ºF (+538 ºC). However, the output generated by 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 output 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 schematic.

Figure 2: Voltage Mode System Schematic

The open circuit (e.g., cable disconnected) voltage sensitivity V1 (Volts per psi, lb or g) of the charge output sensor can be represented mathematically by Equation 1. V₁ = q / C₁ (Equation 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. V₂ = q / (C₁ + C₂ + C₃) (Equation 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 sensors have frequency limits exceeding 1 MHz, making them 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) which may be an order of 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.

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 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 polycrystailine ceramics, which exhibit a high charge output, are used in such systems. V_out = q / C_f (Equation 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 cabling 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, Inc.) Powered by constant current signal conditioners, the result is an easy-to-operate, low-impedance, two-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 output sensors, including: 1.) Fixed voltage sensitivity independent of cable length or capacitance. 2.) Low output impedance (<100 Ohms) allows signals to transmitted over long cables through harsh environments with virtually no loss in signal quality. 3.) Two wire system accommodates standard low cost coaxial or other two conductor cable. 4.) High quality, voltage output compatible with standard readout, recording or acquisition instruments. 5.) Intrinsic sensor self-test feature by monitoring sensor output bias voltage. 6.) Low per channel cost as sensors require only low cost constant current signals conditioners. 7.) Reduced system maintenance. 8.) 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 a basic piezoelectric transduction mechanism (which has an output proportional to force, pressure acceleration, or strain, depending on the sensor type) coupled to a highly reliable integrated circuit.

Figure 6: Basic Quartz and Ceramic ICP^® Sensors

Two types of integrated circuits are generally used in ICP^® sensors: voltage amplifiers 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 measurand, acting upon the piezoelectric sensing element, produces a quantity of charge referred to as ∆q. This charge collects in the crystal capacitance, C, and forms a voltage according to the law of electrostatics: ∆V = ∆q/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 ∆V 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 quality. 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 sensor's 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 eliminated. A quick comparison of integrated circuit voltage and charge amplifiers is provided below:

Note that the schemata in Figure 6 also contain an additional resistor. In both cases, the resistor is used to set the discharge time constant of the RC (resistor-capacitor) circuit. This will be further explained in the section titled "Transducer Discharge Time Constant".

In-line Charge and Voltage Amplifiers

Certain applications (such as high temperature testing) may require integrated circuits to be removed from the sensor. For this reason, a variety of in-line charge amplifiers and in-line voltage amplifiers are available. Operation is identical to that of an ICP^® sensor, except that the cable connecting the sensor to 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 signal conditioner is shown in Figure 7. All ICP^® sensors require a constant current power source for proper operation. The simplicity and the principle of two-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 diode is used instead 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 identical. 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 voltage level shifter.

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: V_B = Sensor Bias Voltage = 10 VDC V_S1 = Supply Voltage 1 = 24 VDC V_E1 = Excitation Voltage 1 = V_S1 -1 = 23 VDC V_S2 = Supply Voltage 2 = 18 VDC V_E2 = Excitation Voltage 2 = V_S2 -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 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 = V_B-2 (Equation 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 = (V_s - 1) - V_B = V_E - V_B (Equation 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.

The sinusoidal output of piezoelectric accelerometers includes positive and negative components resulting from bidirectional motion imparted into the sensor axis.  For clear understanding, conventions must be set for the motion and for the electrical charge.  Axial motion imparted from the base of the sensor, directed into the sensor is considered to be in the positive direction.  Motion directed away from the sensor base is considered the negative direction.  This motion is transferred to the sensor’s seismic mass which loads the piezoelectric sensing elements providing a high impedance electrical signal known as charge output.  

Charge output accelerometers are typically wired with positive motion resulting in a negative output as they are typically converted to low impedance signals via an inverting op-amp.This convention makes charge sensors negative polarity unless specified otherwise.

ICP^® accelerometers have a built in externally powered microelectronics to convert the high-impedance charge signal into a low-impedance voltage signal directly usable by most modern instrumentation without requiring external charge amplifiers. For this reason ICP sensors are typically positive polarity.

Note about Charge amplifiers - different manufacturers have used both inverting and non-inverting op-amps within their amplifier electronics and it is important to understand which type of amplifier you are using with which type of sensor.PCB inline charge amplifiers are of a negative polarity, designed for use with standard charge sensors resulting in a positive signal to readout devices.

Charge sensors in an inverting design are labeled with a P – prefix to indicate the special positive polarity.Be aware that some custom ‘M’ models include the inverting functionality but do not have the P-prefix.Inverted ICP^® sensors are extremely rare.Refer to the accelerometer datasheet to confirm if the specific model has P options available and confirm the P-prefix in the specific sensor’s model number.

Accelerometer output can be confirmed through use of an oscilloscope (possibly with use of a volt meter for sensors with relatively long time constants), hold the sensor in the palm of your hand, and tap gently on the accelerometer’s base (positive input). Similarly, polarity of ICP^® sensors can be confirmed using a PCB battery operated signal conditioner where the needle will move to the right indicating positive polarity and move to the left indicating negative polarity.

All of this holds true for triaxial sensors which also receive symbols etched into each sensor detailing which direction is positive for each axis (x, y, z).There are specific applications where knowing the specific polarity has a greater impact – predominantly tests with multiple sensors or inputs at multiple locations such as modal analysis. Incorrect polarity, direction or inconsistent sign conventions will result in inaccurate results.