Manfred Weber 

Metra Mess- und Frequenztechnik in Radebeul e.K. 

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Transducers with Charge Output

1. Charge Amplifiers

Accelerometers with charge output generate an output signal in the range of some picocoulombs (1 pC = 1000 fC) with a very high impedance. To process this signal by standard AC measuring equipment it needs to be transformed into a low impedance voltage signal.

Preferably, charge amplifiers are used for this purpose. The input stage of a charge amplifier features a capacitive feedback circuit which balances the effect of the applied charge input signal. The feedback signal is then a measure of input charge. Figure 10 shows a typical charge input stage.

Figure 10: Charge amplifier

The input charge q_in is applied to the summing point (inverting input) of the amplifier. It is distributed to the cable capacitance C_c, the amplifier input capacitance C_inp and the feedback capacitor C_f. The node equation of the input is therefore:

Using the electrostatic equation:

and substituting q_c, q_inp and q_f :

Since the voltage difference between the inverting and the non-inverting input of a differential amplifier becomes zero under normal operating conditions, we can assume that the input voltage of the charge amplifier u_inp will be equal to GND potential. With u_inp = 0 we may simplify the equation:

and solving for the output voltage u_out:

The result shows clearly that the output voltage of a charge amplifier depends only on the charge input and the feedback capacitance. Input and cable capacitances have no influence on the output signal. This is a significant fact when measuring with different cable lengths and types.

Referring to Figure 10, the feedback resistor R_f has the function to provide DC stability to the circuit and to define the lower frequency limit of the amplifier. The circuit in Figure 10 represents only the input stage of a charge amplifier. Other stages like voltage amplifiers, buffers filters and integrators are not shown.

Typical charge amplifiers are, for example, the M68 series Signal Conditioners and the ICP100 series Remote Charge Converters made by Metra.

2. High-Impedance Voltage Amplifiers

Instead of charge amplifiers, high impedance voltage amplifiers can be used with charge mode transducers. In this case, however, the capacitances of sensor, cable, and amplifier input must be considered (Figure 11).

Figure 11: Charge mode sensor at voltage amplifier

The voltage sensitivity of an accelerometer with known charge sensitivity B_qa and inner capacitance C_i is calculated to:

B_qa and C_i can be found in the sensor data sheet.

Taking into account the capacitance of the sensor cable C_c and the input capacitance C_inp of the voltage amplifier, the resulting voltage sensitivity B´_ua will become lower than B_ua:

A typical 1.5 m low noise cable Model 009 has a capacitance of approximately 135 pF.

The lower frequency limit f_l will also be influenced by C_c, C_inp and R_inp:

The lower frequency limit increases with decreasing input resistance.

Example: A charge mode accelerometer Model KS50 with inner capacitance C_i = 1.4 nF is connected to a typical scope input with R_inp = 10 MOhms and C_inp = 20 pF. The sensor cable has a capacitance of 135 pF.

Result: The lower frequency limit will be at about 10 Hz.

IEPE Compatible Accelerometers

A special feature of IEPE compatible transducers is that power supply and measuring signal are transmitted via the same cable. So, an IEPE compatible transducer requires, like a transducer with charge output, only one single-ended shielded cable.

Figure 12 shows the principle circuit diagram.

Figure 12: IEPE principle

The integrated sensor electronics is powered with constant current in the range between 2 and 20 mA. A typical value is 4 mA. Some battery powered instruments even work at 1 mA. The constant current I_const is fed into the signal cable of the sensor. The supply current and the length of the cable may influence the upper frequency limit.

The de-coupling capacitor C_c keeps DC components away from the signal conditioning circuit. The combination of C_c and R_inp acts as a high pass filter. Its time constant should be sufficiently high to let all relevant low frequency components of the sensor signal pass.

Important:

• Under no circumstances a voltage source without constant current regulation should be applied to an IEPE compatible transducer.

• False polarization of the sensor cable may immediately destroy the built-in electronics.

Figure 13: Dynamic range of IEPE compatible accelerometers

In Figure 13 can be seen that IEPE compatible transducers provide an intrinsic self-test feature. By means of the bias voltage at the input of the instrument the following operating conditions can be detected:

• U_BIAS < 0.5 to 1 V:     short-circuit or negative overload

• 1 V < U_BIAS < 18 V: O.K., output within the proper range

• U_BIAS > 18 V:             positive overload or input open (cable broken or not connected)

A variety of instruments are equipped with a constant current sensor supply. Examples from Metra are the Signal Conditioners of M68 series, M208 and M32, the Vibration Monitor M12 or the Vibration Calibrating System VC110. The constant current source may also be a separate unit, for example Model M28.

Intelligent Accelerometers to IEEE 1451.4

The standard IEEE 1451, discussed in recent time, complies with the increasing importance of digital data acquisition systems. IEEE 1451 mainly defines the protocol and network structure for sensors with fully digital output. The part IEEE 1451.4, however, deals with "Mixed Mode Sensors", which have a conventional IEPE compatible output, but contain in addition a memory for an "Electronic Data Sheet". This data storage is named "TEDS" (Transducer Electronic Data Sheet). The memory of 256 bits contains all important technical data which are of interest for the user:

• Model and version number

• Serial number

• Manufacturer

• Type of transducer, physical quantity

• Sensitivity

• Last calibration date

In addition to this data, programmed by the manufacturer, the user for itself can store information for identification of the measuring point.

• The Transducer Electronic Data Sheet provides several advantages:

• When measuring at many measuring points it will make it easier to identify the different sensors as belonging to a particular input. It is not necessary to mark and track the cable, which takes up a great deal of time.

• The measuring system reads the calibration data automatically. Till now it was necessary to have a data base with the technical specification of the used transducers, like serial number, measured quantity, sensitivity etc.

• The sensor self-identification allows to change a transducer with a minimum of time and work ("Plug & Play").

• The data sheet of a transducer is a document which often gets lost. The so called TEDS sensor contains all necessary technical specification. Therefore, you are able to execute the measurement, even if the data sheet is just not at hand.

• The standard IEEE 1451.4 is based on the IEPE standard. Therefore, TEDS transducers can be used like common IEPE transducers.

Figure 14 shows the principle of TEDS. If a constant current source is applied, the sensor will act like a normal IEPE compatible sensor. Programming and reading the built-in non-volatile 256 Bit memory DS2430 is also done via the sensor cable. The communication uses Dallas Semiconductor’s 1-Wire^® protocol (www.ibutton.com). For data exchange TTL level with negative polarity is used. This makes it possible to separate analog and digital signals inside the sensor by two simple diodes. 

Figure 14: Accelerometer with TEDS to IEEE 1451.4

The 8 Channel IEPE Conditioner M208 of Metra provides TEDS support with automatic transducer sensitivity normalization.

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Last update: 05.02.10  Jan Burgemeister
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