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Fylde Electronic Laboratories Ltd: Using Head Amplifiers with Piezo-Electric Transducers.


1. Introduction

Charge amplifiers have traditionally been employed to interface transducers which output a charge level in proportion to mechanical stimulus. Such transducers are known as Piezo-Electric and include devices to measure acceleration, pressure, force and sound. In recent years, due to improvements in micro-electronics, a challenge has emerged in the form of transducers with inbuilt charge to voltage conversion. Such devices are generally powered by utilisation of the same (coaxial) cable which conducts the signal to the receiving instrumentation. So advantageous is the technique, being low cost in both cabling and receiving electronics, that many manufacturers offer a complete range of transducers operating on the constant current, Integrated Circuit Piezo (ICP®) principle.

2. Schematic of an Amplifier System

The receiving amplifier, which also contains the power supply to the transducer electronics, is connected via a coaxial cable. Due to the conversion at source of the signal to voltage, the cable type is no longer critical and low cost cables can be substituted for the low noise types normally essential in charge amplifier applications. Power, in the form of a constant current, is carried by the same cable to the transducer. The returned signal is AC coupled into the receiving amplifier which blocks the power supply current.

Because the signal carried by the cable is in voltage form, and because the receiving amplifier presents a high input impedance to the cable, long cables may be employed with minimal detriment to signal integrity, particularly when bandwidth requirements are modest.

3. Mode of Operation

The output of the transducer electronics is biased to a voltage which is approximately half of the power supply capability. As this is typically 20-24VDC , the bias level is often in the region of 10-12VDC.

When the transducer develops a dynamic signal, this is superimposed on the bias level to give an AC voltage which may range from a few milli-volts to several volts in amplitude. The bias level may be monitored by the receiving amplifier in order to provide information on both transducer and cable status.

It should be appreciated that this technique is generally favoured for dynamic signals when AC coupling is acceptable, although receiving electronics with DC performance is available for specialist application.

4. Fylde Receiving Amplifiers

Fylde offers receiving amplifiers in several forms; all have constant current supply for the transducer, high impedance amplification and configurable low pass filter.

In addition, the FE-530-IE and FE-376-IPF are available with a unique input circuit which is able to reject interference which may arise when transducers must be electrically earthed to a structure.

The FE-376-IPF is a Micro Analog 2 module which can be connected by USB to make an acceleration or vibration data acquisistion system. Free FYLDE MADAQ software supplied with all USB systems includes processing of signals to providing the FFT spectrum of the acquired data.

 Blue Panel System  FE530IE  Features LED bias monitoring  Data Sheet
 Micro Analog 2  FE-376-IPF  Low cost dual channel card  
 1800 Series  FE-1817  High integrity dual amplifier for aircraft or automotive apps.  Data Sheet

5. Converting Piezo Transducers

Many applications remain for standard piezo-electric transducers because of :-

 Cost. They may be cheaper than an equivalent integrated electronic type.
 Size. They may be smaller than an equivalent integrated electronic type.
 Environment. They can often withstand a higher operating temperature.

Where it is necessary to instrument these transducers, charge amplifiers are often the first choice. However, the constant current technique may still be used by utilisation of an inline or Head Amplifier. These miniaturised devices are often themselves charge amplifiers which are specially configured to operate on the constant current power supply principle and may be obtained pre-calibrated to exactly convert pico-coulombs to milli-volts.

In this case, the transducer must be connected to the head amplifier using low noise 100% screened charge amplifier cable, and thus it is cost and also performance effective to keep this section of cabling as short as is practicable. Between 1 metre and 5 metres of cable will prove to be sufficient in most installations. These inline amplifiers will often prove to be small enough not to require specific housing, and may be left hanging in the cable harness.

Important points to note here are that the cable type changes either side of the head amplifier, and that calibration is very straightforward when head amplifiers are given exact conversion ratios. For example, figures such as 1 mV/pC and 10mV/pC conversion ratios ensure error free calibration, and importantly, enable amplifiers to be exchanged or swapped at will.

6. Fylde Head Amplifiers

Fylde offers Constant Current Head Amplifiers in several types:-

 True Charge Amplifier  FE-074-HA/C  Pre-calibrated 1mV or 10mV / pC  Data Sheet
 Passive Charge Amplifier  FE-074-HA  0.1mV / pC for high charge levels  
 Buffer Amplifier  FE-665-DIC  Low cost unity gain  

These inline amplifiers are small, rugged and easy to apply. All operate from a constant current supply and will convert a Piezo Electric transducer to voltage output.

7. Notes relating to Cables

7. 1 Cable from Transducer or Head Amp to Receiving Amplifier

The transducer (having built in amplifier), or head amplifier may be required to drive cable lengths up to a hundred metres or more. In these instances, consideration of the drive requirements is worthwhile:-

The cable capacity, coupled with the device output impedance, forms a low pass filter whose cut off frequency may preclude the passage of frequencies of interest according to the formula:

 Fc = 1/2piCR  Where  Fc = Low pass cut off frequency
     C = Cable Capacitance
     R = R1 + R2 = Device output impedance and driver restrictions*

*The device output impedance (R1) is generally designed to match the characteristic impedance of the expected cable type; typically 50 -100. However, the device's cable drive ability is limited by the constant current supplied by the receiving amplifier, and further restricted because only a portion of that current is available to drive the cable, the remainder being consumed within the device electronics. This parameter is described by the formula :-

 R2 = V / I  Where  V = signal pk voltage
      I = constant current (less 1.5mA)

Thus, satisfactory performance of the arrangement depends on : -

 i)  The cable length
 ii)  The peak voltage of the signal
 iii)  The maximum frequency of interest
 iv)  The current available to the transducer or head amplifier
 v)  The output impedance of the transducer or head amplifier

As an example, consider the case of a transducer connected by 100m of miniature RG59 coaxial cable of capacitance 50pF/m. Maximum signal voltage is in the region of 5Vpk and supply current is 4mA

 R2  =  5 / (4-1.5)mA = 2kohm  
 Fc  =  1 / 2pi x 5000pF x (50* + 2k)ohm  *Device o/p impedance
   =  15.5kHz  

Peak voltages at greater than 75kHz may be passed if their amplitudes are restricted to 1V or less.
Of course, many signals are complex and although the fundamental may be relatively low in frequency, inherent harmonics may raise the frequency requirement to many times the fundamental; fortunately, often at greatly reduced amplitudes.

7.2 Input Cables from Piezo Transducers to Head Amplifier

7.2.1 True Charge Head Amplifiers

One of the characteristics of the classic charge amplifier configuration is that it is able to operate using long input cables without calibration error; the only downside being the cost of the special input cables required. Noise performance suffers as cable length increases but typically this is as little as 0.05pC / 10m of additional cable.

Thus where head amplifiers with charge amplifier input stages are in use, cable length will cause little or no change in calibration.

7.2.2 Buffer Head Amplifiers

Caution should be exercised however if buffer head amplifiers are deployed. These simple devices are high impedance voltage buffers and their calibration may be compromised by change of input cable length. Why this should be is explained in the following:-

The operation of buffer type devices relies on the fact that the equivalent circuit of a Piezo Electric transducer can be simplified to a voltage generator in series with a capacitor. Transducer capacitance may range in value from a couple of hundred pF to several thousand pF.

Because the buffer amplifier is sensitive to the voltage developed and not strictly the charge output of the transducer, a voltage divider is developed in the ratio of transducer capacitance CT to cable capacitance Cc. Cable capacitance varies with mechanical parameters but a values of 90pF / metre is typical.

 Example:  Transducer Charge Output  =  31.6pC/g
   Transducer Capacitance ( CT)  =  1000pF
   Cable length 2m. (CC)  =  180pF

The voltage output of the transducer may be calculated by using the formula V = Q/C

 For the example above:  V  =  31.6pC / 1000pF
   V  =  31.6 mV / g

When a 2 m cable is added, the voltage seen at the buffer amplifier (v) becomes :-

 v  =  V x CT / (CT + Cc)
 v  =  31.6 x 1000 / 180
 v  =  26.8 mV / g

This desensitisation is not a problem providing that a calculation similar to the above is performed to determine loss of signal level, but beware, transducer manufacturers may regard the voltage output of their devices as being of secondary importance. Also, the capacitance given for the transducer in the data sheet, if given at all, may be a nominal figure. Calculation is fine for estimation of output level, but for highest accuracy the transducer and buffer amplifier system should be through calibrated using a mechanical method such as a vibration table.

It follows from the above that changing the cable length will change the output voltage and cables should only be replaced by ones of similar length and type to avoid having to recalculate / recalibrate.

On the positive side, buffer amplifiers are low cost and very small size, also their simplicity makes for high reliability.

7.2.3 Passive Charge Amplifiers

When charge levels from piezo transducers are high, a passive charge amplifier may be deployed.
Such devices provide a large receiving capacitor which accepts the source charge with minimum error.

In this configuration, the transducer charge is shared between the transducer capacitance, cable capacitance and amplifier input capacitance. Because the amplifier capacitance is made dominant, the voltage (V) developed at the amplifier input is controlled largely by Ci.

V = Q / C Where C = CT + Cc + Ci

These amplifiers are best employed when charge levels exceed 10000pC; such levels may originate in shock measurements or pressure events.

After through calibration, errors can exist when transducer or cable capacity is changed; these can often be kept small however, as the following example explains:-

 Example:  Consider a pressure transducer transducer which outputs 15pC / psi with an expected maximum pressure of 5000psi. Transducer capacity is 1300pF; cable length is 2 metres at 100 pC / m. The input capacitance of the amplifier is 100nF.

   Therefore  CT  =  1300 pF
     Cc  =  200 pF
     Ci  =  100,000 pF
   Input voltage to the amplifier is:-  V  =  Q/C
       =   (15 x 5000pC) / 101500pF
       =  0.739V

A change in the transducer capacitance from 1300pF to 1500pF, or a change in the input cable length from 2m to 4m would result in an error of only 0.2%.



Note: ICP is a registered trademark of PCB Piezotronic Inc.


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