When the high output impedance of the piezoelectric film is required, a low leakage and high impedance buffer circuit is needed. For example, infrared mobile sensor and accelerometer application, you need to input resistance up to 50G, to get a very low frequency response. At this time, the input impedance of the buffer circuit should be much higher than the output resistance of the piezoelectric film to maintain a very low frequency response. In addition, it is also important to minimize the leakage current of the buffer circuit in order to achieve the highest measurement accuracy. Examples of low leakage buffer circuits include: JFET~4117 (Siliconix, Sprague); operational amplifiers LMC660, LF353 (National semiconductors), OP80 (PMI), and 2201 (Texas instrument company).

Figure 40 shows an example of a universal single gain buffer circuit.

Operational amplifiers used as buffer circuits and amplifiers are very kind

Many. Can be used as the charge type side can also be used for voltage type talent

Amplifier. Figure 41 gives the basic charge and voltage amplifier.

Configuration diagram. The voltage output of the charge amplifier is determined by the Q/Cf.

The Q in the type is the charge on the piezoelectric film, and the Cf is charged.

The feedback capacitance of the large device.

The output voltage of a charge amplifier is determined by a feedback capacitance.

Not the input capacitance. This shows that the output voltage of the charge amplifier is independent of the capacitance of the cable. The main advantages of a charge amplifier are when a long cable is used between a piezoelectric film sensor and an electronic circuit. In addition, it also greatly reduces the charge leakage caused by the parasitic capacitance around the sensor. In addition, a simple voltage amplifier is sufficient to meet most of the applications. The circuit in Figure 41 is a typical non - transformation voltage amplifier.

The advantages of a voltage amplifier are reflected when the ambient temperature must be considered. The change of the voltage sensitivity (g constant) at different temperatures is less than the change of the charge sensitivity (d constant). Therefore, the voltage amplifier with the piezoelectric film is less affected by the temperature. In Figure 41, the time constants of the charge amplifier and the voltage amplifier are determined by RCf and RC, respectively.

As an example of a design, a traffic sensor interface circuit is described. Due to its soft characteristics, the piezoelectric cable is an ideal sensor material for the application of traffic measurement. MSI's BL traffic sensor is composed of a piezoelectric cable that is compressed with a copper sheath and is fitted with a signal cable of different length according to the installation requirements. Its induction length is more than 3 meters. In this case, the BL sensor is 2 meters long. The electrically shielded sensor has a 100 foot coaxial cable. The electrical parameters of the sensor include:

Capacitance =9.5nF (including piezoelectric and signal cable capacitance)

Output =500mV (for a 800 LB wheel at 55mph and 70 F

Load)

Signal to noise ratio =10:1

The basic requirement of the interface circuit is: low end frequency =1.6Hz

Circuit output = digital pulse count

The interface circuit to meet these requirements is shown in Figure 42. This is the circuit.

A comparison circuit. As the cut-off frequency is reduced to 1Hz, with 10M

Input impedance. The actual cut-off frequency of this resistor can be calculated by 1.6Hz.

With a 10M potentiometer to adjust the threshold voltage of V diodes for protection

The element is protected from high pressure. A small passenger car running at 55mph speed

The output signals of the piezoelectric and interface circuits are shown in Figure 42.

Signal sorting

Because the piezoelectric film has the piezoelectric effect and the thermoelectric effect, it is necessary to have the piezoelectric effect.

Preventive measures remove or reduce the effect of no signal. Signal sorting

The basic principles include:

Filter - an electronic filter with the bandpass and band resistance characteristics is given.

Average - if the desired signal is periodic and unwanted signal

It is random, and the single mean value can increase the signal to noise ratio.

Common mode inhibition -

Basic switching circuit

A variety of circuits can be used for the interface of a piezoelectric film, including a field effect transistor

(FET), great operation (OpAmps) and low current digital logic circuit (CMOS).

Since FET has been used in surface mounting technology, it can be used in small size applications

Worry about using FET. The important characteristic that should be considered when using FET is the switching frequency

Rate, piezo film capacitance, FET off state leakage current, input bias resistance

Resistance and electromagnetic interference (EMI) shielding.

Figures 43 and 44 are the FET circuit diagrams of a typical piezoelectric film switch.

Figure 43, the common drain or source follower is very well used in the application.

A simple buffer is important in application. Here, the voltage gain of the circuit

It's about 1.

The low frequency of the common source pole path in Figure 44 for the voltage gain

The application is appropriate. The gain is determined by the impedance Rd and the Rs. When gain increases

At the time of addition, the frequency bandwidth is reduced by the coefficient of each 20dB gain / ten octave.

The operational amplifier provides a great convenience for the application of the piezoelectric film switch.

For a specific application, it is easy to match it. Important operational amplifier

The circuit features include input bias impedance, film switching capacitor and EMI shielding.

The operational amplifier circuit of figure 45, a charge amplifier, is suitable for one.

The application of the measured vibration trigger switch. Also work in small signal applications

It's done very well. A charge amplifier eliminates the piezoelectric and connecting cables.

Time constant effect. A charge amplifier is a zero input impedance of the current

The result is that there is no voltage on the two sides of the membrane. Charge amplifier fast

The charge produced by the rapid absorption of the film. Because there is no charge on the film electrode,

The film shows no time constant.

The capacitance of the film and the connection point cable does not reverse the transmission function of the circuit.

Effect? Therefore, the tolerance control of the film size and cable length should not be strictly controlled.

Lattice. The capacitance that the charge is transported from the film to the amplifier feedback loop, the capacitor

Determine the output voltage: V=Q/Cf.
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