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