Basic circuit principle

A reasonable design of the interface circuit will play a key role in the optimization of the piezoelectric film sensor. The application of piezoelectric film from toys to military sensor. The interface circuit and the application are closely related. In many applications, a piezoelectric film can be directly connected to an electronic circuit without a special interface. But for applications that require an interface circuit, it is suggested that the following three steps be taken into consideration:

1. in the desired dynamic range, the frequency range and signal are considered.

Range requirements.

2. select the appropriate load resistance to ensure the low end working frequency and

The signal loss caused by the load effect is minimal.

3. if the signal level is very low, a buffer circuit is selected. If needed

To a high value of load resistance (such as 22M or above), select a

A buffer amplifier with low leakage and high impedance. Market is sold for buffering

The JFET or CMOS operational amplifier of the circuit.

Simplified equivalent circuit

The first step in the design of the interface circuit is the characteristics of the piezoelectric film

It is understood as part of the equivalent circuit. Figure 34 shows the piezoelectric film

A simplified equivalent circuit. It is connected by a voltage source with a phase.

The capacitance of the piezoelectric film is expressed in series capacitance Cf, and it is with the film.

The dielectric constant is proportional to the area, but inversely proportional to the thickness of the membrane. Voltage

The amplitude of the source is equal to the open circuit voltage of the piezoelectric film, according to the size of the excitation.

The change is different, ranging from a few microvolts to more than 100 volts. equivalent circuit

It is suitable for most applications, but for an ultrasonic vibration at high frequency.

When the little significance.

Figure 35 gives an equivalent circuit for a charge generator.

The film capacitance Cf and internal resistance Rf are given in the equivalent circuit. As before

The induction charge Q is linear to the added force, and the capacitance is Cf

It is proportional to the surface area of the membrane, but inversely proportional to the thickness of the membrane. When applied to

In the case of low frequency, the internal resistance Rf is very high and can be ignored. open circuit

The output voltage can be obtained by the membrane capacitance, that is, V = Q / Cf.

Input resistance

The key part of the interface circuit is the input resistance. The input resistance affects the ability of low frequency measurement and the amplitude of the signal. This is called the "load effect".

The piezoelectric film capacitance can be considered as the equivalent source impedance. The source impedance will increase with the decrease of the membrane capacitance and the decrease of the working frequency. It is important to note that this point is important. The source impedance is combined with the input resistance to form a divider. With the decrease of the ratio of the input resistance to the source impedance, the total output voltage decreases. Therefore, selecting the appropriate input resistance for the electronic interface is the key to minimize the load effect.

time constant

In addition to the input resistance, the input capacitance of the interface circuit is the same.

The impact on the output. The equivalent circuit of a piezoelectric film is given in Figure 36.

The input resistance is Ri, and the input capacitance is Ci. Typical time of a piezoelectric film

The domain response is shown in Figure 37. The charge produced after the addition is Ri (Cf+