Signal conditioning of piezoelectric ultrasonic sensors

Piezoelectric sensor

The application of piezoelectric sensors for sensing and excitation extends to many areas. This article mainly introduces the induction of some physical intensities, namely acceleration, vibration, oscillation and pressure, which can be considered similar from the perspective of the sensor and its required signal adjustment. In terms of acceleration, the ultrasonic sensor sensitivity is usually expressed as a charge proportional to the external force or acceleration (called acceleration due to gravity most of the time). However, in a strict physical sense, the ultrasonic sensor outputs a charge that is actually determined by its deformation/deflection. Showing a piezo ceramic sensor installed at the top position, while the bottom is being pulled by an external force. In the case of using an accelerometer, the fixed end (top) will stick to the object to be measured acceleration, and at the same time The external force is the inertia of the mass adhering to the other end (bottom), and this end constantly wants to remain still. As far as the reference coordinate system is fixed at the top (assuming that the sensor acts as a spring with a high spring constant K), the deflection x will form a reaction force:

Fint = Kx (1)

Ultimately, the mass (ultrasonic sensor ) will stop moving/changing under the following conditions:

Fint = Fext = Kx (2)

Since charge Q is proportional to deflection (first order), and deflection is proportional to force, Q is also proportional to force. A sinusoidal force with a maximum value of Fmax will form a sinusoidal charge with a maximum value of Qmax. In other words, when the sine force is at its maximum value, the current from the sensor can be integrated to get Qmax. Increasing the frequency of the sine wave will increase the current; but it will reach the peak faster, that is, keeping the integral (Qmax) constant. frequency range of the ultrasonic sensor to specify the sensitivity specification. However, due to the mechanical properties of the sensor, the sensor actually has a resonant frequency (above the usable frequency range), and even a small oscillation force will produce a relatively large deflection, resulting in a large output amplitude.If we ignore the effect of resonance, we can model the piezoelectric sensor as a current source in parallel with the parasitic capacitance of the sensor (herein referred to as Cd), or model it as a voltage source in series with Cd . This voltage is the equivalent voltage seen on the anode of the sensor when storing charge. However, we need to pay attention to the fact that the second method is simpler in terms of simulation of many applications. As mentioned earlier, the current is proportional to the rate of skew change; for a sinusoidal AC curve with constant amplitude acceleration, the amplitude of the current generator must be changed according to frequency.

Finally, if such a generator needs to represent the actual physical signal, a transformer can be used. In this example, we modeled a generator with 0.5pC/g sensitivity and 500pF parasitic capacitance. The sine wave generator outputs 1V per unit g to realize simulation. The transformer adjusts it down to 1mV in its secondary coil. The 1mV swing will be injected in the next stage as we expected Q = VC = 0.5 pC.

Charge amplifier analysis

Showing the basic principle of a classic charge amplifier, which can be used as a signal conditioning circuit. In this case, we choose the current source model, indicating that the ultrasonic sensor is mainly a device with high output impedance.

input resistance

The signal conditioning circuit of piezo ceramic disc transducer

must have a non-low input impedance to collect most of the charge output of the sensor. Therefore, a charge amplifier is an ideal solution, because as long as the amplifier maintains a high gain at these signal frequencies, its input will cause the sensor signal to appear virtual ground. In other words, if any charge of the sensor wants to increase on the sensor anode (Cd) or amplifier input parasitic capacitance (Ca), a voltage will be formed at the amplifier input. By pulling or drawing the same amount of negative feedback network charge current, namely RFB and CFB, this voltage is immediately compensated.

Gain

Because the signal input of the amplifier is virtual ground, the input current forms a kind of output voltage swing; and the high-frequency gain is set by the value of CFB (RFB influence is reduced, which will be described later in the "bandwidth" part). Please note that the smaller the capacitance.Also note that the circuit gain does not depend on the capacitance (Cd) of the ultrasonic sensor at all, but it is better to pay attention to the effect of this value on noise.

bandwidth

In order to be able to bias the amplifier correctly (provide a DC path for the amplifier input bias current), a feedback resistor (Rf) is required. At lower frequencies, the capacitive circuit of the feedback path becomes an open circuit, and the feedback resistance becomes the main resistance, thereby effectively reducing the gain. At higher frequencies, the impedance of the capacitor circuit becomes smaller, thereby effectively eliminating the influence of the resistance feedback path. The final circuit response to AC physical excitation (including the parasitic capacitance of the sensor) is the response of the high-pass filter.

The relevant signal bandwidth is determined by the application. Therefore, while reducing the capacitance to increase the gain, it is also necessary to increase the resistance to keep the pole frequency low. Adding resistance will affect other aspects of the solution. In addition to affecting noise (described in detail in the "noise" section), the higher the resistance, the more difficult it is to actually implement it-it is difficult to find a ready-made resistance and to ensure that the PCB trace to trace parasitic resistance is greater than the RFB itself. If the circuit specifications allow the use of resistors of , the surface mount resistors can be used immediately, and advanced layout techniques (such as the use of guard bands, etc.) are not required.

As mentioned earlier, another factor limiting of piezo ceramic cylinder increases in resistance is circuit bias. The input bias current of the amplifier forms an output bias voltage through the resistor. This voltage can be minimized by choosing amplifiers with low input bias currents, such as FET input amplifiers. As long as the feedback resistor value is less than 1GΩ, and the AC coupling between the stages can be used to filter the generated offset, then the input bias current of this amplifier (generally less than 100pA) should be no problem.

Please note that due to the difficulty of keeping the high-pass filter low frequency, it is becoming more and more difficult to use piezoelectric sensors in near DC applications (although the leakage current of the sensor itself is very small).

Although it is not part of the amplification stage, a low-pass filter needs to be added somewhere to reduce the circuit's response to unwanted signals at the ultrsonic  sensor's resonance frequency, while reducing the total digitization and aliasing noise in the relevant frequency band.