Voltage and Current Monitoring
The QPA200 offers two auxiliary outputs for monitoring or feedback control of piezo load voltage and current. Both signals are buffered by an internal op amp. The voltage output is a replica of the load voltage scaled by a factor of 1/50 while the current output provides instantaneous load current at a rate of 10V/A. These outputs should be connected only to an oscilloscope or other high-impedance device >1MΩ.
The user should be aware that the ground signal (outer BNC conductor) of the piezo output is not the same ground as that of the input signal BNC and monitor output BNCs. This is because current sensing is implemented with a low-side sensing element on the piezo load. In other words, the piezo output ground should not be connected to the same system ground as the monitor grounds, such as with a three-channel oscilloscope. While no damage will occur, current monitoring will be disrupted.
Current Limiting and Bandwidth
Piezoelectric actuators behave as complex, reactive loads with considerable non-linear behavior. However, they can be effectively modeled by a simple capacitor. Like any other amplifier, the drive frequency limitation of the QPA200 is a function of its peak current capability, according to the familiar equation:
Since the current limit Ilim is +/-200mA, the ratio of Ilim to load capacitance C equals the maximum rate of output voltage
change. For a pure sinusoid with frequency f amplitude Vo, the maximum rate or derivative is Vo•2π•f. Therefore, the maximum drive frequency as a function of output amplitude Vout and load capacitance C can be expressed by:
The maximum frequency fmax is the frequency at which the output waveform becomes distorted by current-limiting. Sinusoids, for example, will become linear at zero crossings and begin to appear as triangle waves. The user is alerted to an over-current condition by the illumination of the red LED indicator. Operation is permitted beyond this frequency provided that internal temperature limitations are not exceeded. Figure 2 illustrates fmax as a function of load capacitance for various signal amplitudes.
Figure 2: fmax vs. load capacitance for different signal amplitudes
