| | | | | | | | | | | | | | | | | | | | | | | | | | | |
| | | |
| | | Basics of Piezoelectric Measuring Technology | | |
| | | |
| | | | | |
| | | |
| | | Charge amplifiers convert the charge output by a piezoelectric sensor into a proportional voltage, which is used as an input variable for analysis systems, and can be digitized in an analog-to-digital (A/D) converter if necessary. A charge amplifier basically consists of an inverting voltage amplifier with a high open-loop gain and capacitive negative feedback, and a metal oxidesemiconductor field effect transistor (MOSFET) or a junction field effect transistor (JFET) at its input to achieve high insulation resistance and minimize leakage current. Neglecting Rt and Ri, the output voltage is given by: | | |
| | | |
| | | i+ AJCr(ct +Cr +Cc) | | |
| | | Schematic diagram of a measuring chain | | |
| | | |
| | | |
| | | Time constant and drift Two of the important practical properties of charge amplifiers are the time constant and drift. The time constant t is defined as the time required for a capacitor to discharge to 1/e (37%) of its original value. The time constant of a charge amplifier is the product of the capacitance of the range capacitor Cr and the time constant resistance Rt: | | additional time constant resistor connected in parallel, the charge amplifier will only drift very slowly towards the negative or positive limit (MOSFET: <±0,03 pC/s, JFET: <±0,3 pC/s). This drift determines the permissible duration of quasistatic measurement and is independent of the selected measuring range. Frequency and time domain The time constant affects the time domain as well as the frequency range. It determines the lower cut-off frequency fu = 1/2 7t-t at an amplitude attenuation for sinusoidal signals of 3 dB (30%). The longer the time constant, the lower this cut-off frequency and the longer the usable measuring time. For quasistatic measurement the longest possible time constant is therefore always selected. | | |
| | | will approach zero. The cable and sensor capacitance can therefore be neglected, leaving the output voltage dependent on just the charge at the charge amplifier input and the range capacitor. U - — | | |
| | | |
| | | |
| | | The amplifier acts as an integrator that constantly compensates the sensor's electric charge with one of equal magnitude and opposite polarity of the range capacitor. The voltage across this capacitor is proportional to the charge generated by the sensor and to the acting measurand. In effect, the charge amplifier converts an electric input charge Q into a usable proportional output voltage Uo. As most Kistler charge amplifiers allow adjustment of sensor sensitivity and measuring range, the measure ment is displayed in the mechanical units of the measurand and the output signal as an integer multiple of the measured variable. | | |
| | | T = Rt*C | | |
| | | |
| | | Drift is defined as an undesirable change in the output signal over a long period of time that is not a function of the measu-rand. Even the best MOSFET and JFETs cause leakage currents (MOSFET: Ii<10 fA, JFET: l<100 fA), which are the main cause of drift. If the input insulation resistance Ri is too low, it can cause additional drift. However, as long as the input insulation resistance in the negative feedback circuit is sufficiently high (>1013 Q) and there is no | | |
| | | |
| | | |
| | | |
| | | www.kistler.com | | |
| | | 11 | | |
| | | |
| | | |
| | | | | | | | | | | | | | | | | | | | | | | | | | | |