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What is a charge amplifier and why do I need one?
My piezoelectric accelerometer's datasheet says to use it with a charge amplifier. Can't I just connect the accelerometer directly to an oscilloscope?
You could connect it to an oscilloscope and you would get a signal. But Endevco does not recommend this. First, doing so would almost certainly result in a completely uncalibrated measurement. Second, except under the most ideal conditions, such a measurement would be unreliable and unrepeatable. I'll break this down further to explain why, but we have already alluded to the solution: the charge amplifier.
Until the mid-1960s, piezoelectric accelerometers were routinely calibrated using voltage measuring devices, such as a voltmeter or oscilloscope (and in voltage units of mV/g). Because these sensors inherently output charge, however, this voltage calibration had to be stated with the exact load conditions used during calibration, specifically the capacitance of the cable and input impedance of the recording device (such as a voltmeter or oscilloscope).
If different load conditions were used in an actual test, which was very likely, the system was no longer calibrated. The user either had to estimate how far off they were, or use complicated capacitance decade boxes to try to match the calibration lab load conditions. If load conditions changed again due to a change in instrumentation (or accelerometer), the process had to be repeated. The result? Data that is potentially unreliable and unrepeatable.
This all changed with the arrival of the charge amplifier. The charge amplifier performs a charge conversion function by converting charge (in pC units) at the input to the amplifier, to voltage (in mV units) at the output. In fact, the gain of a charge amplifier is stated in units of mV/pC.
Modern charge amplifiers today can be very sophisticated, with highly flexible programmable gains. The significant innovation here, however, is that the gain of the charge amplifier, inherent in its design, is unaffected by capacitance at the amplifier's input (the reason why is beyond the scope of this article). The charge amplifier's gain is set by components inside the amplifier itself (chiefly a feedback capacitor), allowing the measurement system to be calibrated very accurately.
This affords the user quite a bit of flexibility. The user no longer has to worry about the load conditions between the accelerometer and instrumentation. The cable length can be changed easily, with no concern about how it will affect accuracy. Recording devices on the output of the charge amplifier can also be changed easily, as again changing load conditions are no longer a concern. Use of a charge amplifier with piezoelectric accelerometers is considered minimum best practice today.
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