Barry & Chris -- Piezo devices are often used with charge amplifiers, which is just an op-amp with capacitive feedback and a parallel resistor to keep it from drifting off scale. When the input is connected to the Piezo device (a capacitor) the op-amp circuit acts as an amplifier with gain of CZ/C1. The low-frequency limit is given by the time constant of the resistor and C1. Op-amps with very low input bias and offset currents must be used. This circuit provides a low-impedance output to drive other circuitry cabling, etc. With some of today's op-amps having input currents in the picoamp range, this type of thing is quite viable. Use teflon standoffs for the connections at the inverting input of the op-amp. And any cabling used between the piezo device and the circuit should be made with wire having insulation with very low dielectric absorption. Karl +--------------\/\/\/\------------+ | | +----------------||---------------+ | C1 | | |\ | | | \ | | | \ | | | \ | +-------------| - \ | | | \ | | | \ | Piezo --- | >------+------------O CZ --- | / | | / | +-----| + / | | | / | | | / --- --- |/ \ / \ / V V _____________________________________________________________________ Public Seismic Network Mailing List (PSN-L)
Larry Cochrane <cochrane@..............>