Spider DSA User’s Manual
72
Figure 23: AC Input Mode High-Pass Filter Shape
IEPE (ICP)
All Crystal Instruments products support IEPE (Integral Electronic
PiezoElectric) constant current output type input channels. IEPE refers to a
class of transducers that are packaged with built-in voltage amplifiers powered by
a constant current. These circuits are powered by a 4 mA constant current source
at roughly 21 Volts.
IEPE accelerometers are available under several different brands including ICP®
(PCB Piezotronics), Isotron® (Endevco), CCLD or Delta-Tron® (B&K), and
Piezotron® (Kistler).
IEPE sensors are rarely used to measure DC or very low frequency signals. This is
rarely a problem when measuring acceleration in dynamic tests. The IEPE input
mode has the analog high-pass filter enabled with a cutoff of -3dB at 0.3 Hz and -
0.1dB at 0.7 Hz.
Charge Mode
Some sensors provide a high-impedance charge output. Usually, these are high-
sensitivity piezoelectric units that lack a built-in voltage mode amplifier (i.e.
IEPE), allowing them to be used in high-temperature environments. The Spider
front-end module has a built-in charge amplifier that allows the system to read
the output of these sensors.
The charge amplifier in the Spider converts the charge sensor output in
picoCoulombs (pC) to a voltage in millivolts (mA), which is then input to the ADC.
The sensitivity parameter in the software Input Settings sets the composite
sensitivity from sensor output to engineering unit, so this process is transparent
to the user. The analog high-pass filter has a cut-off of -3dB at 0.3 Hz and -0.1dB
at 0.7 Hz.
For the Spider-80X, the input range is 40,000 pC. For the Spider-81 series, there
are two gain settings in the charge channel, one with full range of 10,000pCand
the other 49,000pC. Like that of voltage input mode, the user should try to
maximize the input range of charge signals while making sure no overload will
happen. In other words, choose a charge sensor with higher sensitivity in order to
achieve the best measurement results.
All versions of the Spider-81 are equipped with charge inputs. The charge
amplifier is not available on the version 5.8 and older Spider-81B. It is available
on version 7.x of Spider-81B.
Using an In-Line Charge Converter
An In-Line Charge Converter is an external device designed to convert the high-
impedance source of a charge mode piezoelectric transducer to a low-impedance
voltage signal by means of an ICP ® powered signal conditioner. These units may
be used with either quartz or ceramic charge-mode piezoelectric sensors.
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