PCB Piezotronics 301A11, Installation And Operating Manual

Page: 22 / 77

MANUAL #: 19179
MANUAL REV: B
ECO: 43785
MODEL 482A21 LINE POWERED SIGNAL CONDITIONER
3
When an ICP
®
transducer (or in-line amplifier) is connected to the INPUT jack, the front monitor meter indicates
approximately midscale (green) if the transducer or amplifier is functioning properly. It does not read midscale if the
transducer bias is 3 to 5 volts.
If a cable is faulty (open), or the sensor's built-in amplifier is open, the meter indicates a yellow (full scale) reading.
When a cable or transducer is shorted, the meter indicates zero volts (red).
Immediately after connecting a readout instrument (oscilloscope, meter, or recorder) to the OUTPUT jack, the coupling
capacitor begins charging through the input resistance of the readout instrument. This charging causes an apparent
"drifting" of the output signal until the capacitor is fully charged. The normal amount of leakage through the coupling
capacitor usually results in a <20 mV maximum DC offset.
4.0 Coupling Time Constant, AC Coupled
The coupling time constant (TC) is the product of the coupling capacitor (47 µF) and the input resistance of the readout
instrument, in parallel with the 274 kilohm shunt resistor. Typically, this is ten seconds, assuming the input resistance of
the readout measurement is one megohm.
In most cases, it is desirable to keep the coupling TC long, with respect to the transducer discharge TC, to minimize the
effect of the coupling TC on low frequency response.
4.1 Driving Long Cables
When driving long cables, it may be necessary to increase the constant current drive to the transducers. 482A21 is
normally supplied with the constant current output to the transducer set at 4 mA nominal. This is adequate for most
laboratory and field applications. Special situations, such as driving long cables (beyond 100 ft) with high frequency or
fast rise time pulses, may require increasing the transducer drive current up to 20 mA.
When driving fast rise time pulses over long lines, system performance may be optimized by "tuning" the drive current
to the line; i.e., by finding the best current setting for the particular set of physical parameters established by the
transducer, line length, line termination, pulse rise time, etc.
The optimum current setting is best determined by experimentation with your particular test setup. A good rule of thumb
is to use the lowest current consistent with satisfactory results to minimize transducer self-heating and noise.
Another technique requires the use of impedance matching resistor model 073A connected immediately after the
transducer. This variable resistor is then used to "tune" the current to the line.
One of the best approaches is to test the long line with a signal generator set at a frequency of interest.
4.2 Setting the Constant Current
To set the constant current, locate the current adjust potentiometer on the rear panel. Connect a 0 to 30 mA DC ammeter
(or multimeter) to the INPUT jack (as shown in Figure 3). The constant current value is read directly on the ammeter.
Using a small regular screwdriver, vary the setting of the current adjust pot to set the current at a new level.
CAUTION:
It may be possible to exceed 20 mA slightly. Do not do so; to operate an ICP
®
transducer or amplifier
above 20 mA may damage the equipment.