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Selecting a Pearson Current Monitor.
Obtain a copy of the specification sheet that
lists the standard and special order current
monitors. These monitors are able to measure
pulses, transients and continuous signals. They
do not measure steady-state dc current.
The first four columns of the specification
sheet give model number, output sensitivity
and physical dimension information. The next
four columns list the specs which apply to
measuring pulses, and are referred to as time
domain parameters. The final four columns list
the specs which apply to measuring continuous
sine-wave currents and are referred to as
frequency domain parameters.
Determine if the application is to measure
a pulse or continuous signal. If the signal is
more complex, and rms, peak, dc or other
limiting values are hard to determine, refer to
Pearson Electronics Application Notes, or call
our sales office.
Pulse Signals
If single pulse, determine the approximate
maximum peak current (amps) and the
approximate maximum pulse length (seconds).
Compute the current-time product (amp-seconds).
To use a given model, both the maximum current
and the current-time product should not exceed
the spec sheet values.
If repetitive pulses are to be measured, consult
the Pearson Application Notes to be sure that the
average dc level (zero frequency component) does
not exceed the maximum allowable for that model.
If the rise-time of the signal pulse is known, it
should be compared to the spec sheet value. The
monitor should not be used for pulses that have a
rise-time shorter than the value listed.
For a rectangular pulse the deviation from a
perfectly flat top is given by the droop rate.
Multiplying the pulse length (in microseconds) by
the droop rate will yield the percentage deviation
from the flat-top value at the end of the pulse.
Continuous Signals
Determine the approximate maximum sine-wave
amplitude, I (amps), and the approximate
minimum frequency, f (Hz).
Compute I/f (amps/Hz). To use a given model,
this should not exceed the spec sheet value.
Compute the maximum rms current. To use a
given model, this should not exceed the spec
sheet value.
Determine if the frequency over which the
monitor is to be used is within the range of the
low and high 3 dB points. The accuracy of the
monitor will decrease outside of this range.
If there are several monitors that satisfy the
above criteria, a selection based on voltage
output (sensitivity) and size can now be made. In
the third column is given the output volts per
primary amp to be measured. For example, a
1 milliamp signal will produce a 1 millivolt output
for a monitor that has a 1 volt/amp output.
An often asked question concerns
terminating the output of the monitor with a
resistor. Since the monitor can be modeled as a
voltage source in series with 50 Ohms, the
addition of an external terminating resistance will
decrease the output of the unit. For example, a
50 Ohm external termination would reduce the
output to one-half.
To return to the FAQs & Applications Main Menu, click here...
Further information available from
Alrad Electronics, a trading division of Alrad Instruments Ltd.
Alder House, Turnpike Road Industrial Estate Newbury, Berkshire RG14 2NS
Telephone: +44(0) 1635 30345 Fax No. +44(0) 1635 32630
Email: sales@alrad.co.uk