User`s guide

4-4
1. Choose an LC product that sets the filter cutoff frequency at least a decade
below the frequency of the resonant current demand (f
d
) in the load. That
is, choose L
o
C
o2
≥ 100/(4π
2
f
d
2
);
2. L must be large enough to keep the "effective capacitance" of C
o2
as seen by
the LPT-10 transceiver to less than 5% of the transceiver’s 22µF output
capacitor C
2
. This can be accomplished by ensuring that
L
o
≥ 1mH(C
o2
/1µF);
3. The inductor's series resistance must be small enough so that the load
current flowing through it does not generate excessive voltage drop. Choose
an inductor value L
o
that has a low enough DC resistance (DCR in the
manufacturer data sheets) for your load current. Generally, smaller
inductors cost less, so you will want to choose the smallest value of L
o
that
meets the above criteria;
4. Once L
o
is set, the value of C
o2
can be chosen subject to the conditions in
steps 1 and 2 above.
Finally, it is good design practice to decouple each IC's Vcc pin in the node circuit using
0.1µF or 0.01µF radial or surface mount capacitors. This decoupling helps to reduce Vcc
noise that could lead to logic noise problems and radiated EMI problems (see Chapter 6
for more information on design hints to reduce EMI). The combined capacitance of all
the decoupling capacitors should not exceed 1.0µF.
WARNING: THE COMBINED CAPACITANCE OF ALL THE DECOUPLING
CAPACITORS (FROM VCC TO GND) SHOULD NOT EXCEED 1.0µF. TOTAL
DECOUPLING CAPACITANCE IN EXCESS OF 1.0µF CAN CAUSE UNSTABLE
POWER SUPPLY PERFORMANCE.
Powering Non-Isolated Devices
In order to provide reliable common mode rejection, the link power system operates
with one earth ground connection at the LPI-10 module, and all LPT-10 transceivers are
isolated (floating) relative to ground. The LPT-10 transceiver power supply is designed
to power devices that are also ground isolated. If the power supply is connected to a
grounded device, both network communications and power distribution will be
degraded. For nodes where the application electronics must be connected to earth
ground, the FTT-10 transceiver is the best choice, providing both isolation and
communication with LPT-10 nodes on the same twisted pair network segment.
Optionally, optical isolation may be used between the application electronics and
Neuron Chip.
Note that only floating measurement instruments should be used when working with
link power nodes when power is applied. Only isolated Digital Volt Meters (DVMs),
floating differential probes (e.g. Tektronix P6046 or P6135A with A6902B) or battery-
powered oscilloscopes (e.g. Tektronix 200 series or T200 series oscilloscopes) should be
used when working with link power nodes. The LPT-10 transceiver may be damaged if
its circuitry is inadvertently shorted to earth while power is applied.