Download Power Supply Decoupling

Power Supply Decoupling
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The objective of a power distribution system is:
1.
2.
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Supply a nearly constant dc voltage to all loads under conditions of
varying load currents.
Any ac noise signals generated by the load should not generate an ac
voltage across the dc power bus.
A typical power distribution system as it might appear on a
schematic
Fuse
A local decoupling capacitor
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Power Supply Decoupling
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The actual circuit for a dc power distribution system, including
parasitics
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Power Supply Decoupling
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Circuit of Fig. 4-15, less the decoupling capacitor and noise pickup
voltage
The performance of the power distribution system:
1.
The static or dc performance of the system.
2.
The transient or noise performance of the system.
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Power Supply Decoupling
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The minimum dc voltage is VL (min)  Vdc (min)  I L (max) ( RS  RF  RT ) max .
Transient noise voltages on the power distribution circuit are
produced by sudden changes in the current demand of the load.
VL  I L Z 0
where Z 0 
LT
.
CT
For best noise performance, a power distribution system
transmission line with as low as a Z 0 as possible is desired --typically 1  or less.
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Power Supply Decoupling
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Power Supply Decoupling
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The optimum power distribution line would be one with parallel flat
conductors, as wide as possible, placed one on top of the other,
and as close together as possible.
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Power Supply Decoupling
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Demonstration of the difficulty in providing power distribution
systems with very low Z 0 .
D d  1.5 Teflon dielectric
t  0.0027  in
w  0.02  in
h  0.04  in epoxy glass PCB
w  0.25  in h  0.005  in of Mylar
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Power Supply Decoupling
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To achieve the desired low impedance, decoupling capacitors can
be placed across the power bus at the load end.
Although this is a good approach, a discrete capacitor will not
maintain its low impedance at high frequencies, because of series
inductance.
Low-Frequency Analog Circuit Decoupling
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Power Supply Decoupling
1.
2.
3.
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The inductor used must also be able to pass the direct current
required by the circuit without saturation.
A second capacitor, such as those shown dashed in Fig. 4-18, can be
added to each section to increasing filtering to noise being fed back to
the power supply from the circuit. This turns the filter into a network..
When considering noise, a dissipative filter such as the R-C circuit is
preferred to a reactive filter, such as the L-C circuit.
Amplifier Decoupling
1.
Even if only a single amplifier is connected to a power supply,
consideration of the impedance of the power supply is usually required.
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Power Supply Decoupling
2.
The capacitor should serve as a short circuit across the frequency
range over which the amplifier is capable of producing gain.
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Driving Capacitive Loads
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An emitter follower, which feeds a capacitive load such as a
transmission line, is especially susceptible to high-frequency
oscillation caused by inadequate power-supply decoupling.
Voltage gain 
Zc
Ze
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Driving Capacitive Loads
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Emitter follower decoupled from power supply
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Driving Capacitive Loads
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The feedback can be decreased even more by adding an R-C filter
in the power supply to the first stage.
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Driving Capacitive Loads
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An op-amp is driving a heavy capacitive load.
The Ro and CL from a low-pass filter
that adds phase shift to the output
signal.
Break frequency f B 
One solution:
1
2 Ro CL
Ro
Another solution
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System Bandwidth
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One simple but often overlooked method of minimizing noise in a
system is to limit the system bandwidth to only that required by the
intended signal.
High-speed logic (fast rise time) is much more likely to generate
and be susceptible to high-frequency noise than its lower speed
counterpart.
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Modulation and Coding
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The susceptibility of a system to interference is a function not only
of the shielding, grounding, cabling, and so on, but also of the
coding or modulating scheme used for the signal.
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