Electrostatic discharge and electromagnetic interference protection circuit

The protection circuit of the invention connects differentiated grounds in an electronic system either by a single diode, or by two diodes arranged in a back-to-back, parallel fashion. The differentiated grounds may include a chassis ground, a logic ground and an earth ground. The circuit locally connects two of the differentiated grounds, thereby providing a low inductance path for the fast discharging of electrostatic charge build-ups, with the diode(s) therein providing the electrical separation necessary to provide proper local isolation between the interconnected grounds to enable filtering out of electromagnetic interference potentials.

TECHNICAL FIELD 
This invention relates to circuitry used to protect electronic components 
from electrostatic discharge and electromagnetic interference, and more 
particularly, to specific circuits used to protect against such problems. 
BACKGROUND ART 
Electronic devices or systems such as computers and printers may contain 
various grounds. A ground is a conducting connection used to establish and 
maintain the electrical potential of the earth or other grounding body on 
conductors connected to it. 
For example, a computer printer may be grounded to the earth by a ground 
lead contained in a 3-wire power cord connecting the printer to an 
electrical outlet. Additionally, a computer printer may include a chassis 
ground wherein the printer's chassis is the grounding body. 
An electronic device may also contain a logic or circuit ground which 
provides a reference ground potential to components such as digital logic 
circuitry. A logic ground generally uses the earth ground as the grounding 
body. However, the conductive path from the logic ground to the point of 
connection with the earth ground is often lengthy, resulting in a high 
inductance. Such high inductance, with current flow, creates an 
electromotive force in the circuitry which affects the ability of the 
conductive path to maintain a certain ground potential or to discharge 
certain charges to the earth. 
Electronic devices such as computers and printers are often also exposed to 
electrostatic discharge. Electrostatic discharge is a dissipation of an 
electric charge often causing spurious sharp voltage pulses that can 
damage or interfere with the operation of electronic equipment. Such 
charges may originate from varied sources such as a user touching a 
keyboard or paper being fed into a printer. 
To prevent the possibility of damage from electrostatic discharge, 
electronic devices may be shielded. Shielding may consist of an 
electroplate coating on the surrounding enclosure. However, such shielding 
is expensive and difficult to employ. 
Alternatively, a separate conductive path with a low inductance to earth 
ground may be used. Such a path may be supplied by using electronic 
components such as a four-layer printed circuit board. In such a case, one 
or more layers of the board may be dedicated to provide a ground plane. 
However, additional board layers are also relatively expensive. In 
addition, it is desirable to steer the electrostatic discharge current 
away from the electronic components. Accordingly, a more desirable low 
inductance path would be through the chassis, which is connected to earth 
ground. Such a path is referred to as chassis ground. 
When digital logic or other electronics performs under normal operation, it 
draws time-varying current through logic ground. When there is inductance 
between logic ground and earth ground, the logic ground becomes 
contaminated. Contamination refers to unnecessary high frequency signals 
superimposed on the desirable signal. 
Additionally, electronic devices often create, and are affected by, 
electromagnetic interference. Such interference is undesirable 
contamination of a signal which tends to obscure the information content 
of the signal. To minimize electromagnetic interference, it is desirable 
to filter out the contamination. A clean ground must be used to filter out 
the contamination; if the contaminated ground were directly connected to 
the clean ground, the clean ground would become contaminated, and the 
filter would not function. So the contaminated ground, such as logic 
ground, must remain isolated from the clean ground, such as the chassis 
ground. 
This invention overcomes the limitations of the prior art by providing an 
inexpensive protection circuit which conducts electrostatic discharge by 
way of a low inductance path to a ground capable of dissipating such 
signals, while isolating the "contaminated" ground, such as the logic 
ground, from a "clean" ground in the system, such as the chassis ground. 
DISCLOSURE OF THE INVENTION 
The protection circuit of the invention takes the form of a conductive 
circuit connected between differentiated grounds in an electronic device. 
The conductive circuit includes either a single diode, or two diodes 
arranged in a back-to-back parallel fashion. The differentiated grounds 
may include a chassis ground, a logic ground and an earth ground. The 
conductive circuit locally connects two of the differentiated grounds, 
thereby providing a low inductance fast path for discharging electrostatic 
charge build-ups, with the diode(s) in the circuit providing the 
preconduction threshold potential isolation necessary to provide proper 
local isolation between the grounds to enable filtering out of 
electromagnetic interference potentials.

DETAILED DESCRIPTION AND BEST MODE FOR CARRYING OUT THE INVENTION 
FIG. 1 shows generally at 10 an electronic system employing a protection 
circuit 11 constructed in accordance with the invention. System 10 
includes a logic ground 12 and a chassis ground 14. Often a ground, such 
as logic ground 12, has a high inductance path to earth ground. 
Accordingly, such a ground is unable to dissipate electrostatic discharge 
or electromagnetic interference potentials effectively. 
Circuit 11 provides a low inductance, fast path to ground by locally 
directly connecting logic ground 12 to chassis ground 14 by diodes 16, 18. 
Diodes 16, 18 provide a conductive path whereby an electrostatic charge 
build-up, or an electromagnetic interference potential, at ground 12 may 
be dissipated rapidly to chassis ground 14. Diodes 16, 18 are arranged in 
a back-to-back, parallel configuration to accommodate both positive and 
negative potentials at ground 12. 
A single diode may be used where a device would only be subject to either 
positive or negative charges. For example, FIG. 2 illustrates such a 
modified form of protection circuit employing a single diode 19 which is 
capable of handling expected positive potential rises of logic ground 12 
relative to chassis ground 14. Were a reverse situation expected, with 
unwanted negative potentials anticipated on logic ground 12 relative to 
ground 14, a single diode, such as diode 19, would be used connected in 
the reverse direction. 
System 10 further includes a first system component 20 connected to logic 
ground 12. Ground 12 provides component 20 a reference ground potential. A 
second system component 22 is grounded to chassis ground 14. Ground 14 
provides component 22 a separate ground potential. 
Generally, in an electronic device, an internal ground, such as logic 
ground 12, would not be directly connected to a chassis ground, in order 
to minimize contamination of the chassis ground. Again, contamination 
refers to high frequency signals superimposed on the desired signal. 
Chassis ground 14 is desirably required to be a "clean ground", in order 
to minimize what is known as common mode electromagnetic interference. In 
the preferred embodiment, diodes 16, 18 provide the preconduction 
threshold potential isolation necessary to minimize contamination 
problems. 
When an electrostatic discharge occurs at ground 12, with this discharge 
characterized by a potential which rises above (in an absolute sense) the 
threshold voltage of the back-to-back diodes, charge is conducted to 
chassis ground 14 across diode 16 or 18, depending upon its polarity. 
Under circumstances where no relative potential changes occur between the 
grounds, which changes exceed the threshold potential of the diodes, these 
diodes, in effect, isolate grounds 12, 14 in a bidirectional manner. Put 
another way, diodes 16, 18 set a bidirectional preconduction isolating 
threshold potential. 
FIG. 3 shows generally at 30, within dashed lines, another electronic 
system employing the protection circuit of the invention. System 30 
includes a system logic block 34 which uses a signal on a conductor 36. 
Block 34 is grounded to a logic ground 38. Ground 38 is connected to earth 
ground 40 by way of conductive path 42, which is typically a relatively 
narrow, and therefore a relatively high inductance, route to the earth 
ground. Such inductance is represented in FIG. 3 by an inductor 44. 
System 30 also includes a capacitor 46 which connects conductor 36 to a 
chassis ground 48. Capacitor 46 and chassis ground 48 filter certain 
electromagnetic interference potentials from the signal on conductor 36. 
Chassis ground 48 is also connected to earth ground 40 by a conductive 
path 50. 
Logic block 34 may produce an electromagnetic interference potential 
build-up at logic ground 38 or, alternatively, an electrostatic discharge 
may arise at logic ground 38. Ground 38 may be unable to dissipate any 
such charges or interference potentials to earth ground 40 by means of 
path 42 because of the high inductance symbolized by inductor 44. 
Accordingly, system 30 includes protection circuit 52 to provide a path 
whereby such electromagnetic interference potentials and electrostatic 
charges may be dissipated. Circuit 52, which includes back-to-back, 
parallel diodes 54, 56, connects logic ground 38 to chassis ground 48 and 
to earth ground 40 as shown. 
With the protection circuit of the invention, electrostatic discharge, and 
electromagnetic interference signals, in a system including components and 
elements sensitive to such aberrations are handled, and substantially 
eliminated, in a relatively low-cost manner. In particular, such 
interference conditions are eliminated through the approach of 
diode-connecting potentially offending nodes, typically ground nodes, in a 
system. 
INDUSTRIAL APPLICABILITY 
The protection circuit of the invention is illustrated herein in a setting 
wherein it will most often be useful, namely, in the setting of dealing 
with relative potential changes between differentiated grounds. However, 
one should also recognize that the protection circuit of the invention may 
be used in any instance where it is desired to minimize similar problems 
occurring between any two nodes that nominally bear signals of matching 
potential, whether that potential is expected to be nontime varying or 
time varying. 
Accordingly, while a preferred embodiment, and a modification, of the 
invention have been described herein, other variations and changes may be 
made without departing from the spirit of the invention.