Henry Ott Consultants
Electromagnetic Compatibility Consulting and Training
Chassis to Circuit Ground Connection
1. I am very much interested in the subject of the proper way to
connect the circuit ground to the chassis. We usually have mounting
screw holes on the board that connect to the chassis. Should I tie the
mounting screw holes to the circuit ground plane or leave them as a
chassis ground only and tie it to circuit ground at only one point, say at
the power supply? If I connect them to the ground plane should I tie
just one to the ground plane (if one, which one), or all of them to the
2. A second question involves a mixed-signal PCB, should the chassis
ground connect to the analog ground, the digital ground, or be left as
a chassis ground and only tied at one point, say at the power
Any light that you can shed on this subject will be greatly appreciated.
1. First I will tell you what you should not do, that is to make a
single point connection between the circuit ground and the chassis
ground at the power supply.
In most cases, the circuit ground should be connected to the chassis
with a low inductance connection in the I/O area of the board. In some
cases additional circuit to chassis connections may have to be made,
but these are in addition to the one in the I/O area, not in place of
The best way to understand this is to consider the circuit ground as a
noise voltage generator (as a result of the finite ground impedance and
the logic currents flowing through the ground). This ground noise will
drive a high-frequency common-mode noise current out the cables and
cause them to radiate. If, however, the circuit ground is connected to
the chassis in the I/O area, this noise current will be diverted to the
chassis, and will not flow out on the cables. This circuit ground to
chassis connection must be a very low inductance connection in
order to divert the high-frequency noise currents–this usually
requires multiple, short connections in the I/O area.
Establishing a low inductance connection between the circuit ground and
the chassis in the I/O area is also advantageous with respect to rf and
transient immunity. A high-frequency noise currents induced into the
cables, from external sources, will be diverted to the chassis, instead
of flowing through the PCB ground?
2. When a mixed-signal PCB is involved the circuit to chassis ground
connection should normally be made in the digital section of the board,
still following the procedure explained in 1.
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Henry Ott Consultants
48 Baker Road Livingston, NJ 07039
Phone: 973-992-1793, FAX: 973-533-1442
May 22, 2009