Henry Ott Consultants

Electromagnetic Compatibility Consulting and Training

Chassis to Circuit Ground Connection

Question: 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 ground plane?

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 supply ground?

Any light that you can shed on this subject will be greatly appreciated.

Answer: 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 it.

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