Friday, September 19, 2008

SATA Interface Layout Considerations

Correct separation of the signal and shield grounds is required to isolate the ESD energy. This separation is a physical gap between all the planes on a PCB with no traces crossing and is commonly referred to as a moat. The AC and signal grounds can be shorted together, but only after the main AC ground connection point. The following examples are from actual products designed to meet all international regulatory requirements.

Picture below is a portion of a PCI-X HBA with two SATA ports. Note the large moat between the chassis ground plane and the signal ground plane. These two planes are shorted together at a single point by a SMT zero Ohm resistor near the top PCI bracket mounting hole. This connection point is used because it is the closest to the PCI bracket mounting screw, which will have the lowest impedance to the Chassis AC ground connection. ESD energy on the SATA cable will seek the chassis ground connection rather than the highly inductive path through the zero Ohm resistor and the signal ground plane. Do not use a thermal relief on this mounting hole as it will raise the impedance of this connection.

Picture below shows identical moats cut through the power plane of the same HBA. This technique is used to completely isolate the chassis ground section of the PCB. ESD energy can easily jump between planes that are not adequately physically isolated. According to MIL-P-13949/4C for FR4 material, the average dielectric strength (perpendicular to laminations) is 750V per mil (.001”) minimum (29.25 KV per mm). This value should be de-rated at least 25 to 50% to allow for material tolerances.

Picture below shows a small section of the PCB isolated by a moat. As in the previous examples, this moat separates the shield and signal ground on an SATA port. The large hole at the bottom (labeled M4) is a mounting hole tied to a metal chassis. This product uses an ATX-type PC power supply equipped with a grounded AC receptacle. This means that this mounting hole has very low impedance to AC ground for conducting ESD energy. (Do not use a thermal relief on the chassis ground connection hole). The mounting hole has a SMT 0 Ohm resistor to connect the signal ground to the chassis ground at this one point, sometimes called a Mecca ground. The signal ground has no other connection for ESD energy to flow so the ESD energy is routed into the chassis, away from the SATA port IC.

Picture below is from a product that uses an ungrounded “floor wart” power supply that supplies regulated DC directly to the eSATA device. Notice that the shield and signal grounds are shorted together by a 0 Ohm SMT resistor on the far side of the power input connection, away from the SATA port.

In all the previous examples, the connections between the chassis ground and signal ground is facilitated by a zero Ohm SMT resistor. This component is used instead of a direct trace connection to ground for two reasons:

  1. The small amount of case inductance of the SMT package helps to filter the very fast rise-time ESD bursts
  2. The value of the SMT device can be changed for tuning the response time of this connection
Note: To raise the inductance of this connection, an actual bead inductor can be used instead of the zero Ohm resistor. This connection is also the point where EMI causing common mode current is shunted to ground. Be careful not to significantly raise the high frequency impedance of this connection as it will adversely effect EMI emissions. While using an actual resistor may improve ESD immunity, the impedance increase in the signal ground path would significantly increase EMI emissions.

Proper impedance control of the PCBs and the SATA connections will improve both EMI and ESD performance. Other parameters like the size, number and high frequency response of the signal ground connections, may be inversely related and must be balanced between effective EMI shielding and adequate ESD protection. These parameters are very dependent on the particular implementation.

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