78M6612 Hardware Design Guidelines
AN_6612_007
2
Rev 2
2 Non-isolated Configuration
When using a resistive shunt current sensor, the measurement IC and its power domain are not isolated
from the AC mains. In this configuration, the 3.3 VDC supply rail (V3P3) for the 78M6612 must be directly
connected to AC-Neutral for precision energy measurement. Isolation components, if required, are added
in between the measurement IC and the rest of the system.
The V3P3 connection to AC-Neutral can be eliminated when using current transformers (CT) as the
current sensing elements. Refer to the section on Isolated Connections for designing with CTs.
2.1 Safety Precautions
With V3P3 directly connected to NEUTRAL, the 78M6612’s Ground signal is -3.3 V below earth ground.
Therefore, any external test equipment attached to the 78M6612 will be subject to this -3.3 V ground
reference disparity.
External test equipment
must
be floated from earth ground to avoid equipment damage due to
this ground reference disparity.
An additional safety issue may arise due to mis-wiring of the AC outlets. If the LINE and NEUTRAL wire
connections at the AC plug are reversed between the 78M6612 and the external test equipment, the
78M6612 and external test equipment will see a 120/240 VAC voltage difference rather than -3.3 V. This
scenario can occur when the 78M6612 and external test equipment are powered from different wall
outlets or power strips, one of which is mis-wired. With proper earth grounding, the external equipment is
always referenced to earth ground via their enclosures.
Any systems communication interface (UART, SPI, I
2
C) between the 78M6612 and external
circuitry must be isolated to accommodate the -3.3 V disparity in their GND pins (or in the event
of a LINE reversal).
Refer to the
78M6612 Safety Precautions
document.
