78M6612 Hardware Design Guidelines
AN_6612_007
8
Rev 2
Figure 6 shows a basic connection diagram of a CT-based system.
Figure 6: Basic Connection Diagram of a CT-Based System
The use of a CT allows for the 78M6612’s V3P3 to be isolated from the plant NEUTRAL wiring. This
topology eliminates the safety issues stated earlier regarding shunt-based current sensing. Figure 6
shows a 2 M
Ω
isolation from the LINE voltage via the voltage divider network. In this topology, the line
voltage is a pseudo-differential measurement of VB-VA.
The V3P3 reference point critical to multi-shunt measurement performance is not an issue with CTs. The
output currents generated by the CT’s secondary winding is small enough that the sheet resistance of the
1 oz. copper plating does not present measurement errors from adjacent CTs. Shield the CTs secondary
pins, burden resistor and filter components with top and bottom printed circuit board layer V3P3 plane
surfaces. Insert multiple V3P3 vias to interconnect the top and bottom V3P3 structures for a low
impedance shield. Refer to the
78M6612 Printed Circuit Board Layout Guidelines
application note for
additional layout design recommendations.
See
Using Current Transformers with the 78M661x
for additional information.
3.2 Other Connections
The same power supply decoupling circuit from
can be used for an isolated configuration. The
78M6612 supply and ground connections, however, can be shared with other (isolated) components in
the system when V3P3 is not tied to AC Neutral.
Line voltage sensor recommendations for a non-isolated sensor configuration (resistor divider) can also
be used in a isolated configuration due to the high impedance connections between high voltage AC and
the 78M6612. Alternatively, a voltage transformer (VT) can be used for fully isolated voltage sensing.
NEUTRAL
LINE
VA
VB
750
1M
1M
IA
IB
LOAD
750
78M6612
1M
1M
V3P3
1000pF
RBurden
750
1000pF
0.1
μF
0.1
μF
1000pF
