AN_6612_007
78M6612 Hardware Design Guidelines
Rev 2
5
VA
V3P3A
1M
1M
750
LINE
V3P3
1000pF
0.1µF
78M6612
2.3 Line Voltage Resistor Divider Selection
The input line voltage must be scaled to match the 78M6612’s ADC input signal range of 176.78 mVrms.
In the example of Figure 3, the line voltage is scaled as follows:
The use of two 1 M
Ω
resistors instead of a single 2 M
Ω
resistor is required to meet the maximum voltage
rating of the resistor package and to provided adequate breakdown and arcing clearance. Typically 1206
series surface mount resistors are recommended.
An important aspect to consider is the accuracy of the resistors in the voltage ladder. Another
consideration that can affect the overall measurement accuracy is the resistor temperature coefficient
(TCR in
Ω
ppm/°C).
Figure 3: Input Voltage Divider and Filtering
Initial component tolerance can easily be compensated for during calibration. Depending on the system
accuracy requirements, the incremental cost of a higher precision resistor (i. e. 0.1% versus 5%) may
result in a lower production line calibration cost (shorter calibration time). If 0.1% tolerance resistors are
used, a calibration would only need to be done on one system and then the voltage calibration coefficient
obtained can be used for all others in production thus saving time through the production line. The
variation from board to board would be within the tolerance of the resistors used in the divider.
Additionally, the higher precision resistor will have a smaller temperature coefficient. This eliminates the
source of error that can arise from changes in resistance from self heating as the line voltage changes.
TCR of 50 ppm/
°
C or lower (preferably 25 ppm/
°
C) is recommended.
