Maxim Integrated 78M6610+PSU Hardware Design Guidelines - Manual (Page 4)

78M6610+PSU Hardware Design Guidelines AN_6610_107
4 Rev 0
2.1.1 Line Voltage Resistor Divider Selection
The input AC line voltage is scaled down to match the 78M6610+PSU’s ADC input signal range of
±176.78 mVrms (+/-250mV pk-pk). To support most 2-wire AC voltages with enough overhead for surges,
a peak RMS voltage of 471 VAC is recommended. Using the circuit in the previous figure, the line voltage
is scaled down as follows:
𝑉𝐴𝐷𝐶 =
𝑉𝐿𝐼𝑁𝐸 ∗ 750
1𝑀 + 1𝑀 + 750 = 𝑉𝐿𝐼𝑁𝐸 ∗ 3.7486 ∗ 10
−4
The use of two 1 M
Ω resistors instead of a single 2 MΩ resistor is suggested to meet the maximum
voltage rating of the resistor and to provided adequate breakdown and arcing clearance. A series of 2 or
more resistors may be needed in order to meet the safety standards requirements (UL, IEC etc.) for the
intended system.
A couple of important aspects to consider in the selection of the voltage divider resistors are the tolerance
and temperature coefficient (TCR in
Ωppm/°C).
Component tolerance can easily be compensated for during calibration. Depending on the system
accuracy requirements, however, the incremental cost of higher precision resistors (i. e. 0.1% versus 5%)
may justify lower production costs (shorter calibration time). If 0.1% tolerance resistors are sufficient for
the system accuracy targets, the nominal voltage calibration coefficient obtained from a few calibrated
units can be used for production units saving time calibration time in the manufacturing cycle. The
measurement accuracy variation from board to board would be within the tolerance of the resistors used
in the divider.
More importantly, higher precision resistors will often have a smaller temperature coefficient. This
eliminates the source of error that can arise from changes in resistance caused by temperature changes.
TCR of 50 ppm/°C or lower (preferably 25 ppm/°C) is recommended. To maintain a consistent
temperature coefficient, it is recommended to use similar resistive types in the divider.