Isolated AC voltage sensing using LV25-P

Voltage sensing is one fundamental thing in embedded system design.
The simplest and cheapest way to sense the high voltage is using the voltage divider to scale the voltage into ADC allowable sensing range.

Additional filter capacitor (0.1uF) can be added in between VADC and ground to eliminated the noise.

Voltage divider sensing is easy to implement, has good accuracy, and sensing range can be customised. By adding a trimmer in between R1 and R2, a more precise measurement range can be obtained.

The only drawback of this method is that common ground issue, the ground from sensing voltage needed to be grounded together with microcontroller. In this regard, there might be safety issue, when measuring >100V. Also, by common ground, it is easier to induce noise to your circuitry especially in power converter applications.


Isolated sensing should be used in high voltage and noisy measurement. The LEM LV25-P is a hall-effect voltage sensor. It has galvanic isolation in between the sensing voltage, and signal output. It has safety certification (EN 50178: 1997), thus make it a very expensive device.

The price from Element14 is around RM294.91.

Above is basic layout of LV25 and it's pins descriptions. The signal is connected to +HT and -HT, "+" and "-" sign need +/- 15V supply voltage. M is the output signal. 

The basic circuit connection for LV25-P is as follow:

Although LV25 is a voltage sensor, but it actually sense current. A sense resistor is needed to limit the current before fed into LV25. The equation to select the suitable resistor is dividing the maximum sensing voltage with 10mA. The 10mA is the nominal current rms LV25 can tolerance. So, LV25 can withstand primary current, measuring range from 0 to ± 14mA.

The output of LV25 is current (Secondary nominal current rms 25mA). However,  ADC module only accepts the voltage signal instead of current signal. A resistor is placed at the output side to convert the current to voltage. The resistor value must be less than 300 ohm, else the linearity will change at difference voltage range. It is advisable to add an op-amp to amplify the signal instead of using a trimmer at the output to vary the gain.

If you are sensing DC voltage, the output signal range is from zero to positive value. 

If AC voltage, you will get a bipolar output, an offset circuitry (suggestion: use differential amplifier) is needed to shift the voltage range from +/-, to 0/+.