STPM34 Measuring 240Vrms without Neutral

[HaydenG6]

Hello,

We are designing an EVSE for a North American 120/240-V split-phase supply. The EVSE is connected to L1 and L2, with a nominal voltage of 240 V RMS between them. Neutral is not available at the EVSE.

We would like to use the STPM34 to measure the L1-to-L2 voltage and calculate the load power and energy. Current sensing is implemented separately using current transformers and is not shown in the attached schematic.

Our proposed voltage-sensing circuit uses the primary differential voltage channel as follows:

• L1 is connected to VIP1 through three series 560-kΩ resistors.
• L2 is connected to VIN1 through an identical resistor chain.
• Each input has a 1-kΩ resistor and filter capacitor connected to the local analog ground, iGND_A.
• iGND_A is the local ground of the isolated STPM34 metrology domain. It is not connected to neutral or protective earth.

The intention is to create a symmetrical differential divider so that the voltage between VIP1 and VIN1 is proportional to the L1-to-L2 voltage. The metrology ground would float approximately at the midpoint between L1 and L2.

I reviewed the STPM34 datasheet and the available STPM34 evaluation-board schematics, but I could not find a reference circuit showing how to measure a line-to-line voltage when no neutral connection is available. The evaluation-board implementations appear to use neutral as the metrology reference, which is why we considered the symmetrical differential arrangement shown above.

Could you please advise on the following?

1. Is this a valid and recommended method of measuring a 240-V line-to-line voltage with the STPM34?
2. Could movement of the floating midpoint due to unequal parasitic leakage, resistor tolerances, or line-to-ground voltage imbalance affect measurement accuracy or violate the input common-mode limits?
3. Must VIP1 and VIN1 each remain within the specified input range relative to GNDA, in addition to the VIP1–VIN1 differential voltage remaining within range?
4. Would ST instead recommend referencing the isolated metrology ground directly to one line and using the standard single-ended voltage-divider arrangement?
5. Is there an STPM34 reference design or recommended circuit specifically for a two-wire, 240-V split-phase application without an accessible neutral?

Thank you.