2.4.1.2 Voltage Measurement

The TIDA-00555 design provides three provisions for the voltage channels. The following Figure 10 shows one of the voltage input circuits.

Figure 10. Circuit Diagram of Voltage Input

The voltage divider resistors for the ac input voltage channel is selected to ensure that the input to the MSP430F67791A SD24_B ADC is less than the differential input range (±919 mV) for the entire range of input voltages.

This design uses a 2.4 kΩ for R94.

To meet safety requirements, the input impedance is typically expected to be > 1 MΩ.

So, the divider resistor value, R_UPPER = 332 kΩ × 4 = 1.33 MΩ.

The R_UPPER is the addition of R122, R117, R111, and R112 (see the preceding Figure 10).

The V_R94 voltage divider output must be less than the ADC input range of the MSP430F67791A device.

So, the effective range for V_R94 = 919 mV / 1.414 = 649.9 mV.

Use the following voltage resistor divider calculation for Equation 2:

Use the preceding Equation 2 to calculate the V_INPUT:

V_INPUT = 360.8 V

The TIDA-00454 design can measure a maximum input voltage up to 360 V.