JAJSR83 September 2023 AMC131M02
PRODUCTION DATA
The nominal mains voltage ranges from 100 V to 240 V, so this voltage must be scaled down to be sensed by an ADC. Figure 9-5 shows the analog front-end used for this voltage scaling at the ADC channel 1.
The analog front-end for voltage measurement consists of a voltage divider network (RHI and RLO), and an RC low-pass filter (RFILT and CFILT). Additional protection components (such as spike protection varistors) are usually placed as well, but are not shown in this circuit for simplicity.
Equation 12 shows how to calculate the range of differential voltages fed to the voltage ADC channel for a given mains voltage and the selected voltage divider resistor values.
RHI is 300 kΩ and RLO is 750 Ω in a typical design. For a mains voltage of 240 V (as measured between the line and neutral), the input signal to the voltage-sensing ADC has a voltage swing of ±256 mV (182 mVRMS) based on Equation 12 and the selected resistor values. This voltage is well within the ±1.2-V input voltage range that can be sensed by the AMC131M02 for the selected PGA gain value of 1 that is used for the voltage channels. In addition, overvoltages as high as four times the main voltages (up to 1000 V) can also be measured based on the selected combination of resistor values and gain setting.
The finite input impedance of the ADC channel 1 results in a gain error and offset error from the non-ideal voltage division by RLO. The introduction of a resistor in series with the inverting input of the ADC can reduce the offset and gain errors, as explained in the Isolated Voltage-Measurement Circuit With ±250-mV Input and Differential Output application note. The value of the additional series resistor is recommended to be in the order of RLO, and the design steps are documented in the Isolated Voltage-Measurement Circuit With ±250-mV Input and Differential Output application note.