The voltage from the mains is usually 230 V or 110 V and needs to be brought down to a range of 1 V. The analog front end for voltage consists of spike protection varistors (not shown in this figure) followed by a simple voltage divider and a RC low-pass filter that acts like an anti-alias filter.

Figure 3-2 shows the analog front end for the voltage inputs for a mains voltage of 230 V. The voltage is brought down to approximately 700 mV RMS, which is 990 mV peak and fed to the positive input, adhering to the ΣΔ analog limits. A common-mode voltage of zero can be connected to the negative input of the ΣΔ. In addition, the ΣΔ has an internal reference voltage of 1.2 V that can be used externally and also as a common-mode voltage if needed. GND is referenced to the neutral voltage or line voltage, depending on the placement of the current sensor.

Figure 3-2 Analog Front End for Voltage Inputs

It is important to note that the anti-alias resistors on the positive and negative sides are different, because the input impedance to the positive terminal is much higher and, therefore, a lower value resistor is used for the anti-alias filter. If this is not maintained, a relatively large phase shift of several degrees would result.