SLAA638A august 2014 – may 2023 MSP430I2040 , MSP430I2041
Due to the nature of a sigma-delta AD converter and of the voltage and current signal being measured, an external interface circuit is needed to interface the voltage and current to keep the SD24 ADC to work properly and provide proper filtering for band of interest.
The hardware of the interface circuit is simple, which is composed of passive components apart from the protection diode (D1, D8). Figure 2-2 shows the interface circuit designed. The top circuit is the interface to voltage sensor and the bottom circuit is for the current sensor – the shunt resistor.
In the current sensor interface circuit, R6 being the shunt sensors, D1 is the optional protection diode. The protection diode is not required unless, for example, a high shunt resistor value or a current transformer is in use instead of the current sensor, and the user anticipates a significant current surge to cause enough voltage across the sensor to damage the MCU.
L1, R9 and C9 forms a low-pass filter having bandwidth of a few MHz. The purpose of this filter is to reject radio frequency interference from going into the sigma-delta ADC. The filter formed by L2, R8 and C8 has the same characteristic which is balanced for the differential signal.
The filter formed by R9, R8 and C10 is a filter that gives the bandwidth of about 10 kHz, which is the filter for the band of interest.
It is important to note that the order of magnitude of these capacitors should be observed; even the actual value of the capacitance is not that critical.
The voltage sensor interface is a resistor ladder. The ratio of the resistance in the resistor ladder allows up to AC 308 V and DC 420 V to be measured. It is very similar to the current sensor filter except that the value of R10 is 100 Ω not 1 kΩ. The reason of this is the voltage divider circuit has an equivalent source resistance in series with R10. In order to have the filter balanced for the differential signal, a small R10 value is used. The combine effect is then of similar value to R11. Though there is a slight mismatch using the designed value but there is no observable impact to the accuracy.