Input common mode voltage

When choosing a DC detection solution, the input common mode voltage is the most important specification.It is defined as the average voltage at the input terminal of the amplifier.Figure 1 illustrates the definition of input common mode voltage [1].

This specification is important because it limits our selection of differential amplifiers.For example, operational amplifiers and IA require input of common mode voltage within their power supply.However, differential amplifiers and CSM can typically accommodate input common mode voltages beyond their power supply.This is useful in applications where amplifiers detect parallel voltage in the presence of high common mode voltage and must interface with low-voltage analog-to-digital converters (ADCs).In this case, regardless of the common mode voltage of the system, the same power supply voltage can be used to power the amplifier and ADC.

High end and low-end current sensing

When monitoring load current, designers can choose to place the detection resistor between the power supply voltage (Vbus) and the load, or between the load and ground.The former is called high-end detection, while the latter is called low-end detection.

Due to the common mode voltage being close to ground, low-end sensing is required, which allows the use of single power rail to rail input/output operational amplifiers.The disadvantage of low side sensing is the interference with the grounding potential of the system load and the inability to detect load short circuits.Figure 2 depicts a typical low-end sensing situation.

High end sensing is desirable because it can directly monitor the current supplied by the power supply, thereby detecting load short circuits.The challenge is that the input common mode voltage range of the amplifier must include the power supply voltage or Vbus of the load.This requirement often requires the use of DA or dedicated CSM, which allow for common mode voltages beyond their power supply range.Figure 3 depicts a typical high-end sensing situation.