A general-purpose CSA must work with common-mode voltage that far exceeds the power supply. For example the common-mode voltage could be 80 V while the supply voltage is 5 V. This is achieved with transistors and capacitors that are capable of standing off high voltage, enabling the rest of the circuitry to employ low-voltage devices. A dominant input stage configuration is a common base or common gate where the input signal is coupled to the emitter or source of the transistor pair. The common-mode voltage is used to power the front end amplifier when it is sufficiently high, or higher than the power-supply voltage. A comparator and simple resistor-based common-mode sensing enables the transition between the two power rails. Bias current is drawn from the common-mode voltage source when it is used to power the front amplifier. Figure 4-4 shows a block enclosed by the red dashed box, which represents the common-mode sensing and supply function. This block is sometimes referred to as “Bias” in some data sheets. Normally a pair of resistors of larger value is used to sense the common-mode voltage level, which is passed onto the supply selection circuitry including the comparator. A second pair of much smaller value passes the common-mode voltage to the front amplifier when chosen as power source.

The bias current for this type of input stage usually ranges from 10 μA to 100 μA under nominal working conditions and sets the low limit of the current to be measured. While this level of bias current is negligible in most power supply monitoring applications, it does pose a threshold in applications where small currents need to be measured accurately. When not distinguished, “bias current” refers to both the positive and negative input bias current associated with the positive and negative input pins respectively. Because of the structure of the common-mode sense resistor pair, the two input bias currents are equal only when the differential input voltage equals to zero. They start to diverge as the differential input voltage deviates from zero volts. The increase or decrease is generally linear. However, there are exceptions if there is clamping circuitry and such circuity is activated.

Switched-capacitor architecture offers an alternative in this situation because it eliminates DC bias current entirely. However, the bias current is not zero as a result of capacitor charging and discharging. In this category, it is routine to find CSA with I_b on the order of 10 μA. Recent development has pushed I_b lower considerably, with 1 nA specification commonplace.

Contrasting with CSA, digital power monitors measure both shunt voltage and bus voltage with an integrated ADC. An internal math engine converts these quantities and outputs a bit stream that represents current, voltage, and power. Some are able to keep time and therefore can calculate energy and charge. Most digital power monitors employ a hybrid ADC that samples the input voltage directly, though some comes with a front-end programmable gain amplifier to accommodate a wide input range. Similar to switched capacitor analog CSA, small input bias currents can be achieved with some on the order of 1 nA. However small, it is not zero and may need to be considered in an application circuit.