10.1 Use Case

Using a shunt resistor for measuring a circuit’s current consumption is a common application for microcontrollers with a differential ADC. The ADC can measure the voltage drop over the resistor, and using ohm’s law, the current can be calculated from the measured voltage and the known resistance of the shunt resistor:

I_{m} = \frac{V_{m}}{R_{s}}

Introducing a series resistor into the circuit has two adverse effects:

Increases the power consumption of the circuit
A voltage drop over the added resistor

Both effects can be mitigated by choosing a resistor R_S with as small as possible resistance, especially important when considering low power applications. The downside of choosing a low resistance is that measuring the voltage drop becomes more difficult as a change in current results in a very small change in voltage. Such an issue can be alleviated by using a differential amplifier to amplify the voltage drop across the current sense resistor and thus increasing the resolution of the measurement. The effects of the resistor’s size on the measurement’s resolution and the power consumption can be seen in Table 10-1.

Table 10-1. Relation Between Resistor Size and Power Consumption and Resolution
Resistor_S [Ω]	Voltage Drop [mV]	Power Consumtion [mW]	Resolution no Amplification [mA/bit]	Resolution 7x Amplification [mA/bit]
10	100	1000	50	7.14
1	10	100	500	71.4
0.1	1	10	5000	714

Note: The table assumes a current of 10 mA and a 12-bit differential ADC with a reference voltage of 1.024V.

Figure 10-2 shows how a differential amplifier can be placed in between the shunt resistor and the ADC to measure the amplified signal.

Figure 10-2. Differential Amplifier Used in a Shunt Current Measurement