# 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:

${I}_{m}=\text{\hspace{0.17em}}\frac{{V}_{m}}{{R}_{s}}$- 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.

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 |

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