Voltage Divider with ADC

Sometimes you need to measure voltages higher than the ADC’s reference voltage (3.3V). For example, measuring a 5V power supply, a 9V battery, or a 12V system. Since the RP2350 ADC can only safely measure up to 3.3V, you need to scale down the voltage using a voltage divider.

We have already seen how the voltage divider works and how two resistors can be used to scale a voltage. If you need a refresher on how voltage dividers work, refer back to the voltage divider section. Here, we will focus on how a voltage divider interacts with an ADC and how the divider output turns into an ADC reading.

Tip

Voltage dividers are not just for scaling voltages. Sensors like LDRs and thermistors are resistors whose values change with light or temperature. When used in a voltage divider, these resistance changes turn into voltage changes that the ADC can read.

Using Voltage Dividers with ADC

When using a voltage divider with an ADC, the goal is usually to scale a voltage so that it stays within the ADC input range.Design the voltage divider so that the maximum expected input voltage produces an output voltage below the ADC reference voltage.

For a 3.3 V system, it is good practice to aim for 3.0 V or less to allow some margin for supply variation and noise.

Example: Measuring a 3.3V Supply

Consider a simple example where the input voltage is 3.3V and we use resistor values of R1 = 10 kΩ and R2 = 1 kΩ.

You should already be familiar with the voltage divider formula:

\[ V_{out} = V_{in} \times \frac{R_2}{R_1 + R_2} \]

Using the voltage divider equation:

\[ V_{out} = 3.3V \times \frac{1k}{1k + 10k} = 3.3V \times \frac{1k}{11k} = 0.3V \]

This output voltage is well within the ADC input range. The divider output voltage V_out is connected to an ADC input pin.