Example Data Table
| Vin |
R1 |
R2 |
Vref |
Bits |
ADC Voltage |
Approx Code |
Divider Current |
| 12 V |
30 kΩ |
10 kΩ |
3.3 V |
12 |
3.000 V |
3723 |
0.300 mA |
| 24 V |
100 kΩ |
15 kΩ |
3.3 V |
12 |
3.130 V |
3884 |
0.209 mA |
| 5 V |
10 kΩ |
20 kΩ |
3.3 V |
10 |
3.333 V |
Saturates |
0.167 mA |
Formula Used
Divider ratio: K = R2 / (R1 + R2)
ADC input voltage: Vadc = Vin × K
Top resistor: R1 = R2 × ((Vin / Vadc) - 1)
Bottom resistor: R2 = R1 × Vadc / (Vin - Vadc)
ADC code: Code = round((Vadc / Vref) × (2^N - 1))
Divider current: I = Vin / (R1 + R2)
Resistor power: P = I² × R
Thevenin resistance: Rth = (R1 × R2) / (R1 + R2)
RC settling: Settling = (1 - e^(-t / (Rth × C))) × 100
How To Use This Calculator
- Select the calculation mode.
- Enter the input voltage and resistor values.
- Use target voltage when solving a resistor value.
- Enter ADC reference voltage and resolution.
- Add tolerance, gain error, and offset error if known.
- Enter sampling data for settling checks.
- Press Calculate to see results above the form.
- Download CSV or PDF reports after calculation.
Why ADC Dividers Matter
An ADC voltage divider lets a controller read a signal that is higher than its reference. The divider must protect the pin. It also must keep enough resolution. A poor ratio can waste counts. A very large resistance can slow sampling. A very small resistance can waste power and heat parts.
Design Checks
This calculator combines the main electrical checks in one place. It finds divider output, ADC code, LSB size, input referred resolution, current, resistor power, and Thevenin resistance. It also estimates RC settling from the sample capacitor and acquisition time. These values help you compare a fast sensing design with a low power design.
Tolerance And Error
Real resistors do not match their printed values exactly. Their tolerance can move the divider ratio. That movement changes the code even when the input is stable. The worst case range shows the low and high output from resistor tolerance. Gain error and offset error show how the ADC may shift the displayed count.
Practical Selection Tips
Start with the highest input voltage. Choose a safe ADC output below the reference. Leave margin for spikes, tolerance, and supply drift. Pick standard resistor values near the required ratio. Then check current and power. For battery products, higher values reduce drain. For fast ADCs, lower Thevenin resistance improves settling.
Using Results Safely
The result should not replace the microcontroller data sheet. Many ADC inputs need a maximum source impedance. Others need longer acquisition time. Some designs need a buffer amplifier when resistance is high. Add input protection when the source can exceed normal limits. Use the CSV and PDF reports to record assumptions before building hardware.
Example Workflow
Suppose a 12 V sensor feeds a 3.3 V ADC. A 30 kΩ top resistor and a 10 kΩ bottom resistor make 3 V at the ADC. A 12 bit converter then uses about 3723 counts. The input current is 0.3 mA. The Thevenin resistance is 7.5 kΩ. This is often acceptable, but sampling rules decide the final choice.
Always confirm pin voltage during startup and faults. A divider tied to an unpowered chip can back feed protection diodes. Series resistance and clamps may be needed when required.
FAQs
What is an ADC voltage divider?
It is a two resistor network that scales a higher voltage down to a safe ADC input level. The ADC then converts that scaled voltage into a digital count.
Which resistor is R1?
R1 is the top resistor. It connects between the input signal and the ADC node. R2 is the bottom resistor. It connects between the ADC node and ground.
Why should Vadc stay below Vref?
The ADC usually cannot measure above its reference. If Vadc exceeds Vref, the reading may saturate at the maximum code. The input pin may also need protection.
What does Thevenin resistance mean here?
Thevenin resistance is the effective source resistance seen by the ADC input. A high value can slow sample capacitor charging and cause conversion error.
Why include resistor tolerance?
Tolerance changes the real divider ratio. Even good nominal values can produce a different voltage in hardware. The tolerance range helps estimate worst case readings.
Can I use very large resistors?
Large resistors reduce current drain. They can also increase noise pickup and slow ADC settling. Check the ADC data sheet source impedance limit before choosing them.
What is input referred LSB?
It is the smallest input voltage change represented by one ADC count before the divider. A smaller value gives finer measurement resolution at the original signal.
Does this replace circuit testing?
No. It gives design estimates. Always verify the circuit with the selected microcontroller, reference voltage, resistor tolerance, sampling time, and expected fault conditions.