Voltage divider tolerance and worst-case output error
A nominal divider ratio is rarely the whole design question. Resistor tolerance, load resistance, and source variation can all move the output.
How much can a voltage-divider output move from its nominal value?
The output can move whenever Vin, R1, R2, or the load changes. Worst-case checking pushes each input to the corner that maximises or minimises Vout, then compares that range with the nominal result and the circuit limits.
Model summary
- Nominal divider: Vout = Vin * R2 / (R1 + R2).
- Worst-case high output usually occurs with high Vin, low R1, and high R2.
- Worst-case low output usually occurs with low Vin, high R1, and low R2.
- If a load is present, include R2 parallel Rload before calculating each corner.
Worked example
For Vin = 3.3 V, R1 = 20 kOhm, R2 = 10 kOhm, nominal Vout is 1.1 V.
With 1% resistors and fixed Vin, the high corner uses R1 = 19.8 kOhm and R2 = 10.1 kOhm, giving about 1.115 V.
The low corner uses R1 = 20.2 kOhm and R2 = 9.9 kOhm, giving about 1.086 V.
Design decision
Use nominal output for a first pass. Use corner analysis when the divider output feeds a threshold, ADC range, enable pin, or any circuit where a small error changes behaviour.
Common mistakes
- Assuming two 1% resistors produce a 1% divider output.
- Checking resistor tolerance but forgetting load resistance.
- Using a divider as a threshold without checking input thresholds across tolerance and temperature.
When the approximation breaks down
- The static resistor model does not include capacitor sampling transients, leakage, temperature coefficients, or PCB contamination.
- Precision threshold and reference designs may need resistor ratio tolerance, temperature drift, source tolerance, and input bias current in the same model.