Complete op-amp error budget

Assumptions to check

• Entered terms are referred to the input.
• RSS terms are independent.
• The selected gain is appropriate across the stated operating range.

Build the budget at one reference node

An op-amp accuracy claim only becomes useful after every contribution is referred to the same node. This starter budget refers offset, bias, and common-mode contributions to the output through the entered gain, then shows both RSS and worst-case totals.

Use RSS only for independent, random-like error terms. Bounded specifications, common temperature effects, and shared references can be correlated, so the worst-case sum or a documented correlation model may be the defensible result.

Interpret the dominant term

The largest output-referred contribution usually identifies the first improvement to investigate. A low offset part cannot correct a resistor-induced bias error, and a high gain can magnify every input-referred term. Extend the budget with device-specific temperature, CMRR, PSRR, noise, and dynamic behavior before selecting a precision path.

Build the budget before choosing the part

Start by writing the allowed output error, the signal range, the closed-loop gain, and the temperature range. Refer every term to the input or output before combining it. Offset voltage, input-bias current through source resistance, resistor-ratio error, common-mode error, supply variation, reference error, and noise do not all have the same meaning or distribution. A table with source, condition, reference node, sign or distribution, and worst value is more useful than one appealing total.

Use this compact budget to discover which term is likely to dominate, then extend the calculation only where it changes a decision. For example, a high-impedance source makes bias current relevant, while a precision low-impedance path may be limited by offset drift or resistor ratio. Validate the final range at the gain, common-mode voltage, supply, bandwidth, and temperature conditions used by the real circuit.

Common mistakes

• Combining bounded maximum errors by RSS.
• Mixing input and output-referred terms.
• Using typical data as a guaranteed limit.

Model limit and handoff

Use the selected device data sheet at matching temperature, common-mode, supply, bandwidth, and source-impedance conditions.

FAQs