Electrical & Electronics

Op-Amp Summing & Difference Calculator

Calculate weighted sums or voltage differences, choose matched resistor pairs and check output limits.

Your inputs

Calculation mode

Reference voltage · Vref · V

Input voltage · V1 · V

Input voltage · V2 · V

Input voltage · V3 · V

Input voltage · V4 · V

Number of inputs

Resistance · R1 · kΩ

Resistance · R2 · kΩ

Resistance · R3 · kΩ

Resistance · R4 · kΩ

Feedback resistor · Rf · kΩ

Apply output limits

Minimum · Vout · V

Maximum · Vout · V

Use usable output voltages from the datasheet, not supply rails. The bounded output is an estimate.

Ideal DC model. Check common-mode input range, bandwidth, load and resistor tolerances in the datasheet.

Your results

Ideal output voltage: —

Bounded output estimate: —

G1 · V1 → Vout: —

G2 · V2 → Vout: —

Common-mode gain: —

Formula—

Compare weighted sums and difference-amplifier outputs

Calculate an ideal inverting sum of several voltages or a two-input difference amplifier. Include a reference voltage, examine each input's gain and optionally apply output bounds. Difference mode also shows common-mode gain and whether nominal resistor ratios match. The calculation is a static ideal model; it excludes resistor tolerance, amplifier offset, finite bandwidth, input-range restrictions and load-dependent output swing.

Step by step

Choose summing or difference mode. Enter input voltages and reference voltage in V. Summing mode supports 2 to 4 inputs, each with its own resistor; difference mode uses exactly 2 inputs and the four resistors shown in its diagram.
Enter resistor values in kΩ. In summing mode, choose feedback resistance Rf. In difference mode, enter R1 through R4, or choose a target gain and E12, E24 or E96 series before applying a matched nominal resistor pair.
Review ideal output and individual gains. If output limits are enabled, enter a lower bound below the upper bound and compare limited output with ideal output. In difference mode, also inspect common-mode gain and the matched-ratio message.

Settings and limits

Summing weights and reference: Each summing input has weight −Rf / Ri. Output is Vref − Σ[Rf/Ri × (Vi − Vref)], so changing reference affects the entire expression. Equal input resistors give equal weights; different values let you scale inputs independently. Only the selected number of input rows participates.
Difference ratios and matching: When R2/R1 = R4/R3, ideal output is Vref + (R2/R1) × (V2 − V1). Unequal ratios change the positive input gain and leave a nonzero common-mode gain. Nominal ratio matching does not quantify real common-mode rejection when resistor tolerances and amplifier errors are present.
Preferred pairs and output bounds: The pair search examines preferred values around the entered R1 scale and sets the same nominal pair in both branches. Its rounded gain can miss the target. Output bounds are values you supply, not limits derived from supply rails; the tool clamps ideal output without modeling overload recovery.

Worked example

For summing, use 2 inputs: V1 = 1 V through 10 kΩ and V2 = 2 V through 20 kΩ, with Rf = 10 kΩ and Vref = 0 V. Weights are −1 and −0.5, giving ideal output −2 V. Enable bounds of −1.5 V and 3.3 V to obtain limited output −1.5 V.

Expected result
Σ / Δ	G1	G2	Vout,ideal	Vout	GCM
Σ	−1	−0.5	−2 V	−1.5 V	—
Δ	−2	2	2 V	2 V	0 V/V
Δ; R4 = 22 kΩ	−2	2.0625	2.125 V	2.125 V	0.0625 V/V

For difference mode, keep V1 = 1 V, V2 = 2 V and Vref = 0 V. Set R1 = R3 = 10 kΩ and R2 = R4 = 20 kΩ. Output is 2 V and common-mode gain is 0. Changing only R4 to 22 kΩ gives 2.125 V output and common-mode gain 0.0625 V/V, illustrating how unequal ratios alter the result.

Questions and troubleshooting

Why is the summed output negative?

This is an inverting summing circuit. With zero reference, positive inputs contribute negative weighted terms. A physical single-supply implementation needs a suitable reference and output range; use realistic bounds when checking whether the ideal sum can be reached.

Why do equal difference inputs produce a nonzero output?

With matched ratios the output equals reference when the inputs are equal. Unequal ratios add a common-mode contribution as well. Check reference and all four resistors before interpreting the result as a differential signal.

Does applying a preferred pair preserve my reference and voltages?

The button replaces the four difference-mode resistor values with matched nominal pairs and recalculates using the current inputs, reference and output bounds. Inspect the resulting gains and output again; it does not select a device or adjust those other settings for you.