Instrumentation Amplifier Calculator

This tool computes key design parameters for a classic 3-op-amp instrumentation amplifier: required resistor values (R₁, R₂, R_G), differential gain (A_v), common-mode rejection ratio (CMRR) degradation due to resistor mismatch, output voltage swing limits, and bandwidth estimation based on op-amp GBW and gain. It also flags potential saturation or headroom violations.

Use it during the front-end analog signal conditioning phase—especially for sensor interfaces (e.g., strain gauges, thermocouples, bridge transducers) where high CMRR, precise low-noise gain, and excellent common-mode rejection are critical. It’s ideal for selecting resistor networks and verifying feasibility before PCB layout or simulation.

R_G (the external gain-setting resistor) directly controls the overall differential gain: A_v = 1 + (2R₁/R_G). Smaller R_G yields higher gain but increases sensitivity to resistor tolerance and thermal drift. Values below ~100 Ω become impractical due to PCB parasitics and op-amp output current limitations.

R_trim models worst-case resistor mismatch (e.g., from ±0.1% to ±5% tolerance), which degrades CMRR. The calculator uses it to estimate CMRR loss: CMRR_actual ≈ CMRR_ideal × (100 / R_trim). For precision designs, use 0.01–0.1%; for cost-sensitive applications, 0.5–1% is typical.

V_CM must stay within the op-amps’ common-mode input range—typically 1–2 V below ±V_s. Exceeding this risks clipping or phase reversal. The calculator checks if V_CM is valid and estimates output headroom, ensuring the amplified signal remains within rail-to-rail or output swing limits.

Enter the GBW of the *input-stage* op-amps (not the output stage), as they dominate bandwidth limiting in the IA topology. Bandwidth ≈ GBW / (2 × A_v) for matched R₁/R₂. Underestimating GBW leads to excessive phase shift and reduced effective CMRR at higher frequencies.

Yes—with caveats. Set ±V_s to represent your actual supply (e.g., V_s = 5 V for 0 V to 5 V rails), and ensure V_CM stays within the op-amps’ specified input common-mode range. The calculator flags invalid V_CM relative to supply rails and recommends biasing strategies if needed.

Standard E-series resistors introduce small mismatches in R₁/R₂, reducing actual gain and CMRR. The calculator assumes ideal matching unless R_trim is adjusted. For production, always simulate with real resistor tolerances—or use matched monolithic resistor networks (e.g., LT5400) to preserve performance.

CMRR is estimated as 20·log₁₀(100/R_trim) dB for the resistor network contribution, plus op-amp intrinsic CMRR (assumed ≥100 dB). Accuracy is limited by unmodeled factors like PCB layout imbalance, temperature gradients, and op-amp input offset/CMRR vs. frequency roll-off—always verify with SPICE or bench testing.

R₁ and R₂ are typically 1–10 kΩ for low noise and drive capability, but scale with R_G per A_v = 1 + 2R₁/R_G. Avoid <1 kΩ (excessive current) or >100 kΩ (noise, leakage, capacitance). The calculator selects optimal values balancing gain accuracy, power, and stability.