How the instrumentation amplifier gain calculator works
An instrumentation amplifier amplifies the small difference between two inputs, such as the output of a strain gauge bridge or a load cell, and rejects the voltage common to both. Most single-resistor in-amps set their gain with one external resistor, RG, according to an equation of the form G = 1 + K / RG. K is set by internal resistors and is printed in the datasheet’s gain equation.
The calculator finds the exact RG for your target gain, the standard E24 or E96 values on either side of it, the gain each gives, and the output voltage at both ends of your input range. It checks the output against the supply rails less the output swing, and recommends the value closest to the target that fits.
The gain constant K
K differs between parts and must come from the datasheet. 49.4 kΩ appears in several common in-amps (AD620-type and INA129-type equations), 50 kΩ in others (INA128-type), and 100 kΩ in some low-power parts. A K taken from the wrong datasheet gives a gain error of a few percent, which looks like a calibration fault.
Standard values and gain error
An exact RG is rarely a standard value. For G = 100 and K = 49.4 kΩ the exact value is 498.99 Ω; 499 Ω (E96) gives G = 99.998. A smaller RG gives a higher gain, so when the output is close to the rail the larger standard value is the safe one.
The tolerance of RG enters the gain as (G − 1)/G × tolerance: a 0.1 % resistor adds almost 0.1 % gain error at high gain. The datasheet adds the tolerance of K, typically a few tenths of a percent, and the gain drift with temperature. A two-point calibration removes the initial errors; drift remains.
Output range and the reference pin
The output is G × (V+ − V−) + VREF. On a single supply the output cannot reach 0 V: with a 0 V reference and no input, it sits below the lower swing limit and reads wrong. Lifting the reference pin, with a low-impedance source such as a buffered divider, moves the whole output range up; 0.5 V or mid-supply are common. The calculator shows the largest gain that keeps both ends of your input inside the output limits.
Enter the bridge output from the strain gauge bridge calculator or the load cell calculator, and check the resulting resolution with the ADC resolution calculator.
Frequently asked questions
Why does the input common-mode range matter?
Each internal amplifier has its own voltage limits, so the allowed common-mode voltage depends on the gain and the output voltage. Datasheets show this as a diamond-shaped plot. A bridge excited from the same single supply sits at half the supply; check that point against the plot. This calculator does not.
Can I use two resistors in series for an exact value?
Yes, but each resistor’s tolerance and temperature coefficient add. Two 0.1 % resistors are usually better than one 1 % value, and a single E96 value is often close enough.
Where should RG be placed?
As close to the RG pins as possible, with short, symmetric traces. The RG pins connect to the inputs of the internal amplifiers, and stray capacitance or pickup there is amplified.