Electrical & Electronics

Instrumentation Amplifier Calculator

Calculate gain, choose gain resistors and check internal output voltages.

Your inputs

Your results

Voltage gain: —

Ideal output voltage: —

Voltage · V+ − V−: —

Voltage · VCM: —

VA: —

VB: —

Remaining output margin: —

Formula—

Check differential gain and internal amplifier headroom

Model the displayed three-amplifier instrumentation circuit. Calculate differential gain and ideal output while checking internal stage voltages against user-supplied limits. This simplified headroom test does not establish a real device's complete common-mode range.

Step by step

Enter matched stage resistances, both input voltages and the output reference. Use the diagram to distinguish input and output-stage resistor pairs.
Enter supply limits and headroom allowance. For gain design, choose a target and preferred series, then apply the proposed gain resistor.
Compare ideal output, common-mode input and internal outputs. A plausible final output can still accompany an internal limit warning.

Settings and limits

Gain resistor: RG controls first-stage gain. The final stage also multiplies the difference according to its resistor ratio; inspect achieved total gain after rounding.
Reference voltage: Reference shifts ideal output without changing the differential input. It does not move every internal node equally or automatically solve common-mode limitations.
Headroom allowance: The entered allowance reduces the usable rail interval for all checked nodes. Real input and output ranges depend on the device, load and supply.

Worked example

Use the differential-input example below. The final output and both first-stage outputs fit inside the chosen usable interval.

Example input

R = 10 kΩ; RG = 1 kΩ; Rin = Rout = 10 kΩ
Vref = 0 V; V− = −5 V; V+ = 5 V
h = 0.2 V

Expected result
Vin+ / Vin−	G	Vout	VA	VB	ΔVrail
1.01 / 1 V	21 V/V	0.21 V	0.9 V	1.11 V	3.69 V
4.9 / 4.9 V	21 V/V	0 V	4.9 V	4.9 V	−0.1 V

Then set both inputs to the equal high value shown. Ideal differential output vanishes, yet the internal nodes exceed the chosen headroom limit.

Questions and troubleshooting

Why is there a warning with a small output?

The first stages respond to input common-mode voltage. They can run out of swing even when subtraction would give a small final output.

Are rounded gains exact?

Preferred RG values give the displayed achieved gain. Resistor tolerances, offset and finite common-mode rejection add errors beyond this nominal model.

Can I use these limits for any instrumentation IC?

No. Consult its input common-mode versus output range and reference requirements. An integrated device need not follow this simplified discrete topology.