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Load Cell Amplifier Gain

Calculate load cell output voltage, required amplifier gain, and mV/V sensitivity for Wheatstone bridge weigh scale and force sensor designs.

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Formula

VFS=S×Vex,Vamp=VFS×GV_FS = S × V_ex, V_amp = V_FS × G
S— Sensitivity (mV/V)
V_ex— Excitation voltage (V)
G— Amplifier gain (V/V)

How It Works

This calculator determines instrumentation amplifier gain for load cell signal conditioning, essential for scale designers, industrial automation engineers, and embedded systems developers building weighing systems from grams to tonnes. Load cells are strain gauge Wheatstone bridges that output millivolts per volt of excitation (mV/V) at full-scale load, typically 1-3 mV/V per OIML R60 and NTEP standards. A 2 mV/V load cell on 5V excitation produces only 10 mV full-scale output, requiring 100-500x amplification to match ADC input ranges. Required gain G = Vadc_fs / Vloadcell_fs, where Vadc_fs is ADC full-scale voltage (typically 3.3V or 5V). Instrumentation amplifiers (INA125, INA128, AD620) provide precise differential gain with CMRR >90 dB to reject supply noise. The HX711 24-bit ADC with integrated 128x PGA has become the de facto standard for embedded weighing, providing 0.1 mg resolution at 10 SPS. Load cell accuracy per OIML R60 Class C3 is +/-0.02% (3000 divisions), requiring careful attention to noise and thermal drift.

Worked Example

Problem

Design amplification for a 50 kg S-type load cell (2 mV/V sensitivity, 350 Ohm bridge) with 10V excitation, feeding a 16-bit ADC with 4.096V reference. Target 10 g resolution.

Solution
  1. Full-scale output: Vfs = 2 mV/V * 10V = 20 mV
  2. Required resolution: 50 kg / 10 g = 5000 divisions minimum
  3. ADC step: 4.096V / 65536 = 62.5 uV/LSB
  4. Required gain: G = 4096 mV / 20 mV = 204.8 V/V (use 200 for standard values)
  5. Output at full scale: 20 mV * 200 = 4.0V (within 4.096V range)
  6. Effective resolution: 62.5 uV / 200 / 20 mV * 50 kg = 7.8 g/LSB
  7. INA128 gain resistor: Rg = 50 kOhm / (G-1) = 50k / 199 = 251 Ohm
  8. Bridge power dissipation: V^2/R = 100/350 = 286 mW
Result: Use INA128 with Rg = 249 Ohm (0.1%) for G = 201. Resolution is 7.8 g per ADC count, exceeding the 10 g target.

Practical Tips

  • ✓For cost-effective embedded weighing, the HX711 24-bit ADC provides complete signal conditioning (excitation, amplification, ADC) for <$2; set gain to 128 for 2 mV/V cells or 64 for 4 mV/V cells per Avia Semiconductor datasheet
  • ✓Shield low-level signal wiring between load cell and amplifier to reject 50/60 Hz pickup; twist excitation and signal pairs separately per OIML R76 EMC requirements
  • ✓Zero the bridge output in firmware after mechanical installation; mounting hardware preload shifts the zero point, requiring tare correction of typically 5-20% full scale

Common Mistakes

  • ✗Forgetting to derate gain for amplifier bandwidth: INA128 has 1.3 MHz GBW, so gain of 500 limits bandwidth to 2.6 kHz; for 10 Hz weighing with settling, this is adequate, but dynamic weighing at 100 Hz requires lower gain or faster amplifier
  • ✗Omitting 6-wire sensing: lead resistance in excitation wires causes gain error proportional to Rlead/Rbridge; 5 m of 22 AWG adds 0.42 Ohm, causing 0.12% error on 350 Ohm bridge without remote sense
  • ✗Using single-supply op-amp without level shift: bridge output swings +/-Vfs around Vex/2 common mode; rail-to-rail op-amp needs Vref at mid-supply, or use INA125 with built-in 2.5V reference

Frequently Asked Questions

mV/V (millivolts per volt) is the normalized load cell output: millivolts of bridge output per volt of excitation at full-scale load. A 2 mV/V cell on 10V excitation produces 20 mV at rated capacity. This specification makes sensitivity independent of excitation voltage, enabling direct comparison between load cells. Industrial cells range from 1 mV/V (high-stiffness for dynamic weighing) to 3 mV/V (general purpose) per OIML R60 specifications.
A 12-bit ADC provides 4096 counts; for a 50 kg range, that is 12.2 g resolution. A 24-bit ADC provides 16.7 million counts, theoretically sub-milligram resolution. However, noise, mechanical vibration, and thermal drift limit practical resolution to 16-18 effective bits (65,000-262,000 counts). For legal-for-trade scales requiring 3000+ divisions (OIML Class III), use 24-bit ADCs with averaging. For industrial monitoring at +/-1% accuracy, 12-bit is sufficient and faster (10-100 kSPS vs 10-80 SPS for sigma-delta).
Not recommended for load cells. A Wheatstone bridge has floating differential output; standard op-amp difference amplifiers require 0.01% matched resistors to achieve 80 dB CMRR. Instrumentation amplifiers (INA128, AD620) have laser-trimmed internal matching for CMRR >100 dB and need only one external gain resistor. The cost premium ($2-5 vs $0.50) is justified by eliminating four precision resistors and achieving reliable performance per Analog Devices application note AN-671.

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