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Load Cell Amplifier Gain

Calculate load cell output voltage, required amplifier gain, and sensitivity for Wheatstone bridge load cells.

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Formula

V_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

A load cell is a force transducer that converts mechanical load into an electrical signal using a Wheatstone bridge of strain gauges bonded to a structural element. The bridge output is typically specified as sensitivity in mV/V: the output voltage in millivolts per volt of excitation at full-scale load. A 2 mV/V load cell on 5 V excitation produces 10 mV at full scale. Because this millivolt-level signal must drive an ADC (often 5 V or 3.3 V full scale), a precision instrumentation amplifier is required. The required gain is G = V_ADC_FS / V_FS, where V_ADC_FS is the ADC full-scale voltage and V_FS is the load cell full-scale output in volts. Common ICs include the INA125 (built-in voltage reference), INA128, and dedicated load cell amplifiers such as the HX711 (24-bit, includes ADC). Power dissipation in the load cell bridge is P = V_ex²/(4R_bridge).

Worked Example

Problem
A 50 kg load cell has 2 mV/V sensitivity on 5 V excitation, feeding a 12-bit ADC with 3.3 V reference. What amplifier gain is needed, and what is the resolution in grams?
Solution
1. Full-scale output: V_FS = 2 mV/V × 5 V = 10 mV 2. Required gain: G = 3300 mV / 10 mV = 330 V/V 3. ADC step: LSB = 3.3 V / 4096 = 0.806 mV 4. Output step per LSB: 0.806 mV / 330 = 2.44 μV 5. Load per LSB: (2.44 μV / 10 mV) × 50 kg = 12.2 g/LSB Result: Use a gain of 330 (e.g., INA128 with R_G = 604/329 × 100 Ω ≈ 150 Ω); resolution is about 12 g per ADC step.

Practical Tips

  • ✓For embedded systems, the HX711 24-bit ADC + amplifier module provides a complete solution at very low cost, with a built-in programmable gain of 64 or 128.
  • ✓Shield the low-level signal wiring between the load cell and amplifier to reduce 50/60 Hz pickup; twist the excitation and signal wire pairs separately.
  • ✓Zero the bridge output in firmware after assembly — mechanical preload due to mounting hardware shifts the zero point, requiring a tare correction.

Common Mistakes

  • ✗Forgetting to derate gain for the amplifier's gain-bandwidth product — a gain of 500 with a 1 MHz GBW op-amp leaves only 2 kHz bandwidth, which may cause settling issues.
  • ✗Omitting 4-wire (Kelvin) excitation sensing — lead resistance in the excitation wire causes a gain error proportional to I × R_lead; 6-wire remote sensing eliminates this.
  • ✗Using a single-supply op-amp without a mid-supply reference — the bridge output swings symmetrically around zero, and a rail-to-rail op-amp referenced to V_cc/2 is needed to avoid clipping negative outputs.

Frequently Asked Questions

mV/V (millivolts per volt) means the load cell outputs a specified number of millivolts for every volt of bridge excitation when the full-scale load is applied. A 2 mV/V load cell on 10 V excitation produces 20 mV at full scale. This normalised specification makes the output independent of excitation voltage.
A 12-bit ADC gives 4096 steps; over a 50 kg range that is about 12 g resolution. A 24-bit ADC gives 16 million steps, theoretically sub-milligram resolution, but noise, mechanical vibration, and thermal drift typically limit practical resolution to 14–16 bits. For weighing scales requiring better than 1-in-1000 resolution, use 24-bit ADCs with adequate averaging.
Not for load cells. A Wheatstone bridge has a floating differential output; a standard op-amp in differential configuration requires precisely matched resistors to achieve high common-mode rejection. Instrumentation amplifiers (INAs) have precision internal matching (CMRR > 90 dB) and need only a single external gain resistor.

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