Inverting Op-Amp Application Guide: Improving FSR Linearity and Stability

What Is an Operational Amplifier?

An operational amplifier (Op-Amp) is a common analog component designed to amplify the voltage difference between its two input terminals. It has three main pins:

• Non-inverting input (+)
• Inverting input (–)
• Output

The Op-Amp continuously compares the voltages at the two inputs and adjusts the output to bring the inputs as close to balance as possible.

What Is an Inverting Amplifier?

An inverting amplifier is a basic Op-Amp circuit configuration where the input signal is fed into the inverting (–) terminal, and a feedback resistor controls the gain.

Formula:

V_out = – (R_f / R_in) × V_in

• R_in: Input resistor
• R_f: Feedback resistor
• V_in: Input voltage
• V_out: Output voltage (inverted)

Thanks to the feedback mechanism, the Op-Amp keeps the inverting terminal virtually at 0V, known as a "virtual ground."

Why Not Just Use a Voltage Divider?

Although a voltage divider can convert an FSR's resistance to a voltage, it has limitations:

• Nonlinear output: FSR + resistor produces a nonlinear voltage curve
• Small signal range: Output may range from only 0.5V to 1V, reducing ADC accuracy
• High impedance and noise sensitivity: Weak signals are prone to interference

An inverting Op-Amp addresses all these issues by providing stable, amplified, and more linear outputs.

Circuit Design: Using Inverting Op-Amp with FSR

Basic Connection:

5V ── R1 ──┬── (–) Op-Amp (+) ── GND
           │        │
          FSR      Output → MCU ADC
           │
          GND

Another version puts the FSR in the feedback path, forming a transimpedance amplifier. This setup can translate pressure directly into a near-linear voltage change.

How to Select and Adjust Resistors

• Gain = Rf / Rin: Adjust this ratio to control output range
• For small Vin variations (e.g., 0.1–0.5V), consider 10×–50× gain
• Be mindful of Op-Amp output limitations: single-supply devices may not reach full 0V or 5V

Common Op-Amps:

Conclusion

An inverting amplifier is ideal for improving the signal quality of FSR sensors. It offers wider voltage output ranges and better linearity. For early prototyping, you can start with a voltage divider, and later upgrade to an Op-Amp circuit for enhanced performance. If you're not familiar with voltage dividers, check out our previous article here.