📡 Signal Conditioning Design Helper

🎯 Getting Started

Many sensors output small signals (millivolts) that need to be amplified and scaled for microcontroller ADC inputs. This tool helps you design the conditioning circuit:

• Range Mapper: Convert sensor output range to ADC input range (e.g., 0-50mV → 0-5V)
• Instrumentation Amp: High-precision differential amplifier for bridge sensors and low-level signals
• Non-Inverting Amp: Simple single-ended amplifier for general-purpose signal scaling
• Offset & Gain: Calculate circuits that both shift and scale signals

📏 Range Mapping

The most common signal conditioning task: scaling a sensor's output to match your ADC's input range.

Example 1: Thermistor voltage divider outputs 0-50mV. Your Arduino ADC reads 0-5V.

• Sensor Min: 0mV, Sensor Max: 50mV
• ADC Min: 0V, ADC Max: 5V
• Required Gain: 100× (5V / 50mV)

Example 2: Pressure sensor outputs 1-5V. Your ADC reads 0-3.3V (with offset needed).

• Sensor Min: 1V, Sensor Max: 5V
• ADC Min: 0V, ADC Max: 3.3V
• Requires: Offset (-1V) + Gain (0.825×)

🔌 Instrumentation Amplifier

Used for differential signals from bridge sensors (load cells, strain gauges, RTDs):

• High Input Impedance: Doesn't load down the sensor
• High CMRR: Rejects common-mode noise and interference
• Adjustable Gain: Set with a single resistor (R_gain)
• Common ICs: INA128, AD620, INA826, LT1167

Gain Formula: A_v = 1 + (R_ref / R_gain)

💡 R_ref is internal to the IC (typically 50kΩ for INA128, 49.4kΩ for AD620)

⚡ Non-Inverting Amplifier

Simple op-amp circuit for single-ended signals:

• High Input Impedance: Won't load the sensor
• No Phase Inversion: Output in phase with input
• Minimum Gain: 1× (buffer/follower configuration)

Gain Formula: A_v = 1 + (R_f / R_in)

⚠️ For gains less than 1× (attenuation), use a voltage divider before a buffer

🎛️ Offset & Gain Circuits

When your sensor output doesn't start at 0V, you need offset adjustment:

• Inverting Summing Amp: Combines signal with reference voltage for offset
• Dual Op-Amp Approach: First stage for offset, second for gain
• Difference Amplifier: Single-stage offset and gain (more complex resistor ratios)

💡 Tip: Use precision voltage references (LM4040, TL431) for stable offset voltages

🧮 Resistor Selection

The tool recommends standard E12/E24 series values:

• E12 Series: 10% tolerance, 12 values per decade (1.0, 1.2, 1.5, 1.8, 2.2, 2.7, 3.3, 3.9, 4.7, 5.6, 6.8, 8.2)
• E24 Series: 5% tolerance, 24 values per decade (includes E12 plus 1.1, 1.3, 1.6, 2.0, 2.4, 3.0, 3.6, 4.3, 5.1, 6.2, 7.5, 9.1)

Input Resistor Range: Typically 1kΩ-100kΩ (lower values for low-frequency/DC signals, higher values for better input impedance)

Feedback Resistor: Calculated based on R_in and desired gain

⚠️ Practical Limits: Op-amp gains above 1000× are challenging due to noise and stability. Consider cascaded stages.

📐 Design Considerations

• Input Bias Current: Use FET-input or CMOS op-amps for high-impedance sensors
• Offset Voltage: Low offset op-amps (e.g., OP07, OPA2277) for precision
• Bandwidth: Ensure op-amp bandwidth supports your signal frequency
• Noise: Lower resistor values reduce thermal noise (but increase power consumption)
• Rail-to-Rail: Use rail-to-rail op-amps when output must reach supply rails