Practical case: Decoupling with inductor in sensor supply

Materials

• 1 × DC bench power supply (+5 V or +3.3 V)
• 1 × Inductor L1 = 10 µH (≥150 mA rating, low DCR)
• 1 × Capacitor C1 = 10 µF, X5R/X7R ceramic (≥2× supply voltage)
• 1 × Capacitor C2 = 100 nF, X7R ceramic
• 1 × Resistor R1 = 1 kΩ (for ripple injection)
• 1 × Function generator (sine output)
• 1 × Oscilloscope (2 channels) and probes
• 1 × Sensor module (e.g., analog temperature sensor), 5–20 mA typical load
• 1 × Breadboard or PCB and hookup wires

Wiring guide

• Power input:
• Set the DC bench supply to +5 V (or your sensor’s rated VDD).
• Provide one common GND reference point for all instruments and the circuit.
• Series inductor:
• Place L1 in series between the main +V rail and the sensor’s supply node. This node after L1 is V_SENS.
• Sensor decoupling:
• Connect C1 (10 µF) from V_SENS to GND.
• Connect C2 (100 nF) from V_SENS to GND.
• Connect the sensor module VDD to V_SENS; connect its GND to the common GND.
• Ripple injection (to simulate upstream noise):
• Connect the function generator output (Vinj) through R1 (1 kΩ) to the upstream node before L1 (this node is V_IN).
• Set the generator to sine, 0.2 Vpp, 0 V offset, 100 kHz. Connect generator ground to GND.
• Oscilloscope:
• CH1 to V_IN, CH2 to V_SENS. Use short ground springs for accurate ripple readings.
• Abbreviations used:
• V_IN = Node before L1 (upstream side, noisier rail).
• V_SENS = Node after L1 (sensor supply rail).

Schematic

                                +5 V
                                 │
                                 │
                 ┌───────────────┴─────────────────────────● Vpre
                 │                                         │
                 │                                        ┌┴┐  C1 10 µF
                 │                                        │ │
                 │                                        │ │
Vac ─────────────┬───────────────┐                        └┬┘
                 │               │                         │
                ┌┴┐  R1 100 Ω   │                         │
                │ │             │                         │
                │ │             │                         │
                └┬┘             │                         │
                 │              │                         │
                 └──────────────┴───────────────┬─────────┘
                                                │
                                               ┌┴┐  L1 22 µH
                                               │ │
                                               │ │
                                               └┬┘
                                                │
                                                ├──────────────● Vsns
                                                │
                               ┌┴┐  C2 100 nF   │    ┌───┐  U1 TMP36
                               │ │              │    │   │
                               │ │              ├────┤   │
                               └┬┘              │    │   │
                                │               │    └───┘
                               ┌┴┐  C3 10 µF    │      │
                               │ │              │      └────────────● Vout
                               │ │              │
                               └┬┘              │
                                │               │
────────────────────────────────┴───────────────┴─────────────────── GND

Measurements and tests

• Initial setup:
  • Set supply to rated sensor voltage and verify the sensor powers on.
  • Set function generator to 100 kHz, 0.2 Vpp, 0 V offset; connect through R1 to V_IN.
• Ripple before vs. after the inductor:
  • With the scope:
    • CH1 at V_IN, CH2 at V_SENS.
    • Measure peak-to-peak ripple on both channels. Expect V_SENS ripple to be significantly lower than V_IN.
• Frequency sweep:
  • Sweep generator frequency from 10 kHz to 1 MHz.
  • Observe attenuation increasing around and above the LC corner frequency f_c ≈ 1/(2π√(L1·C1)).
• Bypass sanity check:
  • Temporarily short L1 (jumper across L1 terminals) to bypass the inductor.
  • Measure ripple at V_SENS again; it should rise and nearly match V_IN. Remove the jumper afterward.
• Probe technique:
  • Repeat key measurements using probe spring ground (very short return) to avoid adding ground lead inductance. Ripple readings should become more accurate and typically lower-noise.

Common mistakes

• Using electrolytic-only decoupling: Without a small 100 nF ceramic in parallel, high-frequency attenuation suffers.
• Long leads to C1/C2: Keep decouplers physically close to the sensor VDD–GND pins to minimize ESL/ESR.
• Oversized ripple injection: Too large Vinj can exceed sensor VDD. Keep ≤ 0.2–0.3 Vpp and verify total VDD remains in range.
• Inductor saturation: Choose L1 with current rating above the sensor’s peak draw; a saturated inductor loses filtering.

Safety and handling

• Do not exceed the sensor’s maximum VDD. Confirm worst-case VDD = DC level ± ripple remains within limits.
• Inductors can get warm if undersized; check L1 temperature during operation.
• Tie all grounds together at a solid reference point to avoid ground bounce.

Improvements

• Add a ferrite bead instead of (or in series with) L1 for better high-frequency suppression.
• Use a π-filter: C (upstream) – L – C (downstream) to further attenuate ripple.
• Place a small series resistor (1–2 Ω) with C1 to damp LC resonance if you observe ringing.
• Power the sensor from a post-filter LDO: +V → L1 → LDO → C1/C2 → sensor VDD.

Validation: The schematic shows +5 V at the top, GND at the bottom, all components (R1, L1, C1, C2, Sensor) identified and connected, with measurement dots labeled V_IN and V_SENS at the exact nodes. No loose or disconnected ends are present.

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Carlos Núñez Zorrilla

Electronics & Computer Engineer

Telecommunications Electronics Engineer and Computer Engineer (official degrees in Spain).

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