Precision Current Source

Built a 0–20 mA Howland current pump with 0.05% FS error for sensor excitation and calibration benches.

Approach & key decisions

Topology
Improved Howland pump with matched thin-film resistors, a low-offset amp, and separate force/sense paths so cable resistance does not show up in the loop.
Protection
Series PTFE and clamp diodes keep the output alive through a shorted load or an inductive kick without destroying the amp.

System spec

Amp: OPA189 zero-drift
Range: 0–20 mA, 12-bit DAC set
Board: 2-layer, guard rings around sense
Tools: KiCad, 6.5-digit DMM, load bank

Build gallery

Compliance sweep into a programmable load. Flat to 18 V before the rail limit.

Sense pairs star back to the amp; force traces are thicker and shorter.

Debugging timeline

Bring-up
Output oscillated into capacitive loads above 10 nF.
Added a small isolation resistor and retuned the feedback compensation for 45° phase margin.

Results & final demo

0.05% full-scale error after a one-point trim at 10 mA.
Noise floor under 2 µA RMS into a 500 Ω load, 0.1–10 Hz.

The problem

Lab sensor work needed a quiet, load-independent current source. Bench supplies drifted and most cheap modules lacked Kelvin sense or proper compliance headroom.

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