blog UM DIY Enthusiast Rebuilds Classic Highprecision Test Instrument

For engineers requiring basic semiconductor DC testing and curve tracing capabilities, commercial solutions like the Keithley 2400 Source Measure Unit (SMU) often come with a steep $4,000+ price tag. This financial barrier has inspired an ambitious open-source project to recreate – and potentially surpass – the core functionality of classic SMU instruments at a fraction of the cost.

Source Measure Units remain indispensable tools in semiconductor testing, materials research, and precision electronics. These instruments combine voltage/current sourcing with measurement capabilities in a single integrated package, enabling critical tasks like device characterization, parameter extraction, and leakage current detection with unmatched efficiency.

Vintage instruments like the Keithley 236/237 series set industry standards in the 1980s-90s with their wide measurement ranges (±1.1V to ±1100V) and exceptional precision (down to 1nA current measurements). While technologically outdated, their analog circuit designs and meticulous component selection continue to inform modern instrumentation.

Contemporary alternatives like the Keithley 2400/2600 series offer improved integration and user interfaces but maintain premium pricing. This market gap has motivated the development of an open-source SMU that preserves classic functionality while incorporating modern design advantages.

The project aims to deliver Keithley 236/2400-level core performance at approximately 10% of commercial pricing. Key design specifications include:

• Voltage output: Three ranges (±1.5V, ±15V, ±150V) with 30,000-step resolution
• Current measurement: 1µA to 100mA full-range coverage (expandable to 1A)
• Control architecture: Three independent DACs for voltage output and current clamping
• Modular design: Separable analog, power amplification, and front panel boards
• Cost target: $350 bill-of-materials

The design employs three 16-bit bipolar DACs for independent control of output voltage and current limits. Current measurement utilizes switched precision shunt resistors, with special attention given to minimizing leakage currents below 1nA through careful component selection and layout optimization.

Adapting classic 236 concepts, the output stage uses cascode-configured bipolar/MOSFET combinations to achieve ±170V floating supply capability. Local current limiting protects both the instrument and devices under test.

The design incorporates multiple protective measures including floating circuit isolation, visual high-voltage indicators, automatic capacitor discharge systems, and fail-safe output connectors.

All schematics, PCB layouts, firmware, and documentation will be publicly available, encouraging community participation in development and refinement. This approach aims to create not just a functional instrument, but a platform for ongoing innovation in accessible precision measurement technology.