# The test method would not have worked. We found out before building it | OneStack > Case study: a test bench for electric-field sensors that measures a few picofarads in air and gives a pass/fail result in seconds. URL: https://www.onestack.it/en/case-studies/capacitive-sensor-test [← All case studies](/en/case-studies)Test systems for high-reliability applications # The test method would not have worked. We found out before building it A bench that tells in seconds whether an electric-field sensor works, measuring capacitances of a few picofarads in air. The method planned at the start was ruled out with measurements, before any money went into hardware. ![3D render of the measurement bench: the sensor under test and three boards inside a black anodised aluminium housing, with two BNC connectors and an M12 connector](/_astro/banco-sensore-capacitivo.DpjM9RDJ_2dpT9y.webp)![](/_astro/banco-sensore-capacitivo-xray.CdoncSBR_Z1LXalp.webp) ~1.4% estimated accuracy without calibration (requirement: 4.2%) ≤ 10 fF estimated resolution (requirement: 50 fF) 42% margin on the acceptance threshold Measurement chain **Signal generator**STM32U5 · DAC + interpolation on FMAC **Sensor under test**triaxial cable with active guard **Measurement front-end**auto-balancing bridge with TIA **Acquisition and processing**simultaneous ADCs · I/Q demodulation **Operator verdict**desktop software · go / no-go ## The context The client makes an electric-field sensor that operates under water. Production needed a fast test to tell whether each sensor works, **without immersing it** and without relying on the final product’s electronics. In air, though, the quantities involved are tiny: capacitances of a few picofarads. The sensor goes into a mission-critical system, so the bench had to meet tight tolerances. ## The problem The method in the initial proposal looked reasonable on paper: use the existing measurement chain and compare the response with a threshold. Before designing any board we ran a measurement campaign with commercial instruments, built a circuit model of the sensor and analysed its failure modes. The result was clear: **that method would not reliably tell a good sensor from a faulty one.** The cable’s parasitic capacitance was of the same order as the signal to be measured. We told the client straight away, with the data. ## Technical choices, and why **An auto-balancing bridge with a transimpedance amplifier.** The same architecture as lab LCR meters: it holds the sensor at virtual ground and measures the current flowing through it. For capacitances this small it is far more stable than a charge amplifier. **Triaxial cable with active guard.** The middle shield is driven at the same voltage as the centre conductor, so the cable capacitance carries no current and disappears from the measurement. This is what makes picofarad measurements repeatable. **I/Q demodulation.** Separating the in-phase and quadrature components distinguishes capacitance from losses: a faulty sensor changes not only “how much” it reads but also “how”. **Signal generation on the microcontroller.** A DDS on the STM32U5, with polyphase interpolation computed by the FMAC hardware accelerator, produces a clean sine at several frequencies with no external generator and no CPU load. **Multi-frequency measurement.** Capacitance is estimated at several frequencies and compared: a defect also shows in how the response changes with frequency. ## How we verified it - SPICE simulation of the front-end before layout. - Measurement campaigns with lab instruments driven over SCPI, to compare method and prototype repeatably. - Desktop software with automated tests on the data processing. ## The result Performance estimated from circuit analysis: about **1.4% accuracy without calibration** against a 4.2% requirement, and a resolution of **10 fF or better** against the 50 fF required, with a **42% margin** on the acceptance threshold. The biggest value came at the start: the client invested in a method that works, instead of finding out in production that the test could not tell good parts from faulty ones. ## What we bring to your projects Analog measurement front-ends, signal processing on the microcontroller, automated test benches, and the habit of checking an idea with measurements before building it. ## Services - PCB design - Firmware development ## Technologies - STM32U5 - DDS - FIR on hardware accelerator - Transimpedance amplifier - Active guard - I/Q demodulation - SPICE - C# WPF - SCPI ## More case studies [ ![3D render of the control board: microcontroller in the centre, connectors for the two isolated channels on the side, pluggable terminal blocks and power and service connectors](/_astro/interfaccia-rugged.CkQk4iYb_1A6Ozp.webp)![](/_astro/interfaccia-rugged-xray.CZb6dpZZ_Z1kxhOu.webp) 23 tests in ~75 s Systems for harsh environments ### A rugged interface that updates in the field and never bricks A portable, battery-powered unit that powers and controls two remote devices over isolated channels. The hard part was making it reliable far from any lab. Its firmware updates in the field and, if something goes wrong, rolls back on its own to the last working version. Read the case → ](/en/case-studies/rugged-interface-secure-ota)[ ![3D render of the Bluetooth Channel Sounding anchor board: nRF54L15 SoC with chip antenna, accelerometer, power from USB-C or a 12–24 V terminal block](/_astro/ancora-ble.Ckke5SRn_1319HC.webp)![](/_astro/ancora-ble-xray.yah8tbey_Z2nf2oG.webp) 3 layers Retail and large public spaces ### Indoor localization in shopping centres, without collecting personal data Counting and following people flows in large public spaces, and locating with centimetre precision only what needs it. A three-layer architecture, each layer with the right radio technology. Read the case → ](/en/case-studies/indoor-localization)[ ![3D render of the test bench: two steel plates clamped by tie rods with the discs and the boards inside, handles on the sides and the actuator on top](/_astro/banco-mems.BVjo7je3_Z2tCAuW.webp)![](/_astro/banco-mems-xray.CArss8sQ_Z1wwNRX.webp) \> 40 dB Test systems for high-reliability applications ### Ten seconds to know whether a MEMS accelerometer really works A bench and firmware that say yes or no on whether a MEMS accelerometer is present and well coupled to the structure, even in the hands of a non-specialist operator and with the machine running. Read the case → ](/en/case-studies/mems-sensor-10s-test) ## Got a deadline? Tell us about your project: within 48 hours you get a proposal with phases and dates. If needed, we sign an NDA before seeing your files. [Request a proposal](/en/contact)[Urgent? Call +39 030 535 7183](tel:+390305357183)