Geoelectrical surveying works by injecting a controlled electric current into the ground through one pair of electrodes, labeled A and B, and measuring the resulting voltage with a second pair, M and N. Because different earth materials conduct electricity differently, the measured response reveals the spatial distribution of electrical resistivity beneath the surface. This simple principle underpins an astonishing range of applications: locating groundwater aquifers, tracking contamination plumes and landfill leachates, characterizing soil for civil engineering foundations, exploring for minerals, monitoring dams and levees, studying archaeological sites, and even watching dynamic subsurface processes unfold over time through electrical resistivity tomography. The technique is non-invasive, relatively fast, and, with the right instrumentation, remarkably precise.

The problem, the researchers argue, is that the right instrumentation has historically been expensive and closed. Commercial resistivity meters prioritize robustness and automation, but their proprietary electronics and firmware make it nearly impossible for scientists to modify the architecture or adapt it to unusual experimental needs. Open-hardware initiatives such as OhmPi have begun to change that landscape, demonstrating a growing appetite for accessible, reproducible, and customizable instruments. PRISM pushes this trend further by pairing an unusually high-resolution analog-to-digital converter with automatic polarity reversal and a modern browser-based control interface, all under permissive open-source licenses: GPL-3.0 for the software and CERN-OHL-S-2.0 for the hardware.

At the heart of the instrument sits the LTC2440, a 24-bit delta-sigma analog-to-digital converter chosen for its high resolution and low noise. In geoelectrical work, both the injected currents and the measured potential differences can be vanishingly small, so conversion resolution matters enormously. A 24-bit converter reduces quantization effects and allows the system to resolve fine variations in signal that a conventional 16-bit converter would blur into noise. To keep the conversion honest, the designers paired the ADC with an LT1236-5 precision voltage reference providing a stable 5.000 volts with minimal thermal drift, and they built a virtual ground circuit around an LTC2051 operational amplifier that shifts the signal’s DC level to 2.5 volts, allowing bipolar measurements in a single-supply system without violating the converter’s input limits.

The measurement chain continues with an LT1007 low-noise operational amplifier configured as a high-impedance voltage follower for the voltage coming from the ground, complete with a 10-kilohm multiturn potentiometer for offset compensation, and a resistive divider that can attenuate signals by a factor of five when excitation levels climb. Current is measured indirectly through a precision 22-ohm shunt resistor with a tolerance of just 0.1 percent, switched into the circuit by a relay only when needed to prevent parasitic effects. An Arduino Pro Micro board based on the ATmega32U4 microcontroller orchestrates everything, coordinating relay switching, ADC readout over SPI, and communication with the user’s computer at 115200 baud.

Perhaps the most elegant feature is the automatic polarity reversal mechanism. Every measurement point is sampled twice: once with current flowing in the forward direction and once in reverse. The voltage measured at the electrodes is the superposition of the true resistive response and the spontaneous potential that the ground naturally generates. When current flows one way, the measured voltage equals the response plus the spontaneous potential; reversed, it equals the response minus the spontaneous potential. Summing the two readings cancels the spurious component entirely, yielding twice the true resistive voltage. Crucially, the system reverses the polarity of the potential electrodes simultaneously with the current electrodes, guaranteeing that the final recorded value is always positive regardless of injection direction.

The team validated the instrument rigorously in the laboratory. Calibration curves were constructed for voltage and current using a Siglent SDM3055 digital multimeter as reference, a precision LT1021-based voltage source, and a Newport Model 505 laser diode current source, with linear regression coefficients of 0.9999 across all ranges. Statistical testing with 10,000 consecutive measurements of a 0.999091-volt reference produced a mean of 0.999139 volts, a standard deviation of just 2.57 microvolts, and an effective number of bits, or ENOB, of 19, meaning real-world noise degrades the nominal 24-bit resolution to a still-extraordinary 19 bits. Signals on the order of 100 microvolts sit comfortably above the noise floor, and the error distribution followed a clean Gaussian profile, confirming that fluctuations stem from random thermal noise rather than systematic drift.

The head-to-head comparison with commercial hardware is where the story becomes striking. Measuring precision resistors spanning 10 ohms to 68 kilohms under conditions mimicking field surveys, PRISM kept its maximum measurement error to just 0.95 percent, below the nominal 1 percent tolerance of the test resistors themselves, with a mean relative error of 0.39 percent. The commercial PASI MOD. 16GL-N, by contrast, stayed below 1 percent error only up to roughly 2200 ohms, then degraded progressively, reaching approximately 27 percent error at 68 kilohms. The researchers are careful to note that the advantage cannot be credited to the 24-bit converter alone; it emerges from the complete measurement chain, including the low-noise reference, analog conditioning, printed circuit board design, firmware, and polarity-reversal technique working in concert.

The operating envelope suits field practice well. The instrument measures bipolar voltages from 100 microvolts to 10 volts and currents from 100 microamps to 100 milliamps, the latter capped by the 2.5-volt ADC input limit across the 22-ohm shunt. Injected currents in typical geoelectrical surveys rarely exceed 100 milliamps, so the range covers practical conditions. The browser-based graphical user interface, built with HTML, CSS, and JavaScript and communicating through the Web Serial API, lets users configure Wenner, Schlumberger, or Dipole-Dipole electrode arrays, computes resistance, geometric factor, and apparent resistivity in real time, and exports data to CSV for inversion processing. The firmware even discards the first ADC conversion after each relay switch to avoid transient artifacts and waits for the soil’s electrical response to stabilize before sampling.

The authors are candid about limitations: the system is designed exclusively for direct-current resistivity and cannot perform AC impedance or induced polarization surveys, its acquisition speed favors precision over rapidity, and field validation under real survey conditions remains future work. Still, the implications extend well beyond geophysics. The same four-point probe architecture can characterize the resistivity of graphene and other two-dimensional materials, perovskite thin films, and thermoelectric compounds such as bismuth telluride. With complete design files, bills of materials, assembly instructions, and firmware hosted openly on the Open Science Framework, PRISM invites a global community of researchers, educators, and tinkerers to replicate, modify, and improve it. In a field where a single commercial instrument can cost as much as a car, a 360-dollar, 19-effective-bit, fully open alternative may prove to be one of the most consequential pieces of scientific hardware published this year.

Subject of Research: An open-source, high-resolution analog-to-digital converter-based data acquisition system for geoelectrical resistivity prospecting

Article Title: High-resolution analog-to-digital converter-based data acquisition system for geoelectrical prospecting

Article References: Jiménez, M. L., Ruiz, A. G., Martínez, P. P., & Navarro, J. Á. (2026). High-resolution analog-to-digital converter-based data acquisition system for geoelectrical prospecting. HardwareX, 28, Article e00837. https://doi.org/10.1016/j.ohx.2026.e00837

Image Credits: AI Generated

DOI: 10.1016/j.ohx.2026.e00837

Keywords: geoelectrical prospecting, electrical resistivity, open-source hardware, analog-to-digital converter, 24-bit ADC, PRISM instrument, subsurface imaging, resistivity meter, polarity reversal, HardwareX, low-cost instrumentation, electrical resistivity tomography