The evolution of electrodermal activity (EDA) circuitry in polygraph data acquisition systems has played a central role in enhancing both the accuracy and safety of modern lie detection. Two key products in this progression are Lafayette Instrument Company’s LX4000 and LX5000 systems. Both have demonstrated exceptional reliability in diverse and sometimes challenging environments, setting a standard for functionality, safety, and innovation in the polygraph profession.
Introduction to the LX4000
The LX4000 was introduced in 2002 as a robust digital data acquisition system (DAS). Its original design featured circuits for both skin conductance and skin resistance, although only the skin resistance circuit was actively used. The initial configuration employed a 3.6 µA current and supported measurements across 10 kΩ to 1 MΩ. While this range exceeded the typical laboratory EDA span (50 kΩ to 500 kΩ), customer demand for broader capabilities prompted redesigns.
By 2004, the EDA range was expanded to 2 MΩ and current increased to 10 µA, allowing the system to capture a wider spectrum of physiological responses observed in field conditions. This update required an auxiliary daughter board, later integrated directly into the main board by 2006. Importantly, the changes had no effect on safety or performance—both versions operated identically.
Minor refinements followed, including the LX4000A (2009), which streamlined manufacturing, and the LX4000B (2010), which incorporated dedicated auxiliary channels for seat, hands, and feet sensors. The LX4000B also adopted the EDA skin-resistance circuit from the LX5000, using a constant 6.7 µA current over a 10 kΩ to 2 MΩ range.
The LX5000: Modular Innovation
Launched in 2008, the LX5000 was conceived as a modular polygraph system with detachable sensor modules, including the option for wireless use. While wireless solutions saw limited adoption, demand for the LX5000’s core design endured, and the system was later consolidated into a single DAS unit.
The LX5000 EDA circuit distinguished itself by supporting both skin resistance and skin conductance modes, selectable via software. In resistance mode, the circuit applied a 4 µA current and measured from 10 kΩ to 2.3 MΩ. In conductance mode, it used a constant voltage circuit with automatic ranging and a maximum current of 10 µA, accommodating values from 5 kΩ to 4 MΩ. Both modes delivered linear, reliable responses, reinforcing its reputation as a versatile field and research tool.
Safety by Design
Polygraph instruments like the LX4000 and LX5000 are engineered for intrinsic safety. Operating at only 5 volts via USB, they fall far below the 60-volt DC threshold outlined in IEC 60601-1, the international medical equipment safety standard. Importantly, the only subject-contact electrodes—the EDA sensors—are electrically isolated, ensuring no risk of electrical shock even if exposed to external high-voltage sources.
Protective measures such as insulation, anti-static enclosures, and improved USB converters further reduced risks associated with electrostatic discharge. Safety in both systems is therefore grounded in sound engineering principles rather than aesthetics or circuit layout.
Enhancing Stability
Field reports from early LX4000 users revealed occasional disconnections between the DAS and computer. Investigation traced these to software timeouts and electrostatic discharge vulnerabilities. Lafayette engineers addressed the issues by:
-
Eliminating software timeouts for continuous operation.
-
Switching from metallic to anti-static plastic enclosures.
-
Replacing sensitive USB converter components with more robust alternatives.
-
Introducing pneumatic activity sensors to replace piezoelectric versions.
These refinements, combined with testing against repeated electrostatic discharge, rendered both LX4000B and LX5000 highly stable and reliable in long-term operation.
EDA Signal Processing Advances
Both systems digitize EDA signals using 24-bit analog-to-digital conversion before transmitting raw data to the computer. While firmware facilitates data handling, no signal processing occurs within the devices themselves. Instead, signal management evolved through LXSoftware.
Key developments included:
-
Legacy Auto EDA (2002–2007): Returned traces to baseline within set timeframes but sometimes produced scoring inconsistencies.
-
Legacy Detrended EDA (2007–2010): Reduced scoring differences by mathematically stabilizing baselines.
-
Present Detrended EDA (2010–present): Displays sympathetic responses without filtering, offering near-perfect correlation with manual mode.
-
Current Auto EDA (2010–present): Provides smooth, stable baselines with tonic and phasic responses visible. Later refinements optimized filter frequencies, balancing diagnostic clarity with natural waveform preservation.
These innovations reduced the likelihood of numerical scoring differences between manual and automatic EDA modes—a long-discussed challenge within the polygraph profession.
Conclusion
The LX4000 and LX5000 exemplify Lafayette Instrument Company’s commitment to engineering excellence, safety, and responsiveness to professional feedback. From expanded EDA ranges and modular innovations to safety-focused circuit isolation and advanced signal processing, these systems have continually evolved to meet the needs of examiners in law enforcement, government, research, and private practice worldwide.
As the polygraph profession continues to evolve, Lafayette Instrument Company remains dedicated to innovation, rigorous safety standards, and the development of instrumentation that professionals can trust in any environment.