Source Measurement Units (SMU) and Per Pin Parametric Measurement Units (PPMU) are essential electronic test and measurement instruments used in automated test equipment (ATE) applications. These devices provide stimulus to a device under test and precise measurement of voltage and current, which is important in semiconductor characterization, component testing, and system validation. As the demand for higher throughput and greater measurement accuracy grows, designers face increasing challenges in balancing speed, resolution, and system latency.

Analog vs Digital Control Loops

Historically, SMU and PPMU designs relied on fast analog control loops. While these analog loops excelled in speed, they struggled with drift, limited resolution, and 1/f noise at low frequencies. As a result, users experienced difficulties in achieving stable and repeatable measurements, particularly when testing advanced semiconductor devices. The transition to digital control loops brought improved accuracy, repeatability, and programmability, but introduced new challenges, such as quantization noise and sampling artifacts but mainly increased loop latency.

Latency in Digital Loops

Every element in the signal chain, from Analog-to-Digital conversion, through digital processing to the Digital-to-Analog output, contributes to the overall loop delay. Excess latency degrades system responsiveness, limits bandwidth, and reduces measurement throughput. It is essential to consider both the inherent latency of each device and the cumulative effect across the entire signal chain. Strategies for minimizing latency include selecting ADCs and DACs with fast sampling and update rates, optimizing FPGA firmware for efficient data handling, and reducing unnecessary processing stages.

Data Converter Requirements

The need for high precision with low latency drives stringent requirements for ADCs and DACs used in digital control loops. Data converters with a high resolution (typically 16–20 bits) enable precision measurement, while absolute accuracy (low INL/DNL, minimal offset and gain error) underpins reliable test results. A converter’s sampling speed influences loop latency. Higher speeds allow for faster system response but may increase power consumption and compromise precision. For example, an ADC with a pipeline architecture may introduce additional latency compared to a SAR-type ADC, which offers lower result latency and lower power than a pipeline ADC.

Analog Devices Hardware in the Loop Kits

Analog Devices offers a complete reference design for hardware-in-the-loop testing. CN0584 and CN0585 integrate four channels of precision, low latency ADCs and DACs, precision signal conditioning, and flexible interfaces, enabling rapid prototyping and deployment in latency-critical environments. The four sense channels, based on the ADAQ23876 16-bit 15 MSPS ADC enables analog signal capture to data output in 120 ns and the four drive channels, using the AD3552R 16-bit 16.5 MSPS DAC gives signal generation from data write to settled output in less than 200 ns. The CN0585 board offers a complete interface to an FPGA that includes HDL example code and integration with MATLAB and Python.

CN0585 and CN0584 Evaluation Kits.

Upgrade Path

For systems currently utilizing the ADAQ23876, upgrading to the AD4080 20-bit, 40 MSPS ADC provides a significant reduction in conversion latency to 46.25 ns and enhances measurement resolution from 16 to 20 bits, with excellent linearity, low 1/f noise and low drift. Further improvements can be made to the force circuitry by time-interleaving the AD3552R to provide a faster update rate.

Conclusion

SMU and PPMU designers face a complex landscape, balancing speed, accuracy, and latency in digital control loops. Minimizing loop latency requires careful component selection, optimized FPGA design, and system-level analysis. Analog Devices’ CN0584 and CN0585 kits offer practical solutions for hardware-in-the-loop test scenarios, while the transition to the AD4080 ADC provides a clear upgrade path for latency and performance improvements. By addressing these challenges, engineers can deliver reliable, high-speed test systems that meet the evolving demands of electronic measurement applications.

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