Calibration
via an SPI. It has a UART interface that connects to the on-board USB bridge (FT234XD) and a receiver/ transmitter pair for the touchscreen display. A Nextion NX4024K032 3.2” 400 × 240 pixels touchscreen colour display enables user control and feedback.
THERMAL STABILITY
As expected for a high performance measurement instrument, the MAXREFDES183# incorporates multiple design techniques to maximise thermal performance and improve the overall accuracy and stability of the calibrator. Temperature monitoring is provided by two instances of a low power I2C temperature sensor, the MAX31875, which has a high accuracy of ±1°C between 0°C and 70°C. One of these ICs is placed adjacent to the MAX22000, while the other is placed in close proximity to the source terminals. This enables temperature compensation of the voltage gradient between the terminals. A 50Ω resistor (used to set I/O current) is selected to have high accuracy (0.1 per cent tolerance), and temperature stability (0.2ppm/°C). Also, four FET-driven heating resistors are located close to the MAX22000, allowing the ambient temperature to be set and stabilised before measurements are made. For added thermal stability, a small metal enclosure (approximately 1 square inch in area) is used to cover the MAX22000 along with its local temperature sensor IC and the heating resistors.
POWER MANAGEMENT The calibrator is powered by two 3.6V, 3500mAh capacity Li-Ion batteries. A MAX17320 fuel gauge monitors and manages battery states including voltage, current, and temperature, and uses external high-side FETs to guard against over/undervoltage, overcurrent, short-circuit, temperature extremes, overcharging, and internal self-discharge. Charging prescription ensures that the batteries operate under safe conditions, prolonging life between recharging. The IC automatically compensates for cell aging, temperature, and discharge rate, and provides accurate state of charge in milliampere-hours (mAh) or percentage ( per cent) over a wide range of operating conditions. A thermistor is used to measure battery temperature. A MAX17498 flyback converter boosts the USB voltage (5V
Instrumentation Monthly June 2025
Figure 7. The MAX220000 precision calibrator in a 3D print case.
nominal) to the battery pack voltage (nominally 7.2V), providing voltage input to the multiple DC-to-DC converters that generate the voltage rails required to power other components. The MAXREFDES183# consumes 110mA (typical), providing up to 31 hours of normal usage between battery charging cycles.
HIGH ACCURACY
Figure 4 shows the results for voltage and current I/O measurements recorded for temperatures between –20°C and +70°C, which demonstrate that the accuracy of the reference design to be within 0.01 per cent FSR at 25°C (room temperature).
STABILITY
While accuracy over temperature is important, calibration equipment is usually used (and stored) in environments that have a relatively stable temperature. Therefore, it is also important that they exhibit a high degree of repeatability over longer time intervals at a fixed temperature. Figure 5 shows the ±4ppm accuracy (drift) of the MAXREFDES183# when configured for a 5V analogue output in voltage mode, recorded at 15-minute intervals over a 7-day (168 hours) period, using a National Instruments PXIe 1073 DMM (7.5 digits), at an
ambient temperature of 25°C.
CALIBRATING THE CALIBRATOR Even precision calibrators require periodic calibration. The MAXREFDES183# includes a pre-programmed calibration routine (see Figure 6) that clearly explains to the user how external meters and sources should be connected and provides feedback to the user, interpreting results to determine if they are within acceptable boundaries.
MAXREFDES183# 3D printer files can be downloaded to create an enclosure for the calibrator. The 3D print case is pictured in Figure 7.
CONCLUSION
The MAXREFDES183# precision calibrator reference design demonstrates a lightweight, ultra-portable, battery-powered calibrator with the functionality, performance, and accuracy of much larger, heavier, and more expensive solutions. Potential applications for products based on this design include laboratory equipment calibration, adjustment of industrial control equipment, and field calibration of smart sensors and actuators.
Analog Devices
www.analog.com 43
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