The Data Acquisition System (DAQ) is an indispensable part of modern electronic equipment. It is mainly used to capture signals generated by electronic devices and sensors with high precision, to support applications such as real-time processing, hardware-in-the-loop simulation, automatic testing, and data recording.
For data acquisition systems that need to achieve 7.5-digit or even higher precision, the industry has traditionally mostly adopted multi-slope integrating ADCs based on discrete components. Although this type of ADC can provide reasonable measurement accuracy, its design and debugging processes are usually more complex. Over the past decade, 24-bit Σ-Δ ADCs have been widely used in the design of 6.5-digit Digital Multimeters (DMMs), while higher-performance ADCs have become a bottleneck for achieving 7.5-digit precision and linearity. At the same time, another significant challenge comes from the voltage reference. To achieve ultra-low temperature drift, traditional nanovolt-level voltage references usually require complex external signal conditioning circuits.
To solve these design bottlenecks of high-precision DAQs, ADI has launched an innovative solution. By combining the 24-bit, 2 MSPS AD4630-24 with a maximum INL of ±0.9 ppm, paired with the fully integrated ultra-low drift precision reference ADR1001, the precision-matched resistor network LT5400, and the zero-drift low-noise amplifier ADA4523-1, a high-precision signal chain solution with low temperature drift and low noise can be constructed. This article will introduce this solution in detail.
Part.01 Device Selection
- ADC: The AD463X series from ADI includes versions with sampling rates of 2 MSPS or 500 kSPS. It features industry-leading INL performance and high cost-effectiveness, making it ideal for instrumentation applications. The series integrates the Easy Drive function for convenient driving, a wide common-mode input range that further relaxes the requirements for drivers, an on-chip averaging filter, and supports data output up to 30 bits. In addition, ADI also offers the single-channel AD403X series, which has 3dB better noise performance than the AD463X series while maintaining similar other specifications.
- Voltage Reference:
- ADR1399: Features an ultra-low temperature drift of 0.2 ppm/°C, ultra-low noise of 1.44 μV p-p, and excellent long-term drift performance. Its low dynamic impedance makes it highly suitable as a reference for DMMs above 6.5 digits.
- ADR1000: Compared with the LTZ1000, the ADR1000 has improved noise performance and an initial accuracy of ±50 μV (a notable improvement over the LTZ1000). It also has ultra-low long-term drift, ultra-low noise of 0.9 μV p-p, and built-in on-chip heating and temperature sensors, widely used in DMMs and high-precision measurement systems.
- ADR1001: The newly launched ADR1001 integrates a buffer and precision resistors, eliminating the need for a large number of discrete devices and greatly improving user convenience. Its temperature drift performance is comparable to that of the LTZ1000, while also delivering ultra-low noise performance.
- Summary: The latest ADR1001 ranks at the top in terms of output noise, temperature drift, and long-term drift, and its high integration simplifies usage. The other references also offer industry-leading performance and are widely used in 6.5-digit to 8.5-digit DMMs, allowing users to choose flexibly according to their needs.
- Amplifier:
- ADA4522: A high-voltage zero-drift low-noise amplifier with a maximum supply voltage of 55V. It has a voltage noise density of 5.8 nV/√Hz and an ultra-low offset voltage drift of 22 nV/°C, widely used in handheld and benchtop measurement instruments.
- ADA4523-1: The newly launched amplifier, known as the industry’s lowest-noise zero-drift amplifier, has a voltage noise density of 4.2 nV/√Hz and low-frequency noise of 88 nV p-p. Its offset voltage drift is only 10 nV/°C, and its gain-bandwidth product is 5 MHz, which is wider than that of the ADA4522.
- ADA4620: Features ultra-low voltage noise while maintaining excellent DC and AC performance. Its JFET input stage is highly suitable as the first-stage high-impedance input amplifier for instrumentation.
- Precision Resistor: ADI also provides the distinctive precision resistor network LT5400, which has a resistor matching accuracy of 0.01%, a matching temperature drift of 0.2 ppm/°C, excellent long-term stability, and a variety of resistor ratio matching options.
Part.02 Experimental Setup
This 7.5-digit DAQ solution uses the AD4630-24 as the ADC and the ADR1001 as the reference. The signal conditioning section uses the ADA4523 and LT5400 to convert the ±10 V input range to the 0-5 V input range required by the ADC. The power supply section adopts a combination of DC-DC and LDO to power the analog and digital parts separately. Specifically, it uses ADI’s Silent Switcher 2 LT8609S, along with the ultra-low-noise LDOs LT3045 and LT3093. Alternatively, the dual-channel ultra-low-noise LDO LT3097 can be used to replace the latter two devices.
Part.03 Test Results
- Noise: Noise tests were conducted with the input short-circuited, with sampling rates set to 62.5 kHz and 1 MHz, and an output rate of 10 PLC. At a 62.5 kHz sampling rate, the DAQ using the ADA4523 has a noise of approximately 500 nV_RMS, corresponding to a 0.05 ppm noise level. At a higher sampling rate, the same output rate results in lower noise, so the noise at a 1 MHz sampling rate is about a quarter of that at 62.5 kHz.
- Linearity: Tests were first conducted at an output rate of 100 PLC, showing that the INL does not exceed 0.2 ppm. At an output rate of 10 PLC, the INL does not exceed 0.32 ppm, which is mainly affected by noise. When changing the reference, the INL is approximately 0.3 ppm when using the ADR1399, and about 0.4 ppm when using the AD4550D.
- Temperature Coefficient: The temperature coefficient test was carried out at an output rate of 100 PLC, with test temperatures set to 40°C, 23°C, and 0°C. Different input voltages were applied, and the changes at different temperatures were compared. Since the LT5400 determines the offset error, the offset error results are similar across different references. Meanwhile, the LT5400 and the reference have a significant impact on the gain error, and the DAQ using the ADR1001 as the reference shows better gain error characteristics.
- 24-Hour Stability: The 24-hour stability test results also show that the DAQ using the ADR1001 as the reference delivers better performance.
- Comparison with Commercial DMMs: A comparison between existing commercial DMMs and the tested DAQ shows that the performance of this DAQ falls between 7.5-digit and 8.5-digit levels. It should be noted that real DMMs include more complex circuits such as input protection, voltage/current/resistance measurement functions. The test results of this DAQ only verify the performance level that can be achieved by the signal chain constructed with these high-performance chips.
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