[Shishuo Chip Products] Comparative analysis of several classic high-performance voltage references, seeking solutions with optimized performance and features

Of all the available voltage reference types, bandgap voltage references have long been the best choice due to their combination of features and performance. However, they have not always provided the best performance compared to buried Zener references. Alternatively, the bandgap architecture makes it possible to design references with many useful features, including low dropout voltage, wide temperature range, low power, small size, wide operating range, and wide load current capability.

The Role of Voltage References: Relating Data Acquisition to the Real World

These features have led to the emergence of a large number of precision LDO regulators and high-precision voltage references. Over time, the performance of bandgap references has improved and in some cases has surpassed the stability and noise of buried Zener references while maintaining the flexibility derived from the increasing features. This article will introduce several classic voltage references and compare their performance.

The temperature characteristics are shown in the figure below, with the graph showing a 1ppm/°C box. Over a wide temperature range, the output voltage of the LT6657 is well within this box, allowing the 1.5ppm/°C specification to be tested in automated production. The LT6657 specifies low drift over the -40°C to 125°C temperature range, providing a maximum guaranteed error of <250ppm over the entire temperature range.

High-order temperature compensation maintains low and predictable output voltage errors over temperature, while state-of-the-art manufacturing processes provide consistent performance from part to part and from lot to lot. To ensure that every LT6657 shipped meets this high performance level, the LT6657's temperature coefficient is guaranteed by 100% five-temperature testing.

LT6657 Output Voltage Temperature Curve

For comparison, the LTC6655 has long been one of the highest performing bandgap voltage references on the market. Its 2ppm/°C drift specification from -40°C to 125°C outperforms almost any other voltage reference. Like the LT6657, this performance is difficult to test on the bench and incredibly challenging to guarantee during manufacturing.

While many similar products may guarantee stability through characterization or sample testing, the LTC6655 is guaranteed using 100% three-temperature testing. This level of care in the manufacturing process shows the level of quality that must be maintained to produce a truly high-stability product. Compared to the LTC6655, the LT6657 is even more stable, with 1.5ppm/°C drift, and this performance is guaranteed using 100% five-temperature testing.

This provides a new level of precision for automotive and extended temperature industrial applications in addition to laboratory instrumentation. Adding to the overall stability is low thermal hysteresis and excellent long-term stability, which is a measure of how well a system will remain within specification over time and temperature cycling. Testing has verified that drift over time and temperature cycling is low and consistent. When these characteristics are well characterized, drift can be easily predicted over the life of the product.

With the LT6657, noise performance is on par with some of the best low noise buried Zener references – the LT6657 has a noise of only 0.5ppm, making it ideal for many high dynamic range systems. For a 2.5V reference, the LT6657 produces only 1.25μV peak-to-peak noise from 0.1 to 10Hz. Wideband noise is also low at 0.8ppm RMS to 1kHz.

In addition to the impressive stability of the LTC6655 mentioned above, the LTC6655 also has unparalleled noise performance, which distinguishes it from other high-stability bandgap voltage references, including the LT6657. The LTC6655 has 0.25ppm peak-to-peak noise from 0.1 to 10Hz, providing the low noise floor required for high dynamic range measurements. It is worth mentioning that although the LTC6655 has high precision and world-leading noise performance, as well as many very good features, these are at the expense of certain flexibility.

Comparison of Key Performance of LTC6654, LTC6655 and LT6657

In particular, the input voltage range and output current range of the LTC6655 are limited compared to products such as the LT6654. An example of the many features available in a bandgap reference is the LT6654. This 10ppm/°C voltage reference is an excellent general purpose product that performs well enough for a wide range of applications and has many features that make it compatible with almost any application.

Features of the LT6654 include low drift, low noise, low dropout voltage and a wide supply range to 36V, a wide temperature range, low thermal hysteresis and excellent long-term stability, excellent line and load regulation, and 7 available output voltage options. The device is rugged, with 10mA load current source and sink capability, as well as overcurrent and thermal limiting circuitry.

The LT6654 has a very low output impedance, providing good rejection of disturbances that may be present in the application circuit due to environmental conditions or testing such as high current injection. These features allow its use in low voltage or high voltage systems with the ability to bias sensors or signal conditioning circuits while maintaining a precise output voltage.

Voltage references often have to make design trade-offs and optimization choices among many features in pursuit of higher performance, which inevitably leads to a lack of flexibility and compatibility. How to come up with more targeted products is what solution providers need to consider. In general, LT6657 is the "optimal choice" in many aspects.

In addition to high stability and low noise, the LT6657 adds many features. Like most of ADI's voltage reference products, the LT6657 has a very low output impedance over the entire frequency range, which mitigates the effects of load changes with frequency and helps prevent signals from feeding back to the reference and causing interference, errors or noise. This is also crucial for reference stability when driving a high-performance analog-to-digital converter or safely withstanding operational tests such as high current injection in automotive systems.

The LT6657 also has strong drive capability. The load current range is 10mA (source and sink), the load regulation error is less than 1ppm/mA, and the LT6657 has overcurrent and thermal protection functions to avoid damage to the device under fault conditions. The LT6657 can be used to bias a variety of sensors, drive difficult ADC reference inputs, drive multiple ADCs and DACs, or power a small subsystem with reference-level accuracy.

It is easy to see from the comparison chart that the LT6657 has performance comparable to any available voltage reference and has a rich and comprehensive set of features. In addition to the common features of voltage references, the LT6657 has several more advantages, including protection against reverse power and reverse output, which can provide more robust performance in difficult environments. These features are often seen in LDO power supplies, but are not common in precision products.

Finally, the LT6657 can act as a shunt as well as a series reference. Shunt mode operation makes it easy to configure a negative voltage reference to operate from a very high supply voltage or to retrofit an older design to provide better performance.

In summary, the LT6657 has high precision, low noise and high stability, as well as the flexibility provided by its many features. The other features and performance of the LT6657 combined with the differentiated manufacturing will surely make it the best choice for many application circuits, including laboratory test equipment, automated test, and even automotive and industrial systems, in which the combination of high performance and many features makes it compatible with a wide range of system requirements.

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