Innovations in buried Zener voltage references

Many types of voltage references provide ultra-low noise voltage levels. However, buried Zener voltage references stand apart as having particularly low noise. Buried Zener voltage references usually do not require gain to generate an output voltage, thus reducing the noise. Buried Zener voltage references often are used to provide a “golden” voltage level for a high precision system. The voltage reference is used for calibration or with ultra-precise data converters, such as DAC11001B, as shown in Figure 8.

When using buried Zener devices for calibration, there are three main parameters that are important to consider: temperature drift, long-term drift, and noise. During calibration, system data converters use the stable and low-noise voltage provided by a buried Zener voltage reference, such as REF80, to determine the gain and offset error of an ADC or DAC. REF80 has an ultra-low, 0.16ppm_p-p noise specification. For accurate calibration, the voltage level cannot vary over time or temperature, and the value provided must be low noise to ensure the errors observed during calibration can be effectively compensated for.

When REF80 is used with DAC11001B, it must be buffered to allow for good dynamic performance. These buffers will add more noise to the reference circuit, and thus the overall signal chain. For this reason, low noise operation amplifiers must be used to maintain low noise. OPA828 is a low noise operational amplifier with 4nV/√Hz noise at 1kHz that is often used in the reference buffer circuit.

In order to ensure that the noise from REF80 is mostly only coming from the voltage reference, it is also important to power REF80 using low noise low dropouts (LDOs). REF80 is unique in that it has an internal heater. This heater holds the die at a consistent temperature regardless of the surrounding environment. This heater enables the low drift specifications of REF80. The heater and reference power are separate from each other. For this reason, both the heater (HEATP) and VDD (drain supply) need a power supply. The REF80 pinout is shown in Figure 9.

The heater of REF80 typically draws up to 335mA upon startup, settling to 18mA to 75mA while VDD only typically requires 15mA of quiescent current. Additionally, the noise on the voltage output of REF80 (REF_Z) is dependent on the buried Zener circuit, not the heater. The architecture of REF80 is such that only the buried Zener reference has a major impact on the output noise while the heater has minimal impact. Figure 10 shows a simplified block diagram.

Therefore, for the lowest noise possible, the VDD pin, which powers the buried Zener reference, must use a low noise LDO to provide power. In the REF80 evaluation module, REF8EVM, the wide input voltage, ultralow noise LDO TPS7A49 is used for VDD. For the heater, the higher current output, but higher noise, LM317 is used. Figure 11 demonstrates a block diagram of the REF8EVM power supply configuration.

It is also an option to use one LDO for both VDD and the heater. If this is desired, another good option is TPSA4701, which has ultralow noise and a higher output current capability to power both VDD and HEATP.

For the highest precision of technologies and applications, buried Zener voltage references are one of the best options to consider. The low drift and low noise of buried Zener devices such as REF80 are essential where signal chain and calibration noise are a major concern.