Tutorial
Voltage Reference ICs
Voltage reference provide an accurate, temperature compensated voltage source for use in a variety of applications. These devices usually come in families of parts that provide specific accurate voltages. Some families can have up to six different values with output voltages ranging from 1.225V to 5.000V. Initial output voltage accuracy and temperature coefficient are two of the more important characteristics.
Voltage references are available with fixed and adjustable reference voltage outputs. Adjustable output is set by a resistor divider connected to a reference pin. These references are either shunt (two-terminal) or series (three-terminal) types.
The ideal voltage reference has a perfect initial accuracy and maintain its voltage output independent of changes in temperature, load current, and time. However, the ideal characteristics are virtually impossible to attain, so the designer must consider the following factors:
Shunt references are similar to zener diodes in operation because both require an external resistor that determines the maximum current that can be supplied to the load. The external resistor also sets the minimum biasing current to maintain regulation. Consider shunt references when the load is nearly constant and power supply variations are minimal.
Series references do not require any external components and they should be considered when the load is variable and lower voltage overhead is important. They are also more immune to the power supply changes than shunt references.
Temperature Drift is the change in output voltage due to the temperature change, expressed in ppm/°C. Buried zener type references typically have a lower temperature drift than bandgap voltage references. Temperature drift can be specified in several ways (slope, butterfly and box) but the most common way is the box method.
Output Voltage Temperature Hysteresis is the change in the output voltage at the reference temperature, usually 25°C, due to sequential but opposite temperature excursions, i.e., cold-to-hot and hot-to-cold. Negative effects can occur due to this effect because its amplitude is directly proportional the temperature excursions of the associated system. In some systems this parameter is not repeatable. This parameter is a function of the circuit and the semiconductor package. Hysteresis is given in ppm (parts per million).
Initial Accuracy is important in systems where calibration is impossible or inconvenient. Usually, it is accomplished by the calibration of the overall system. Initial accuracy is specified with fixed input voltage and no load current (for series type) or fixed bias current (for shunt type).
Long-Term Stability affects the output of a voltage reference, which changes gradually with time. The largest change occurs in the first 500 hours. This parameter is important in high-performance applications or in applications where periodic calibration is not acceptable. Long-term stability data may be based on the observation over 1000 hours at room temperature.
Power dissipation depends on the voltage and current required to maintain proper operating characteristics.
Output Noise is usually specified over two frequency ranges: 0.1 Hz to 10 Hz (peak-to-peak noise) and 10 Hz to 1 kHz (RMS noise). Noise can be important because it can reduce dynamic range of the acquisition system. High-resolution data acquisition systems may experience "dither" in the LSBs solely due to reference noise. Noise can be reduced by external capacitor filtering of a noise reduction pin.
Following are descriptions of various voltage reference ICs.
1. This precision two-terminal shunt mode, bandgap voltage reference IC is available in fixed reverse breakdown voltages. The device behaves similarly to an ideal zener diode; a fixed voltage is maint