The REF50xx is family of low-noise, precision bandgap voltage references that are specifically designed for excellent initial voltage accuracy and drift. See the Functional Block Diagram for a simplified block diagram of the REF50xx.
The temperature output terminal (TEMP, pin 3) provides a temperature-dependent voltage output with approximately 60-kΩ source impedance. As illustrated in Figure 8, the output voltage follows the nominal relationship:
This pin indicates general chip temperature, accurate to approximately ±15°C. Although not generally suitable for accurate temperature measurements, this pin can be used to indicate temperature changes or for temperature compensation of analog circuitry. A temperature change of 30°C corresponds to an approximate 79-mV change in voltage at the TEMP pin.
The TEMP pin has high-output impedance (see the Functional Block Diagram). Loading this pin with a low-impedance circuit induces a measurement error; however, this pin does not have any effect on VOUT accuracy.
The REF50xx is designed for minimal drift error, which is defined as the change in output voltage over temperature. The drift is calculated using the box method, as described in Equation 2.
The REF50xx features a maximum drift coefficient of 3 ppm/°C for the high-grade version, and 8 ppm/°C for the standard-grade.
Thermal hysteresis for the REF50xx is defined as the change in output voltage after operating the device at 25°C, cycling the device through the specified temperature range, and returning to 25°C. Thermal hysteresis can be expressed as Equation 3:
Typical 0.1-Hz to 10-Hz voltage noise for each member of the REF50xx family is specified in the Electrical Characteristics table. The noise voltage increases with output voltage and operating temperature. Additional filtering can be used to improve output noise levels, although take care to ensure the output impedance does not degrade performance.
For additional information about how to minimize noise and maximize performance in mixed-signal applications such as data converters, refer to the series of Analog Applications Journal articles entitled, How a Voltage Reference Affects ADC Performance. This three-part series is available for download from the TI website under three literature numbers: SLYT331, SLYT339, and SLYT355 for Part I, Part II, and Part III, respectively.
The REF50xx provides a very accurate, factory-trimmed voltage output. However, VOUT can be adjusted using the trim and noise reduction pin (TRIM/NR, pin 5). Figure 39 shows a typical circuit that allows an output adjustment of ±15mV
The REF50xx allows access to the bandgap through the TRIM/NR pin. Placing a capacitor from the TRIM/NR pin to GND (Figure 38) in combination with the internal R3 and R4 resistors creates a low-pass filter. A capacitance of 1μF creates a low-pass filter with the corner frequency from 10 Hz to 20 Hz. Such a filter decreases the overall noise measured on the VOUT pin by half. Higher capacitance results in a lower filter cutoff frequency, further reducing output noise. Using this capacitor increases start-up time.
Figure 40 shows the typical connections for the REF50xx. TI recommends a supply bypass capacitor ranging from 1 μF to 10 μF. A 1-μF to 50-μF output capacitor (CL) must be connected from VOUT to GND. The equivalent series resistance (ESR) value of CL must be less than or equal to 1.5 Ω to ensure output stability. To minimize noise, the recommended ESR of CL is from 1 Ω and 1.5 Ω.
The REF50xx family of voltage references features extremely low dropout voltage. With the exception of the REF5020, which has a minimum supply requirement of 2.7 V, these references can be operated with a supply of 200 mV more than the output voltage in an unloaded condition. For loaded conditions, a typical dropout voltage versus load plot is provided in Figure 6 in the Typical Characteristics.
For applications requiring a negative and positive reference voltage, the REF50xx and OPA735 can be used to provide a dual-supply reference from a 5-V supply. Figure 41 shows the REF5025 used to provide a 2.5-V supply reference voltage. The low drift performance of the REF50xx complements the low offset voltage and zero drift of the OPA735 to provide an accurate solution for split-supply applications. Take care to match the temperature coefficients of R1 and R2.