Datasheet ADR1581 (Analog Devices) - 6

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ADR1581 THEORY OF OPERATION
The ADR1581 uses the band gap concept to produce a stable, Figure 11 shows a typical connection of the ADR1581BRT low temperature coefficient voltage reference suitable for high operating at a minimum of 100 μA. This connection can accuracy data acquisition components and systems. The device provide ±1 mA to the load while accommodating ±10% makes use of the underlying physical nature of a silicon transistor power supply variations. base emitter voltage in the forward-biased operating region. All
VS
such transistors have an approximately −2 mV/°C temperature coefficient, which is unsuitable for use directly as a low TC
R I S R + IL
reference; however, extrapolation of the temperature characteristic
IL VR
of any one of these devices to absolute zero (with collector current
IR V
proportional to absolute temperature) reveals that its V
OUT
BE goes 10 to approximately the silicon band gap voltage. Therefore, if a -0 72 66 voltage could be developed with an opposing temperature 0 Figure 10. Typical Connection Diagram coefficient to sum with VBE, a zero TC reference would result. The ADR1581 circuit in Figure 9 provides such a compensating
+5V(+3V) ±10%
voltage, V1, by driving two transistors at different current densities
2.94kΩ RS
and amplifying the resultant V
(1.30kΩ)
BE difference (ΔVBE), which has a positive TC. The sum of VBE and V1 provides a stable voltage
VR
reference.
VOUT
11
V+
-0 72 66 0 Figure 11. Typical Connection Diagram
V1 TEMPERATURE PERFORMANCE
The ADR1581 is designed for reference applications where stable temperature performance is important. Extensive temperature testing and characterization ensure that the device’s performance is maintained over the specified temperature range.
ΔVBE
Some confusion exists in the area of defining and specifying refer- ence voltage error over temperature. Historically, references have
V
09
BE
-0 been characterized using a maximum deviation per degree Celsius, 72
V–
066 for example, 50 ppm/°C. However, because of nonlinearities in Figure 9. Schematic Diagram temperature characteristics that originated in standard Zener references (such as S type characteristics), most manufacturers
APPLYING THE ADR1581
now use a maximum limit error band approach to specify devices. The ADR1581 is simple to use in virtually all applications. This technique involves the measurement of the output at three To operate the ADR1581 as a conventional shunt regulator (see or more temperatures to guarantee that the voltage falls within Figure 10), an external series resistor is connected between the the given error band. The proprietary curvature correction design supply voltage and the ADR1581. For a given supply voltage, the techniques used to minimize the ADR1581 nonlinearities allow series resistor, RS, determines the reverse current flowing through the temperature performance to be guaranteed using the maximum the ADR1581. The value of RS must be chosen to accommodate deviation method. This method is more useful to a designer than the expected variations of the supply voltage (VS), load current one that simply guarantees the maximum error band over the (IL), and the ADR1581 reverse voltage (VR) while maintaining an entire temperature change. acceptable reverse current (IR) through the ADR1581. Figure 12 shows a typical output voltage drift for the ADR1581 The minimum value for RS should be chosen when VS is at its and illustrates the methodology. The maximum slope of the two minimum and IL and VR are at their maximum while maintaining diagonals drawn from the initial output value at +25°C to the the minimum acceptable reverse current. output values at +85°C and −40°C determines the performance The value of RS should be large enough to limit IR to 10 mA grade of the device. For a given grade of the ADR1581, the designer when VS is at its maximum and IL and VR are at their minimum. can easily determine the maximum total error from the initial tolerance plus the temperature variation. The equation for selecting RS is as follows: RS = (VS − VR)/(IR + IL) Rev. 0 | Page 6 of 12 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ABSOLUTE MAXIMUM RATINGS ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLYING THE ADR1581 TEMPERATURE PERFORMANCE VOLTAGE OUTPUT NONLINEARITY VS. TEMPERATURE REVERSE VOLTAGE HYSTERESIS OUTPUT IMPEDANCE VS. FREQUENCY NOISE PERFORMANCE AND REDUCTION TURN-ON TIME TRANSIENT RESPONSE PRECISION MICROPOWER LOW DROPOUT REFERENCE USING THE ADR1581 WITH 3 V DATA CONVERTERS OUTLINE DIMENSIONS ORDERING GUIDE