AD1580 View Datasheet(PDF) - Analog Devices
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AD1580 Datasheet PDF : 12 Pages
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AD1580
THEORY OF OPERATION
The AD1580 uses the band gap concept to produce a stable,
low temperature coefficient voltage reference suitable for high
accuracy data acquisition components and systems. The device
makes use of the underlying physical nature of a silicon
transistor base emitter voltage in the forward biased operating
region. All such transistors have an approximately −2 mV/°C
temperature coefficient, which is unsuitable for use directly as a
low TC reference; however, extrapolation of the temperature
characteristic of any one of these devices to absolute zero (with
collector current proportional to absolute temperature) reveals
that its VBE goes to approximately the silicon band gap voltage.
Thus, if a voltage could be developed with an opposing
temperature coefficient to sum with VBE, a zero TC reference
would result. The AD1580 circuit in Figure 10 provides such a
compensating voltage, V1, by driving two transistors at different
current densities and amplifying the resultant VBE difference
(ΔVBE, which has a positive TC). The sum of VBE and V1
provides a stable voltage reference.
V+
Figure 12 shows a typical connection of the AD1580BRT
operating at a minimum of 100 μA. This connection can
provide ±1 mA to the load while accommodating ±10%
power supply variations.
VS
RS
VR
IR
IR + IL
IL
VOUT
Figure 11. Typical Connection Diagram
+5V(+3V) ±10%
RS
2.94kΩ
(1.30kΩ)
VR
VOUT
V1
Figure 12. Typical Connection Diagram
TEMPERATURE PERFORMANCE
The AD1580 is designed for reference applications where stable
temperature performance is important. Extensive temperature
ΔVBE
testing and characterization ensure that the device’s performance is
maintained over the specified temperature range.
VBE
V–
Figure 10. Schematic Diagram
APPLYING THE AD1580
The AD1580 is simple to use in virtually all applications.
To operate the AD1580 as a conventional shunt regulator (see
Figure 11), an external series resistor is connected between the
supply voltage and the AD1580. For a given supply voltage, the
series resistor, RS, determines the reverse current flowing
through the AD1580. The value of RS must be chosen to
accommodate the expected variations of the supply voltage, VS;
load current, IL; and the AD1580 reverse voltage, VR; while
maintaining an acceptable reverse current, IR, through the AD1580.
The minimum value for RS should be chosen when VS is at its
minimum and IL and VR are at their maximum, while
maintaining the minimum acceptable reverse current.
The value of RS should be large enough to limit IR to 10 mA
when VS is at its maximum and IL and VR are at their minimum.
The equation for selecting RS is as follows:
RS = (VS − VR)/(IR + IL)
Some confusion exists in the area of defining and specifying
reference voltage error over temperature. Historically, references
have been characterized using a maximum deviation per degree
Celsius, for example, 50 ppm/°C. However, because of
nonlinearities in temperature characteristics that originated in
standard Zener references (such as S type characteristics), most
manufacturers now use a maximum limit error band approach
to specify devices. This technique involves the measurement of
the output at three or more different temperatures to guarantee
that the voltage falls within the given error band. The
proprietary curvature correction design techniques used to
minimize the AD1580 nonlinearities allow the temperature
performance to be guaranteed using the maximum deviation
method. This method is of more use to a designer than the one
that simply guarantees the maximum error band over the entire
temperature change.
Figure 13 shows a typical output voltage drift for the AD1580
and illustrates the methodology. The maximum slope of the two
diagonals drawn from the initial output value at +25°C to the
output values at +85°C and −40°C determines the performance
grade of the device. For a given grade of the AD1580, the
Rev. C | Page 6 of 12
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