ADR1581
THEORY OF OPERATION
The ADR1581 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. Therefore, if a
voltage could be developed with an opposing temperature
coefficient to sum with VBE, a zero TC reference would result.
The ADR1581 circuit in Figure 9 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.
Figure 11 shows a typical connection of the ADR1581BRT
operating at a minimum of 100 ꢀA. This connection can
provide 1 mA to the load while accommodating 10ꢁ
power supply variations.
V
S
R
I + I
R L
S
I
L
V
R
I
R
V
OUT
Figure 10. Typical Connection Diagram
+5V(+3V) ±10%
2.94kΩ
(1.30kΩ)
R
V
S
R
V
OUT
V+
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.
ΔV
BE
Some confusion exists in the area of defining and specifying refer-
ence 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 temperatures to guarantee that the voltage falls within
the given error band. The proprietary curvature correction design
techniques used to minimize the ADR1581 nonlinearities allow
the temperature performance to be guaranteed using the maximum
deviation method. This method is more useful to a designer than
one that simply guarantees the maximum error band over the
entire temperature change.
V
BE
V–
Figure 9. Schematic Diagram
APPLYING THE ADR1±81
The ADR1581 is simple to use in virtually all applications.
To operate the ADR1581 as a conventional shunt regulator (see
Figure 10), an external series resistor is connected between the
supply voltage and the ADR1581. For a given supply voltage, the
series resistor, RS, determines the reverse current flowing through
the ADR1581. The value of RS must be chosen to accommodate
the expected variations of the supply voltage (VS), load current
(IL), and the ADR1581 reverse voltage (VR) while maintaining an
acceptable reverse current (IR) through the ADR1581.
Figure 12 shows a typical output voltage drift for the ADR1581
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 ADR1581, the designer
can easily determine the maximum total error from the initial
tolerance plus the temperature variation.
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)
Rev. 0 | Page 6 of 12
ADI [ ADI ]
