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ADR1500 Datasheet(PDF) 6 Page - Analog Devices

No. de Pieza. ADR1500
Descripción  1.2875 V Micropower, Shunt Voltage Reference
Descarga  8 Pages
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Fabricante  AD [Analog Devices]
Página de inicio  http://www.analog.com
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ADR1500 Datasheet(HTML) 6 Page - Analog Devices

   
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ADR1500
Rev. 0 | Page 6 of 8
THEORY OF OPERATION
The ADR1500 uses the band gap concept to produce a stable
voltage reference suitable for high accuracy data acquisition
components and systems. This device makes use of the underlying
physical nature of the silicon transistor base emitter voltage in
the forward-biased operating region. All such transistors have
an approximate −2 mV/°C temperature coefficient, which is not
suitable for use 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 VBE goes to approximately the silicon
band gap voltage. Therefore, if a voltage could be developed
with an opposing temperature coefficient to the sum with the
VBE, than a zero TC reference would result. The ADR1500
circuit in Figure 7 provides such a compensating voltage, V1,
by deriving two transistors at different current densities and
amplifying the resultant VBE difference (ΔVBE, which has a
positive TC). The sum of the VBE and V1 provides a stable
voltage reference.
V+
V–
+
ΔVBE
VBE
+
V1
+
Figure 7. Schematic Diagram
APPLYING THE ADR1500
The ADR1500 is simple to use in virtually all applications. To
operate the ADR1500 as a conventional shunt reference, see
Figure 8. An external series resistor is connected between the
supply voltage and the ADR1500.
For a given supply voltage, the series resistor, RS, determines the
reverse current flowing through the ADR1500. The value of RS
must be chosen to accommodate the expected variations of the
supply voltage, VS, load current, IL, and the ADR1500 reverse
voltage, VR, while maintaining an acceptable reverse current, IR,
through the ADR1500.
The minimum value for RS should be 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
)
(
)
(
L
R
R
S
S
I
I
V
V
R
+
=
(A)
RS
VS
VR
IR + IL
VOUT
IR
+
IL
(B)
RS
+5V (+3V) ±10%
VR
2.94kΩ
(1.30kΩ)
VOUT
+
Figure 8. Typical Connection Diagram
Figure 8 shows a typical connection of the ADR1500 operating
at a minimum of 100 μA. This connection can provide ±1 mA
to the load, while accommodating ±10% power supply
variations.
TURN-ON TIME
The turn-on time is a critical parameter for applications
demanding a large amount of processing. Figure 9 shows
the turn-on characteristics of the ADR1500.
250mV/DIV
5µs/DIV
2.4V
0V
VIN
CL = 200pF
Figure 9. Response Time
Upon application of power (cold start), the time required for
the output voltage to reach its final value within a specified
error is the turn-on settling time. Tow components are normally
associated with the time for active circuits to settle and the time
for the thermal gradients on the chip to stabilize. This characteristic
is generated from cold start operation and represents the true
turn-on waveform after power up. Figure 10 shows both the
course and fine turn-on settling characteristics of the device;
the total settling time to within 1.0 mV is about 6 μs, and there
is no long thermal tail when the horizontal scale is expanded to
2 μs/DIV. The output turn-on time is modified when an
external noise reduction filter is used. When present, the time
constant of the filter dominates the overall settling.


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