Datasheet ADR525, ADR530, ADR550 (Analog Devices) - 9

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ADR525/ADR530/ADR550 THEORY OF OPERATION
The ADR525/ADR530/ADR550 use the band gap concept to
VS
produce a stable, low temperature coefficient voltage reference
R IIN + IL
suitable for high accuracy data acquisition components and systems. The devices use the physical nature of a silicon transistor
VOUT I
base-emitter voltage (V
L
BE) in the forward-biased operating region.
IIN
3
ADR550
-00 All such transistors have approximately a −2 mV/°C tempera- 1 • 50 04 ture coefficient (TC), making them unsuitable for direct use as Figure 15. Shunt Reference low temperature coefficient references. Extrapolation of the temperature characteristics of any one of these devices to Given these conditions, RBIAS is determined by the supply absolute zero (with the collector current proportional to the voltage (VS), the load and operating currents (IL and IIN) of absolute temperature), however, reveals that its VBE approaches the ADR525/ADR530/ADR550, and the output voltage (VOUT) approximately the silicon band gap voltage. Thus, if a voltage of the ADR525/ADR530/ADR550. develops with an opposing temperature coefficient to sum the V −V S OUT V R = (3) BE, a zero temperature coefficient reference results. The BIAS I + I ADR525/ADR530/ADR550 circuit shown in Figure 14 provides L IN such a compensating voltage (V1) by driving two transistors at
Precision Negative Voltage Reference
different current densities and amplifying the resultant VBE The ADR525/ADR530/ADR550 are suitable for applications difference (ΔVBE, which has a positive temperature coefficient). where a precise negative voltage is desired. Figure 16 shows the The sum of VBE and V1 provides a stable voltage reference over ADR525 configured to provide a negative output. temperature.
V+ + ADR525 –2.5V V1 R
4 -00
VS
501 04
–
Figure 16. Negative Precision Reference Configuration
+ ∆VBE– Output Voltage Trim + VBE
The trim terminal of the ADR525/ADR530/ADR550 can be 02 0
–
1-
V–
50 used to adjust the output voltage over a range of ±0.5%. This 04 Figure 14. Circuit Schematic allows systems designers to trim small system errors by setting the reference to a voltage other than the preset output voltage.
APPLICATIONS
An external mechanical or electrical potentiometer can be used The ADR525/ADR530/ADR550 are a series of precision shunt for this adjustment. Figure 17 illustrates how the output voltage voltage references. They are designed to operate without an can be trimmed using the AD5273, an Analog Devices, Inc., external capacitor between the positive and negative terminals. 10 kΩ potentiometer. If a bypass capacitor is used to filter the supply, the references
VS
remain stable. All shunt voltage references require an external bias resistor (R
R
BIAS) between the supply voltage and the reference (see Figure 15).
VOUT
R
R1
BIAS sets the current that flows through the load (IL) and the
470kΩ AD5273
reference (IIN). Because the load and the supply voltage can vary,
ADR530 POTENTIOMETER 10kΩ
RBIAS needs to be chosen based on the following considerations: 05 0 1- • 50 RBIAS must be small enough to supply the minimum IIN 04 current to the ADR525/ADR530/ADR550, even when the Figure 17. Output Voltage Trim supply voltage is at its minimum value and the load current is at its maximum value. • RBIAS must be large enough so that IIN does not exceed 15 mA when the supply voltage is at its maximum value and the load current is at its minimum value. Rev. F | Page 9 of 12 Document Outline FEATURES APPLICATIONS PIN CONFIGURATION GENERAL DESCRIPTION TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ADR525 ELECTRICAL CHARACTERISTICS ADR530 ELECTRICAL CHARACTERISTICS ADR550 ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION PARAMETER DEFINITIONS TEMPERATURE COEFFICIENT THERMAL HYSTERESIS TYPICAL PERFORMANCE CHARACTERISTICS THEORY OF OPERATION APPLICATIONS Precision Negative Voltage Reference Output Voltage Trim Stacking the ADR525/ADR530/ADR550 for User-Definable Outputs Adjustable Precision Voltage Source OUTLINE DIMENSIONS ORDERING GUIDE