Datasheet ADR420, ADR421, ADR423, ADR425 (Analog Devices) - 19

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Data Sheet ADR420/ADR421/ADR423/ADR425 KELVIN CONNECTIONS DUAL-POLARITY REFERENCES
In many portable instrumentation applications where PC board Dual-polarity references can easily be made with an op amp and cost and area are important considerations, circuit intercon- a pair of resistors. In order not to defeat the accuracy obtained nects are often narrow. These narrow lines can cause large by the ADR42x, it is imperative to match the resistance toler- voltage drops if the voltage reference is required to provide load ance and the temperature coefficient of al components. currents to various functions. In fact, a circuit’s interconnects
VIN
can exhibit a typical line resistance of 0.45 mΩ/square (1 oz. Cu,
1µF 0.1µF 2
for example). Force and sense connections, also referred to as
VIN V 6 OUT +5V R1
Kelvin connections, offer a convenient method of eliminating
R2 10kΩ 10kΩ U1
the effects of voltage drops in circuit wires. Load currents flow-
ADR425 +10V
ing through wiring resistance produce an error (VERROR = R × IL)
GND TRIM
at the load. However, the Kelvin connection in Figure 43
5 V+ U2 4 –5V
overcomes the problem by including the wiring resistance
OP1177 V–
within the forcing loop of the op amp. Because the op amp 046
R3 5kΩ
senses the load voltage, op amp loop control forces the output to 02432-
–10V
compensate for the wiring error and to produce the correct Figure 44. +5 V and −5 V Reference Using ADR425 voltage at the load.
VIN +2.5V 2 +10V RLW V ADR420/ OUT 2 ADR421/ V SENSE V IN IN VOUT 6 ADR423/ R U1 LW ADR425 V R1 A1 OUT ADR425 FORCE V 5.6kΩ OUT 6 GND GND TRIM 5 RL 4 4
045
R2 A1 = OP191 5.6kΩ V +
02432-
U2
Figure 43. Advantage of Kelvin Connection
OP1177 V – –2.5V
047
–10V
02432- Figure 45. +2.5 V and −2.5 V Reference Using ADR425 Rev. J | Page 19 of 24 Document Outline Features Applications Pin Configuration General Description ADR42x Products Revision History Specifications ADR420 Electrical Specifications ADR421 Electrical Specifications ADR423 Electrical Specifications ADR425 Electrical Specifications Absolute Maximum Ratings Thermal Resistance ESD Caution Pin Configurations and Function Descriptions Typical Performance Characteristics Terminology Theory of Operation Device Power Dissipation Considerations Basic Voltage Reference Connections Noise Performance Turn-On Time Applications Output Adjustment Reference for Converters in Optical Network Control Circuits High Voltage Floating Current Source Kelvin Connections Dual-Polarity References Programmable Current Source Programmable DAC Reference Voltage Precision Voltage Reference for Data Converters Precision Boosted Output Regulator Outline Dimensions Ordering Guide