ADR421ARMZ datasheet(18/24 Pages) AD

18 / 24 page

ADR420/ADR421/ADR423/ADR425

Rev. I | Page 18 of 24

KELVIN CONNECTIONS

In many portable instrumentation applications where PC board

cost and area are important considerations, circuit intercon-

nects are often narrow. These narrow lines can cause large

voltage drops if the voltage reference is required to provide load

currents to various functions. In fact, a circuit’s interconnects

can exhibit a typical line resistance of 0.45 mΩ/square (1 oz. Cu,

for example). Force and sense connections, also referred to as

Kelvin connections, offer a convenient method of eliminating

the effects of voltage drops in circuit wires. Load currents flow-

ing through wiring resistance produce an error (VERROR = R × IL)

at the load. However, the Kelvin connection in Figure 43

overcomes the problem by including the wiring resistance

within the forcing loop of the op amp. Because the op amp

senses the load voltage, op amp loop control forces the output to

compensate for the wiring error and to produce the correct

voltage at the load.

VIN

RLW

A1 = OP191

RLW

VOUT

SENSE

VOUT

FORCE

GND

VOUT

ADR420/

ADR421/

ADR423/

ADR425

Figure 43. Advantage of Kelvin Connection

DUAL-POLARITY REFERENCES

Dual-polarity references can easily be made with an op amp and

a pair of resistors. In order not to defeat the accuracy obtained

by the ADR42x, it is imperative to match the resistance toler-

ance and the temperature coefficient of all components.

VIN

1µF

0.1µF

10kΩ

5kΩ

10kΩ

+5V

–5V

+10V

–10V

V+

V–

ADR425

VOUT

VIN

TRIM

GND

Figure 44. +5 V and −5 V Reference Using ADR425

OP1177

5.6kΩ

+2.5V

+10V

–10V

–2.5V

ADR425

V+

V–

VOUT

VIN

TRIM

GND

OP1177

Figure 45. +2.5 V and −2.5 V Reference Using ADR425

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