Datasheet

Data Sheet ADR420/ADR421/ADR423/ADR425
Rev. J | Page 19 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 (V
ERROR
= R × I
L
)
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.
02432-045
A1
V
IN
V
IN
R
L
W
A1 = OP191
R
L
W
R
L
V
OUT
SENSE
V
OUT
FORCE
GND
V
OUT
ADR420/
ADR421/
ADR423/
ADR425
2
4
6
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.
02432-046
V
IN
1µ
F
0.1µ
F
R1
10kΩ
R3
5k
Ω
R2
10k
Ω
+5V
–5V
+10V
–10V
6
2
4
5
V+
V–
U1
ADR425
U2
OP
1
177
V
OUT
V
IN
TRIM
GND
Figure 44. +5 V and −5 V Reference Using ADR425
02432-047
R1
5.6kΩ
R2
5.6kΩ
+2.5V
+10V
–10V
–2.5V
U1
ADR425
6
2
4
5
V+
V–
V
OUT
V
IN
TRIM
GND
U2
OP1177
Figure 45. +2.5 V and −2.5 V Reference Using ADR425