ADR430 Analog Devices, Inc., ADR430 Datasheet - Page 17

• Manufacturer Part Number: ADR430
• Description: Ultralow Noise Xfet Voltage References With Current Sink And Source Capability
• Manufacturer: Analog Devices, Inc.

HIGH VOLTAGE FLOATING CURRENT SOURCE
The circuit in Figure 34 can be used to generate a floating
current source with minimal self heating. This particular
configuration can operate on high supply voltages determined
by the breakdown voltage of the N-channel JFET.
KELVIN CONNECTION
In many portable instrumentation applications, where PC board
cost and area go hand in hand, circuit interconnects are very
often of dimensionally minimum width. These narrow lines can
cause large voltage drops if the voltage reference is required to
provide load currents to various functions. In fact, circuit inter-
connects 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 flowing through wiring resistance produce an error
( V
Figure 35 overcomes the problem by including the wiring
resistance within the forcing loop of the operational amplifier.
ERROR
= R × I
Figure 34. High Voltage Floating Current Source
ADR43x
L
) at the load. However, the Kelvin connection of
GND
–V
V
4
A1
2
V
IN
DD
OUT
Figure 33. Negative Reference
6
6
V
OP90
OUT
ADR43x
+V
GND
V
2
4
DD
IN
+V
S
–V
2N3904
–V
VISHAY
SST111
S
R
2.1kΩ
REF
L
ADR430/ADR431/ADR433/ADR434/ADR435/ADR439
Rev. D | Page 17 of 24
Because the amplifier senses the load voltage, the operational
amplifier loop control forces the output to compensate for the
wiring error and to produce the correct voltage at the load.
DUAL-POLARITY REFERENCES
Dual-polarity references can easily be made with an operational
amplifier and a pair of resistors. To avoid defeating the accuracy
obtained by ADR43x, it is imperative to match the resistance
tolerance as well as the temperature coefficient of all the
components.
V
IN
1µF
+10V
Figure 37. +2.5 V and −2.5 V References Using ADR435
Figure 36. +5 V and −5 V References Using ADR435
Figure 35. Advantage of Kelvin Connection
ADR43x
0.1µF
GND
V
4
2
IN
GND
V
V
2
IN
OUT
4
ADR435
GND
V
2
4
U1
6
IN
ADR435
TRIM
V
U1
OUT
V
OP191
TRIM
+
OUT
A1
5
6
V
5.6kΩ
5.6kΩ
IN
6
5
R
R2
+2.5V
R1
–2.5V
LW
10kΩ
5kΩ
R3
R
R1
LW
OP1177
OP1177
+10V
– 10V
V+
V
SENSE
V
FORCE
R
V–
–10V
U2
U2
OUT
OUT
L
V+
V–
10kW
R2
+5V
–5V