Datasheet

ADR3412/ADR3420/ADR3425/ADR3430/ADR3433/ADR3440/ADR3450
Rev. B | Page 21 of 24
SAMPLE APPLICATIONS
Negative Reference
Figure 42 shows how to connect the ADR3450 and a standard
CMOS op amp, such as the AD8663, to provide a negative
reference voltage. This configuration provides two main
advantages: first, it only requires two devices and, therefore,
does not require excessive board space; second, and more
importantly, it does not require any external resistors, meaning
that the performance of this circuit does not rely on choosing
expensive parts with low temperature coefficients to ensure
accuracy.
AD8663
0.1µF1µF
+VDD
–VDD
0.1µF
0.1µF
–5V
08440-049
ADR3450
V
IN
ENABLE
V
OUT
FORCE
V
OUT
SENSE
GND FORCE
GND SENSE
4
3
6
5
2
1
Figure 42. ADR3450 Negative Reference
In this configuration, the V
OUT
pins of the reference sit at virtual
ground, and the negative reference voltage and load current are
taken directly from the output of the operational amplifier. Note
that in applications where the negative supply voltage is close to
the reference output voltage, a dual-supply, low offset, rail-to-
rail output amplifier must be used to ensure an accurate output
voltage. The operational amplifier must also be able to source or
sink an appropriate amount of current for the application.
Bipolar Output Reference
Figure 43 shows a bipolar reference configuration. By connecting
the output of the ADR3450 to the inverting terminal of an
operational amplifier, it is possible to obtain both positive and
negative reference voltages. R1 and R2 must be matched as
closely as possible to ensure minimal difference between the
negative and positive outputs. Resistors with low temperature
coefficients must also be used if the circuit is used in environments
with large temperature swings; otherwise, a voltage difference
develops between the two outputs as the ambient temperature
changes.
V
IN
+15V
–15V
–5V
+5V
ADA4000-1
0.1µF1µF
0.1µF
R1
10k
R2
10k
R3
5k
08440-050
ADR3450
V
IN
ENABLE
V
OUT
FORCE
V
OUT
SENSE
GND FORCE
GND SENSE
4
3
6
5
2
1
Figure 43. ADR3450 Bipolar Output Reference
Boosted Output Current Reference
Figure 44 shows a configuration for obtaining higher current
drive capability from the ADR34xx references without
sacrificing accuracy. The op amp regulates the current flow
through the MOSFET until V
OUT
equals the output voltage of
the reference; current is then drawn directly from V
IN
instead of
from the reference itself, allowing increased current drive
capability.
0.1µF
C
L
0.1µF
08440-051
2N7002
AD8663
V
IN
U6
V
OUT
+16V
0.1µF1µF
R1
100
R
L
200
ADR34xx
V
IN
ENABLE
V
OUT
FORCE
V
OUT
SENSE
GND FORCE
GND SENSE
4
3
6
5
2
1
Figure 44. Boosted Output Current Reference
Because the current-sourcing capability of this circuit depends
only on the I
D
rating of the MOSFET, the output drive capability
can be adjusted to the application simply by choosing an
appropriate MOSFET. In all cases, the V
OUT
SENSE pin should
be tied directly to the load device to maintain maximum output
voltage accuracy.