Datasheet

Data Sheet ADR3525/ADR3530/ADR3533/ADR3540/ADR3550
Rev. 0 | Page 19 of 20
SAMPLE APPLICATIONS
Negative Reference
Figure 39 shows how to connect the ADR3550 and a standard
CMOS op amp, such as the AD8663, to provide a negative
reference voltage. This configuration provides two main
advantages: first, it requires only 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
09594-039
ADR3550
V
IN
ENABLE
V
OUT
FORCE
V
OUT
SENSE
GND FORCE
GND SENSE
8
1
6
7
2
3
Figure 39. ADR3550 Negative Reference
In this configuration, the V
OUT
FORCE and V
OUT
SENSE 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 40 shows a bipolar reference configuration. By connecting
the output of the ADR3550 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
0
9594-040
ADR3550
V
IN
ENABLE
V
OUT
FORCE
V
OUT
SENSE
GND FORCE
GND SENSE
8
1
6
7
2
3
Figure 40. ADR3550 Bipolar Output Reference
Boosted Output Current Reference
Figure 41 shows a configuration for obtaining higher current
drive capability from the ADR35xx 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
09594-041
2N7002
AD8663
V
IN
U6
V
OUT
+16V
0.1µF1µF
R1
100
R
L
200
V
IN
ENABLE
V
OUT
FORCE
V
OUT
SENSE
GND FORCE
GND SENSE
8
1
6
7
2
3
AD3525/ADR3530/
ARD3533/ADR3540/
ADR3550
Figure 41. 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.