adr391 Analog Devices, Inc., adr391 Datasheet - Page 17

• Manufacturer Part Number: adr391
• Description: Micropower, Low Noise Precision Voltage References With Shutdown
• Manufacturer: Analog Devices, Inc.

APPLICATIONS
BASIC VOLTAGE REFERENCE CONNECTION
The circuit shown in Figure 41 illustrates the basic configuration
for the ADR39x family. Decoupling capacitors are not required
for circuit stability. The ADR39x family is capable of driving
capacitive loads from 0 µF to 10 µF. However, a 0.1 µF ceramic
output capacitor is recommended to absorb and deliver the
charge, as required by a dynamic load.
Stacking Reference ICs for Arbitrary Outputs
Some applications may require two reference voltage sources,
which are a combined sum of standard outputs. Figure 42 shows
how this stacked output reference can be implemented.
SHUTDOWN
*NOT REQUIRED
V
IN
INPUT
Figure 41. Basic Configuration for the ADR39x Family
Figure 42. Stacking Voltage References with the
C
B
0.1 µ F
ADR390/ADR390
ADR391/ADR391
ADR392/ADR392
ADR395/ADR395
0.1 µ F
*
0.1 µ F
ADR390/ADR391/ADR392/ADR395
C2
C2
U1/U2
OUTPUT TABLE
1
1
SHDN
SHDN
SHDN
V
V
IN
OUT(S)
V
2.048
2.5
4.096
5
ADR39x
OUT1
GND
GND
2
5
2
5
V
V
IN
IN
V
V
V
V
(V)
OUT(F)
OUT(S)
OUT(F)
OUT(S)
V
U2
U1
OUT(F)
GND
4.096
5.0
8.192
10
V
OUT2
4
3
4
3
C
B
(V)
*
0.1 µ F
OUTPUT
V
V
OUT2
OUT1
Rev. F | Page 17 of 20
Two reference ICs are used, fed from an unregulated input, V
The outputs of the individual ICs are simply connected in
series, which provides two output voltages, V
V
this voltage and the terminal voltage of U2. U1 and U2 are
simply chosen for the two voltages that supply the required
outputs (see the Output Table in Figure 42). For example, if
both U1 and U2 are ADR391s, V
While this concept is simple, a precaution is required. Since the
lower reference circuit must sink a small bias current from U2
plus the base current from the series PNP output transistor in
U2, either the external load of U1 or R1 must provide a path for
this current. If the U1 minimum load is not well defined, the R1
resistor should be used and set to a value that will conservatively
pass 600 µA of current with the applicable V
that the two U1 and U2 reference circuits are treated locally as
macrocells; each has its own bypasses at input and output for
best stability. Both U1 and U2 in this circuit can source dc
currents up to their full rating. The minimum input voltage,
V
dropout voltage of U2.
A Negative Precision Reference without Precision
Resistors
A negative reference can be easily generated by adding an A1 op
amp and is configured as shown in Figure 43. V
are at virtual ground and, therefore, the negative reference can
be taken directly from the output of the op amp. The op amp
must be dual-supply, low offset, and rail-to-rail if the negative
supply voltage is close to the reference output.
OUT1
IN
, is determined by the sum of the outputs, V
is the terminal voltage of U1, while V
ADR390/ADR391/ADR392/ADR395
A1
–V
DD
Figure 43. Negative Reference
4
3
V
V
OUT(F)
OUT(S)
+V
GND
5
2
V
DD
IN
OUT1
SHDN
is 2.5 V and V
1
–V
OUT2
OUT1
OUT1
REF
OUT2
is the sum of
OUTF
across it. Note
and V
, plus the
OUT2
and V
OUT2
is 5.0 V.
OUTS
.
IN
.