ref195gru-reel Analog Devices, Inc., ref195gru-reel Datasheet - Page 21

• Manufacturer Part Number: ref195gru-reel
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: Analog Devices, Inc.

One caveat to this approach is that although rail-to-rail output
amplifiers work best in the application, these operational amplifiers
require a finite amount (mV) of headroom when required to
provide any load current. The choice for the circuit’s negative
supply should take this issue into account.
TTL/CMOS
STACKING REFERENCE ICS FOR
ARBITRARY OUTPUTS
Some applications may require two reference voltage sources
that are a combined sum of standard outputs. The circuit shown
in Figure 25 shows how this stacked output reference can be
implemented.
Two reference ICs are used, fed from a common unregulated
input, V
series, as shown in Figure 25, which provide two output
voltages, V
while V
of U2. U1 and U2 are chosen for the two voltages that supply
the required outputs (see Output Table in Figure 25). If, for
example, both U1 and U2 are REF192s, the two outputs are 2.5 V
and 5.0 V.
Figure 24. Negative Precision Voltage Reference Uses No Precision Resistors
+VS
V
V
COMMON
SLEEP
S
IN
> V
OUT2
10kΩ
OUT2
S
. The outputs of the individual ICs are connected in
10kΩ
Figure 25. Stacking Voltage References with the REF19x
0.1µF
0.1µF
+ 0.15V
OUT1
C1
C3
is the sum of this voltage and the terminal voltage
2N3906
and V
3
SLEEP
3
3
REF19x
(SEE TABLE)
(SEE TABLE)
REF19x
REF19x
GND
OUT2
V
V
2
4
IN
IN
U2
U1
2
4
2
4
V
OUT
. V
100kΩ
U1/U2
REF192/REF192
REF192/REF194
REF192/REF195
OUT1
6
6
6
V
V
O
O
is the terminal voltage of U1,
(U2)
(U1)
1kΩ
1µF
OUTPUT TABLE
+
+
C2
1µF
C4
1µF
V
2.5
2.5
2.5
A1
A1 = 1/2 OP295,
OUT1
+5V
–5V
R1
3.9kΩ
(SEE TEXT)
1µF
1/2 OP291
(V)
V
5.0
7.0
7.5
100Ω
OUT2
+V
+V
V
COMMON
OUT
(V)
OUT2
OUT1
–V
REF
Rev. I | Page 21 of 28
While this concept is simple, some cautions are needed. Since
the lower reference circuit must sink a small bias current from
U2 (50 μA to 100 μA), 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, Resistor R1 should be used, set to a
value that conservatively passes 600 μA of current with the
applicable V
reference circuits are locally treated as macrocells, each having
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
sum of the outputs, V
A related variation on stacking two 3-terminal references is
shown in Figure 26, where U1, a REF192, is stacked with a
2-terminal reference diode, such as the AD589. Like the
3-terminal stacked reference above, this circuit provides two
outputs, VOUT1 and VOUT2, which are the individual terminal
voltages of D1 and U1, respectively. Here this is 1.235 V and 2.5 V,
which provides a V
reference diodes, such as D1, the rated minimum and maximum
device currents must be observed, and the maximum load
current from V
R1 and V
500 μA bias to D1, so the maximum load current available at
V
PRECISION CURRENT SOURCE
In low power applications, the need often arises for a precision
current source that can operate on low supply voltages. As shown
in Figure 27, any one of the devices in the REF19x family of
references can be configured as a precision current source.
The circuit configuration illustrated is a floating current source
with a grounded load. The output voltage of the reference is
bootstrapped across R
load. With this configuration, circuit precision is maintained for
load currents in the range from the reference’s supply current
(typically 30 μA) to approximately 30 mA. The low dropout
voltage of these devices maximizes the current source’s output
voltage compliance without excess headroom.
OUT1
+V
V
V
COMMON
S
IN
S
> V
is 450 μA or less.
0.1µF
OUT2
Figure 26. Stacking Voltage References with the REF192
O
C1
(U1). When V
+ 0.15V
OUT1
OUT1
AD589
3
across it. Note that the two U1 and U2
REF192
D1
OUT2
can be no greater than the current setup by
U1
2
4
OUT2
SET
of 3.735 V. When using 2-terminal
, which sets the output current into the
, plus the dropout voltage of U2.
O
6
(U1) is equal to 2.5 V, R1 provides a
V
V
O
O
(U1)
(D1)
+
+
S
, is determined by the
C2
1µF
C3
1µF
REF19x Series
R1
4.99kΩ
(SEE TEXT)
+V
3.735V
+V
1.235V
V
COMMON
OUT
OUT2
OUT1