Datasheet
Data Sheet REF19x Series
Rev. L | Page 25 of 28
R5
100kΩ
R6
100Ω
R3
10MΩ
U3
AD820
C4
0.1µF
R2
100kΩ
R1
1.1MΩ
Q1
2N3904
+
V
BAT
V
S
, V
BAT
COMMON
V
OUT
COMMON
C2
0.1µF
C1
0.1µF
5.000V
C3
1µF
U1
REF195
(SEE TABLE)
2
3
4
U2
REF195
(SEE TABLE)
2
3
4
+
6
6
+V
S
R4
900kΩ
4
2
7
6
3
+
–
0
0371-030
Figure 32. Fail-Safe 5 V Reference
For V
S
levels lower than the lower threshold, the output of U3 is
low, thus U2 and Q1 are off and U1 is on. For V
S
levels higher
than the upper threshold, the situation reverses, with U1 off and
both U2 and Q1 on. In the interest of battery power conserva-
tion, all of the comparison switching circuitry is powered from
V
S
and is arranged so that when V
S
fails, the default output
comes from U1.
For the R1 to R3 values, as shown in Figure 32, lower/upper V
S
switching thresholds are approximately 5.5 V and 6 V, respec-
tively. These can be changed to suit other V
S
supplies, as can the
REF19x devices used for U1 and U2, over a range of 2.5 V to
5 V of output. U3 can operate down to a V
S
of 3.3 V, which is
generally compatible with all REF19x family devices.
LOW POWER, STRAIN GAGE CIRCUIT
As shown in Figure 33, the REF19x family of references can be
used in conjunction with low supply voltage operational ampli-
fiers, such as the OP492 or the OP747, in a self-contained strain
gage circuit in which the REF195 is used as the core. Other
references can be easily accommodated by changing circuit
element values. The references play a dual role, first as the
voltage regulator to provide the supply voltage requirements of
the strain gage and the operational amplifiers, and second as a
precision voltage reference for the current source used to
stimulate the bridge. A distinct feature of the circuit is that it
can be remotely controlled on or off by digital means via
the
SLEEP
pin.
500Ω
0.1%
57kΩ
1%
0.1μF
10kΩ
1%
0.1μF
1/4
OP492
10μF
REF195
2.21kΩ
20kΩ
1%
10kΩ
1%
20kΩ
1%
20kΩ
1%
20kΩ
1%
0.01μF
10μF
100Ω
OUTPUT
2N2222
+
1μF
+
62
4
+
–
1
4
11
2
3
1/4
OP492
+
–
7
5
6
1/4
OP492
+
–
14
12
13
1/4
OP492
+
–
8
10
9
10kΩ
1%
00371-031
Figure 33. Low Power, Strain Gage Circuit