REF193 AD [Analog Devices], REF193 Datasheet - Page 21

• Manufacturer Part Number: REF193
• Description: Precision Micropower, Low Dropout Voltage References
• Manufacturer: AD [Analog Devices]

Kelvin Connections
In many portable instrumentation applications where PC board
cost and area go hand-in-hand, circuit interconnects are very
often of dimensionally minimum width. These narrow lines
can cause large voltage drops if the voltage reference is required
to provide load currents to various functions. In fact, a cir-
cuit’s interconnects can exhibit a typical line resistance of
0.45 mΩ/square (1 oz. Cu, for example). In those applications
where these devices are configured as low dropout voltage regu-
lators, these wiring voltage drops can become a large source of
error. To circumvent this problem, force and sense connections
can be made to the reference through the use of an operational
amplifier, as shown in Figure 10. This method provides a means
by which the effects of wiring resistance voltage drops can be
eliminated. Load currents flowing through wiring resistance
produce an I-R error (I
Kelvin connection overcomes the problem by including the
wiring resistance within the forcing loop of the op amp. Since the
op amp senses the load voltage, op amp loop control forces the
output to compensate for the wiring error and to produce the
correct voltage at the load. Depending on the reference device
chosen, operational amplifiers that can be used in this application
are the OP295, the OP291, and the OP183/OP283.
REV. E
SLEEP
Figure 10. A Low Dropout, Kelvin Connected
Voltage Reference
REF19x
GND
V
V
IN
IN
V
OUT
COMMON
V
LOAD
S
+V
, V
+V
BAT
BAT
S
1 F
R
WIRE
100k
3
2
R2
100k
R1
1.1M
A1
) at the load. However, the
V
A1 = 1/2 OP295
IN
C4
0.1 F
1/2 OP292
1/2 OP283
1
10M
R3
R
R
R5
100k
LW
LW
3
2
Figure 11. A Fail-Safe 5 V Reference
4
SENSE
R
+V
FORCE
+V
7
AD820
L
OUT
OUT
U3
6
900k
100
R4
R6
–21–
Q1
2N3904
A Fail-Safe 5 V Reference
Some critical applications require a reference voltage to be main-
tained constant, even with a loss of primary power. The low standby
power of the REF19x series and the switched output capability
allow a “fail-safe” reference configuration to be implemented
rather easily. This reference maintains a tight output voltage
tolerance for either a primary power source (ac line derived) or
a standby (battery derived) power source, automatically switching
between the two as the power conditions change.
The circuit in Figure 11 illustrates the concept, which borrows
from the switched output idea of Figure 8, again using the REF19x
device family output “wire-OR” capability. In this case, since a
constant 5 V reference voltage is desired for all conditions, two
REF195 devices are used for U1 and U2, with their ON/OFF
switching controlled by the presence or absence of the primary
dc supply source, V
plies power to the load only when V
power conditions, V
drain of U1 in its OFF state.
In operation, it is assumed that for all conditions either U1 or U2
is ON and a 5 V reference output is available. With this voltage
constant, a scaled down version is applied to the comparator IC
U3, providing a fixed 0.5 V input to the (–) input for all power
conditions. The R1–R2 divider provides a signal to the U3 (+)
input proportional to V
upon the absolute level of V
a comparator with hysteresis, which provides for clean, noise free
output switching. This hysteresis is important to eliminate rapid
switching at the threshold due to V
chosen is the AD820, a rail-to-rail output device that provides
HI and LO output states within a few mV of V
accurate thresholds and compatible drive for U2 for all V
tions. R3 provides positive feedback for circuit hysteresis, changing
the threshold at the (+) input as a function of U3’s output.
C2
0.1 F
C1
0.1 F
REF195
REF195
U1
U2
S
. V
BAT
S
BAT
, which switches U3 and U1/U2 dependent
sees only the 15 µA (max) standby current
is a 6 V battery backup source that sup-
S
. Op amp U3 is configured here as
C3
1 F
S
fails. For normal (V
S
COMMON
V
5.000V
OUT
REF19x Series
ripple. Further, the device
S
and ground for
S
present)
S
condi-