REV. E

REF19x Series

–21–

U1

REF195

C1

0.1 F

COMMON

COMMON

5.000V

C3

1 F

U2

REF195

R5

100k

R6

100

C2

0.1 F

6

4

2

3

U3

AD820

R4

900k

C4

0.1 F

R2

100k

R3

10M

R1

1.1M

Q1

2N3904

7

Figure 11. A Fail-Safe 5 V Reference

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 m

Ω/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

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.

1 F

REF19x

GND

100k

SLEEP

2

3

1

A1

SENSE

FORCE

A1 = 1/2 OP295

1/2 OP292

1/2 OP283

Figure 10. A Low Dropout, Kelvin Connected

Voltage Reference

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

µA (max) standby current

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 (+)

a comparator with hysteresis, which provides for clean, noise free

output switching. This hysteresis is important to eliminate rapid

chosen is the AD820, a rail-to-rail output device that provides

tions. R3 provides positive feedback for circuit hysteresis, changing

the threshold at the (+) input as a function of U3’s output.