ADR380/ADR381

Rev. C | Page 12 of 16

APPLICATIONS INFORMATION

STACKING REFERENCE ICs FOR ARBITRARY

OUTPUTS

Some applications may require two reference voltage sources,

which are a combined sum of standard outputs. The following

circuit shows how this stacked output reference can be

implemented:

GND

3

GND

C2

1µF

C1

0.1µF

C3

0.1µF

C4

1µF

3

R1

3.9k

Ω

2

2

1

1

U2

ADR380/

ADR381

U1

ADR380/

ADR381

Figure 27. Stacking Voltage References with the ADR380/ADR381

Two ADR380s or ADR381s are used; the outputs of the individ-

ual references are simply cascaded to reduce the supply current.

sum of this voltage and the terminal voltage of U2. U1 and U2

can be chosen for the two different voltages that supply the

required outputs.

While this concept is simple, a precaution is in order. Because

the lower reference circuit must sink a small bias current from

U2, plus the base current from the series PNP output transistor

in U2, the external load of either 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

that the two U1 and U2 reference circuits are locally treated as

macrocells, each having its own bypasses at input and output for

optimum stability. Both U1 and U2 in this circuit can source dc

dropout voltage of U2.

A NEGATIVE PRECISION REFERENCE WITHOUT

PRECISION RESISTORS

In many current-output CMOS DAC applications where the

output signal voltage must be of the same polarity as the

reference voltage, it is often required to reconfigure a current-

switching DAC into a voltage-switching DAC through the use

of a 1.25 V reference, an op amp, and a pair of resistors. Using

a current switching DAC directly requires an additional opera-

tional amplifier at the output to reinvert the signal. A negative

voltage reference is then desirable from the point that an additional

operational amplifier is not required for either reinversion

(current-switching mode) or amplification (voltage-switching

mode) of the DAC output voltage. In general, any positive voltage

reference can be converted into a negative voltage reference

through the use of an operational amplifier and a pair of matched

resistors in an inverting configuration. The disadvantage to this

approach is that the largest single source of error in the circuit is

the relative matching of the resistors used.

The circuit in Figure 28 avoids the need for tightly matched

resistors with the use of an active integrator circuit. In this

circuit, the output of the voltage reference provides the input

drive for the integrator. The integrator, to maintain circuit

equilibrium, adjusts its output to establish the proper relation-

output voltage desired can be chosen by substituting for the

appropriate reference IC. A precaution should be noted with

this approach: 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.

GND

C2

0.1µF

3

+5V

2

A1

1

U2

–5V

OP195

–V

+V

C1

1µF

U1

ADR380/

ADR381

R4

1k

Ω

R3

100k

Ω

C3

1µF

C4

1µF

R5

100

Ω

Figure 28. Negative Precision Voltage Reference Using No Precision Resistors

PRECISION CURRENT SOURCE

Many times in low power applications, the need arises for a

precision current source that can operate on low supply voltages.

As shown in Figure 29, the ADR380/ADR381 can be configured

as a precision current source. The circuit configuration illustrated

is a floating current source with a grounded load. The reference

the output current into the load. With this configuration, circuit

precision is maintained for load currents in the range from the

reference supply current, typically 90 μA to approximately 5 mA.

GND

3

2

R1

1

P1

ADJUST

U1

ADR380/

ADR381

C3

1µF

C1

1µF

C2

0.1µF

Figure 29. Precision Current Source