REV. A

ADR380/ADR381

–11–

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 refer-

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

matched resistors in an inverting configuration. The disadvan-

tage 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 3 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 relationship between

voltage desired can be chosen by simply 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

U1

ADR380

C2

0.1 F

3

+5V

R4

1k

C1

1 F

C3

1 F

2

R3

100k

A1

1

C4

1 F

U2

–5V

OP195

R5

100

–V

+V

Figure 3. A Negative Precision Voltage Reference

Uses No Precision Resistors

Precision Current Source

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

cision current source that can operate on low supply voltages.

As shown in Figure 4, 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’s

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

precision is maintained for load currents in the range from the

reference’s supply current, typically 90

µA to approximately 5 mA.

GND

U1

ADR380

C2

0.1 F

3

C1

1 F

C3

1 F

2

R1

1

P1

ADJUST

Figure 4. A Precision Current Source

Precision High Current Voltage Source

In some cases, the user may want higher output current delivered

to a load and still achieve better than 0.5% accuracy out of the

ADR380/ADR381. The accuracy for a reference is normally

specified on the data sheet with no load. However, the output

voltage changes with load current.

The circuit in Figure 5 provides high current without compro-

mising the accuracy of the ADR380/ADR381. By op amp action,

equilibrium, the op amp also drives the N-Ch MOSFET Q1 into

saturation to maintain the current needed at different loads. R2

is optional to prevent oscillation at Q1. In such an approach, hun-

dreds of milliamps of load current can be achieved and the current

GND

U1

ADR380/

ADR381

3

2

R2

100

1

C1

0.001 F

Q1

2N7002

+8 –15V

R1

100k

A1

–V

+V

AD820

Figure 5. ADR380/ADR381 for Precision High

Current Voltage Source