ADR293

REV. 0

–9–

APPLICATIONS

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 refer-

ence 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 the need for an

additional operational 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 amplifica-

tion (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 configu-

ration. The disadvantage to that approach is that the largest single

source of error in the circuit is the relative matching of the resis-

tors used.

The circuit illustrated in Figure 20 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-

this approach should be mentioned: although rail-to-rail output

amplifiers work best in the application, these operational ampli-

fiers 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.

ADR293

1 F

4

6

2

GND

100k

1 F

–5V

+5V

100

1k

1/2 OP295

Figure 20. A Negative Precision Voltage Reference Uses

No Precision Resistors

A Precision Current Source

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

sion current source that can operate on low supply voltages. As

shown in Figure 21, the ADR293 is configured as a precision

current source. The circuit configuration illustrated is a floating

current source with a grounded load. The reference’s output

into the load. With this configuration, circuit precision is main-

tained for load currents in the range from the reference’s supply

current, typically 15

µA to approximately 5 mA.

ADR293

4

6

2

GND

1 F

ADJUST

Figure 21. A Precision Current Source