ADR290/ADR291/ADR292

REV. B

–13–

APPLICATIONS SECTION

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 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 amplification (voltage-

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

positive voltage reference can be converted into a negative volt-

age reference through the use of an operational amplifier and a

pair of matched resistors in an inverting configuration. The dis-

advantage to that approach is that the largest single source of

error in the circuit is the relative matching of the resistors used.

The circuit illustrated 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

output voltage desired can be chosen by simply substituting for

the appropriate reference IC. One caveat with this approach

should be mentioned: 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.

A1

100

+5V

–5V

1k

1 F

100k

GND

ADR29x

A1 = 1/2 OP291,

1/2 OP295

1 F

Figure 3. A Negative Precision Voltage Reference Uses No

Precision Resistors

A 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, any one of the devices in the ADR29x

family of references can be configured as a precision current

source. The circuit configuration illustrated is a floating current

source with a grounded load. The reference’s output voltage is

load. With this configuration, circuit precision is maintained for

load currents in the range from the reference’s supply current,

typically 12

µA to approximately 5 mA.

1 F

GND

ADR29x

ADJUST

R1

P1

Figure 4. A Precision Current Source

High Voltage Floating Current Source

The circuit of Figure 5 can be used to generate a floating

current source with minimal self heating. This particular con-

figuration can operate on high supply voltages determined by

the breakdown voltage of the N-channel JFET.

OP90

GND

E231

SILICONIX

2N3904

2.10k

Figure 5. High Voltage Floating Current Source

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 circuit’s interconnects

can exhibit a typical line resistance of 0.45 m

Ω/square (1 oz. Cu,

for example). Force and sense connections also referred to as

Kelvin connections, offer a convenient method of eliminating the

effects of voltage drops in circuit wires. Load currents flowing

the load. However, the Kelvin connection of Figure 6, 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.