ADR291FR-REEL Просмотр технического описания (PDF) - Analog Devices

Номер в каталоге

Компоненты Описание

производитель

ADR291FR-REEL

Low Noise Micropower Precision Voltage References

Analog Devices 

ADR290/ADR291/ADR292

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 32 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 the reference’s VOUT and GND. Thus, any negative

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.

VIN

2

ADR29x 6

VOUT

GND

4

100k⍀

1k⍀

1␮F

1␮F

+5V

A1

–5V

100⍀

–VREF

A1 = 1/2 OP291,

1/2 OP295

Figure 32. 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 33, 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

bootstrapped across RSET, which sets 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 12 µA to approximately 5 mA.

VIN

2

ADR29x 6

VOUT

GND

4

1␮F

ISY

ADJUST

·R1

RSET

P1

IOUT

RL

Figure 33. A Precision Current Source

High Voltage Floating Current Source

The circuit of Figure 34 can be used to generate a floating cur-

rent source with minimal self heating. This particular configura-

tion can operate on high supply voltages determined by the

breakdown voltage of the N-channel JFET.

+VS

E231

SILICONIX

VIN

ADR290

GND

OP90

2N3904

2.10k⍀

–VS

Figure 34. 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 mW/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

through wiring resistance produce an error (VERROR = R ϫ IL ) at

the load. However, the Kelvin connection of Figure 35, 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.

REV. A

–13–

Share Link: