ADR291 datasheet(14/20 Pages) AD

14 / 20 page

ADR291/ADR292

Rev. F | Page 14 of 20

APPLICATIONS INFORMATION

HIGH VOLTAGE FLOATING CURRENT SOURCE

The circuit shown in Figure 33 can be used to generate a

floating current source with minimal self-heating. This

particular configuration operates on high supply voltages

determined by the breakdown voltage of the N-channel JFET.

GND

+VS

ADR291/

ADR292

VIN

E231

SILICONIX

2N3904

2.10kΩ

–VS

OP90

Figure 33. High Voltage Floating Current Source

KELVIN CONNECTIONS

In many portable instrumentation applications, the PC board

area is directly related to cost; therefore, circuit interconnects

are reduced to a minimal width. These narrow lines can cause

large voltage drops if the voltage reference is required to provide

load currents to various functions. In fact, circuit interconnects

can exhibit a typical line resistance of 0.45 mΩ/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 × IL) at

the load. However, the Kelvin connection shown in Figure 34

overcomes the problem by including the wiring resistance

within the forcing loop of the op amp. Since the op amp senses

the load voltage, the op amp loop control forces the output to

compensate for the wiring error producing the correct voltage

at the load.

1µF

100kΩ

+VOUT

SENSE

A1 = 1/2 OP295

VIN

RLW

+VOUT

FORCE

VOUT

GND

VIN

ADR291/

ADR292

Figure 34. Advantage of Kelvin Connection

LOW POWER, LOW VOLTAGE REFERENCE FOR

DATA CONVERTERS

The ADR291/ADR292 family has a number of features that

makes it ideally suited for use with analog-to-digital and digital-

to-analog converters. Because of its low supply voltage, the

ADR291 can be used with converters that run on 3 V supplies

without having to add a higher supply voltage for the reference.

The low quiescent current (12 μA maximum) and low noise,

tight temperature coefficient, combined with the high accuracy

of the ADR291/ADR292, make it ideal for low power applica-

tions such as handheld, battery-operated equipment.

One such ADC for which the ADR291 is well suited is the

AD7701. Figure 35 shows the ADR291 used as the reference

for this converter. The AD7701 is a 16-bit ADC with on-chip

digital filtering intended for the measurement of wide dynamic

range, low frequency signals such as those representing chemical,

physical, or biological processes. It contains a charge balancing

(Σ-Δ) ADC, calibration microcontroller with on-chip static

RAM, a clock oscillator, and a serial communications port.

This entire circuit runs on ±5 V supplies. The power dissipation

of the AD7701 is typically 25 mW and, when

combined with the power dissipation of the ADR291 (60 μW),

the entire circuit still consumes about 25 mW.

BP/UP

CAL

VREF

AIN

AGND

AVSS

AVDD

DVDD

SLEEP

MODE

DRDY

SCLK

SDATA

CLKIN

CLKOUT

SC1

SC2

DGND

DVSS

0.1µF

DATA READY

READ (TRANSMIT)

SERIAL CLOCK

0.1µF

10µF

0.1µF

–5V

ANALOG

SUPPLY

+5V

ANALOG

SUPPLY

ANALOG

GROUND

ANALOG

INPUT

CALIBRATE

RANGES

SELECT

0.1µF

ADR291

0.1µF

GND

VIN

VOUT

10µF

0.1µF

AD7701

Figure 35. Low Power, Low Voltage Supply Reference for the AD7701

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