REV. B

ADR420/ADR421/ADR423/ADR425

–15–

DACA

DACB

DACC

DACD

AD7398

ADR425

R1

0.1%

R2

0.1%

Figure 11. Programmable DAC Reference

Precision Voltage Reference for Data Converters

The ADR42x family has a number of features that make it ideal

for use with A/D and D/A converters. The exceptional low noise,

tight temperature coefficient, and high accuracy characteristics

make the ADR42x ideal for low noise applications such as cellular

base station applications.

Another example of ADC for which the ADR421 is also well-suited

is the AD7701. Figure 12 shows the ADR421 used as the precision

reference for this converter. The AD7701 is a 16-bit A/D converter

with on-chip digital filtering intended for the measurement of wide

dynamic range and low frequency signals such as those representing

chemical, physical, or biological processes. It contains a charge-

balancing (sigma-delta) ADC, calibration microcontroller with on-chip

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

GND

ADR42x

0.1 F

SC1

MODE

CLKOUT

0.1 F

10 F

0.1 F

0.1 F

0.1 F

0.1 F10 F

AD7701

–5V

ANALOG

SUPPLY

AGND

CAL

BP/

UP

DGND

SC2

CLKIN

SCLK

SDATA

DRDV

CS

SLEEP

DATA READY

READ (TRANSMIT)

SERIAL CLOCK

SERIAL CLOCK

+5V

ANALOG

SUPPLY

ANALOG

GROUND

ANALOG

INPUT

CALIBRATE

RANGES

SELECT

Figure 12. Voltage Reference for 16-Bit A/D Converter

AD7701

Precision Boosted Output Regulator

A precision voltage output with boosted current capability can

be realized with the circuit shown in Figure 13. In this circuit,

ate heat will be generated on the MOSFET and higher current

can be achieved with a replacement of the larger device. In

addition, for heavy capacitive load with step input, a buffer may

be added at the output to enhance the transient response.

AD8601

25

N1

2U1

GND

4

5

6

U2

2N7002

5V

V+

V–

ADR421

TRIM

Figure 13. Precision Boosted Output Regulator

R1, R2

Digital Code

R1 = R2

0000 0000 0000

R1 = R2

1000 0000 0000

R1 = R2

1111 1111 1111

R1 = 3R2

0000 0000 0000

R1 = 3R2

1000 0000 0000

R1 = 3R2

1111 1111 1111