Datasheet

Data Sheet ADR420/ADR421/ADR423/ADR425
Rev. J | Page 21 of 24
PRECISION VOLTAGE REFERENCE FOR DATA
CONVERTERS
The ADR42x family has a number of features that make it ideal
for use with ADCs and DACs. The exceptionally low noise,
tight temperature coefficient, and high accuracy characteristics
make the ADR42x ideal for low noise applications such as
cellular base station applications.
AD7701 is an example of an ADC that is well suited for the
ADR42x. The ADR421 is used as the precision reference for
the converter in Figure 48. The AD7701 is a 16-bit ADC with
on-chip digital filtering intended for measuring wide dynamic
range and low frequency signals, such as those representing
chemical, physical, or biological processes. It contains a charge-
balancing (Σ-∆) ADC, calibration microcontroller with on-chip
static RAM, clock oscillator, and serial communications port.
02432-050
AD7701
ADR420/
ADR421/
ADR423/
ADR425
V
IN
AV
DD
DV
DD
SLEE
P
MODE
DRDY
CS
SCLK
SDATA
DATA
READY
READ (TRANSMIT)
SERIAL CLOCK
SERIAL
CLOCK
CLKIN
CLKOUT
SC1
SC2
DGND
DV
SS
V
REF
BP/U
P
CA
L
A
IN
AGND
A
V
SS
0.1
µF
0.1
µF 10
µF
+5V
ANALOG
SUPPLY
RANGES
SELECT
CALIBR
ATE
ANALOG
INPUT
ANALOG
GROUND
–5V
ANALOG
SUPPLY
GND
V
OUT
0.1µ
F
0.1µF
0.1
µF
0.1
µF
10µF
Figure 48. Voltage Reference for 16-Bit ADC AD7701
PRECISION BOOSTED OUTPUT REGULATOR
A precision voltage output with boosted current capability
can be realized with the circuit shown in Figure 49. In this
circuit, U2 forces V
OUT
to be equal to V
REF
by regulating the turn
on of N1. Therefore, the load current is furnished by V
IN
. In
this configuration, a 50 mA load is achievable at V
IN
of 5 V.
Moderate heat is generated on the MOSFET, and higher current
can be achieved by replacing the larger device. In addition, for
a heavy capacitive load with step input, a buffer may be added
at the output to enhance the transient response.
02432-051
V
IN
V
OUT
R
L
25
Ω
N1
5
6
U1
2
4
U2
AD8601
5V
2N7002
V+
V–
+
–
ADR421
V
IN
V
OUT
TRIM
GND
Figure 49. Precision Boosted Output Regulator