Datasheet

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
2
4
+
V
S
ADR291/
ADR292
V
IN
E231
SILICONIX
2N3904
2.10kΩ
–V
S
OP90
00163-032
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 (V
ERROR
= R × I
L
) 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.
A1
1µF
100kΩ
+V
OUT
SENSE
A1 = 1/2 OP295
V
IN
R
LW
R
L
R
LW
+V
OUT
FORCE
V
OUT
GND
V
IN
2
6
4
00163-033
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
V
REF
A
IN
AGND
AV
SS
AV
DD
DV
DD
SLEEP
MODE
DRDY
SCLK
CS
SDATA
CLKIN
CLKOUT
SC1
SC2
DGND
DV
SS
0.1µF
DATA READY
READ (TRANSMIT)
SERIAL CLOCK
SERIAL CLOCK
0.1µF
10µF0.1µF
–5V
ANALOG
SUPPLY
+5
V
ANALOG
SUPPLY
ANALOG
GROUND
ANALOG
INPUT
CALIBRATE
RANGES
SELECT
0.1µF
ADR291
0.1µF
GND
V
IN
V
OUT
10µF
0.1µF
AD7701
00163-034
Figure 35. Low Power, Low Voltage Supply Reference for the AD7701