AD581
Rev. C | Page 9 of 12
THE AD581 AS A CURRENT LIMITER
The AD581 represents an alternative to current limiter diodes
that require factory selection to achieve a desired current. This
approach often results in temperature coefficients of 1%/°C.
The AD581 approach is not limited to a defined set current
limit; it can be programmed from 0.75 mA to 5 mA with the
insertion of a single external resistor. Of course, the minimum
voltage required to drive the connection is 13 V. The AD580,
which is a 2.5 V reference, can be used in this type of circuit
with compliance voltage down to 4.5 V.
+V
S
V
OUT
GND
D581
R
LOAD
i =
R
10V
~ ___ + 0.75mA
10V =
BOTTOM VIEW OF
10V PRECISION
REFERENCE CIRCUIT
IN TO-5 CASE
08014-012
Figure 12. A Two-Component Precision Current Limiter
NEGATIVE 10 V REFERENCE
The AD581 can also be used in a 2-pin Zener mode to provide a
precision −10.00 V reference. As shown in Figure 13, the +V
S
and
V
OUT
pins are connected together to the high supply (in this
case, ground). The ground pin is connected through a resistor
to the negative supply. Thus, the output is taken from the
ground pin instead of V
OUT
. With 1 mA flowing through the
AD581 in this mode, a typical unit shows a 2 mV increase in
output level over that produced in the 3-pin mode. Note also
that the effective output impedance in this connection increases
from 0.2 typical to 2 . It is essential to arrange the output
load and the supply resistor, R
S
, so that the net current through
the AD581 is always between 1 mA and 5 mA. For operation to
+125°C, the net current should be between 2 mA and 5 mA. The
temperature characteristics and long-term stability of the device
are essentially the same as that of a unit used in the standard
3-pin mode.
The AD581 can also be used in a 2-pin mode to develop a
positive reference. +V
S
and V
OUT
are tied together and to the
positive supply through an appropriate supply resistor. The
performance characteristics are similar to those of the negative
2-pin connection. The only advantage of this connection over
the standard 3-pin connection is that a lower primary supply
can be used, as low as 10.5 V. This type of operation requires con-
siderable attention to load and primary supply regulation to
maintain the AD581 within its regulating range of 1 mA to 5 mA
(2 mA to 5 mA for operation beyond +85°C).
AD581
GND
+V
S
V
OUT
0.1µF
–15V
1.2kΩ 5%
V
REF
–10V
NALOG
GND
8014-013
Figure 13. 2-Pin −10 V Reference
10 V REFERENCE WITH MULTIPLYING CMOS DACs
OR ADCs
The AD581 is ideal for application with the entire AD7533
series of 10- and 12-bit multiplying CMOS DACs, especially for
low power applications. It is equally suitable for the AD7574
8-bit ADC. In the standard hook-up, as shown in Figure 15, the
+10 V reference is inverted by the amplifier/DAC configuration
to produce a 0 V to −10 V range. If an OP1177 amplifier is used,
total quiescent supply current is typically 2 mA. If a 0 V to +10 V
full-scale range is desired, the AD581 can be connected to the
CMOS DAC in its −10 V Zener mode, as shown in Figure 13
(the −10 V
REF
output is connected directly to the V
REF IN
of the
CMOS DAC). The AD581 is normally used in the −10 V mode
with the AD7574 to give a 0 V to +10 V ADC range. This is
shown in Figure 15. Bipolar output applications and other
operating details can be found in the data sheets for the CMOS
products.
AD581
+15
+10V
V
REF IN
BIT 1 (MSB)
BIT 10 (LSB)
GND
DIGITAL
INPUT
+15V
–15V
V
OUT
0V TO –10V
R
FEEDBACK
I
OUT1
I
OUT2
15 14
3
4
5
13
16
1
2
08014-014
Figure 14. Low Power 10-Bit CMOS DAC Application
PRECISION 12-BIT DAC REFERENCE
AD565A, like most DACs, is designed to operate with a +10 V
reference element. In the AD565A, this 10 V reference voltage is
converted into a reference current of approximately 0.5 mA via
the internal 19.95 k resistor (in series with the external 100
trimmer). The gain temperature coefficient of the AD565A is
primarily governed by the temperature tracking of the 19.95 k
resistor and the 5 kΩ to10 kΩ span resistors; gain TC is guaran-
teed to 3 ppm/°C. Thus, using the AD581L (at 5 ppm/°C) as the
