AD590
V+
+
–
20pF
4mA = 17°C
12mA = 25°C
20mA = 33°C
AD581
V
OUT
35.7kΩ
R
T
5kΩ
30pF
–
1.25kΩ
–15V
DAC OUT
BIT 1
500Ω
REF
+
AD590
MC
1408/1508
+5V
1.15kΩ
BIT 8
BIT 7
BIT 6
BIT 5
200Ω, 15T
+5V
+2.5V
–
AD707A
+
12.7kΩ
5kΩ
BIT 2
BIT 3
BIT 4
00533-C-018
AD580
0.01µF
10kΩ
10Ω
200Ω
V–
6.98kΩ
1kΩ, 15T
+
3
2
+5V
+5V
1kΩ
Figure 18. 4 mA to 20 mA Current Transmitter
8
LM311
1
4
–15V
6.8kΩ
7
OUTPUT HIGH-
TEMPERATURE ABOVE SET POINT
OUTPUT LOW-
TEMPERATURE BELOW SET POINT
5.1MΩ
00533-C-020
(thermostat) using the AD590. R
H
and R
L
are selected to set the
high and low limits for R
SET
. R
SET
could be a simple pot, a
calibrated multiturn pot, or a switched resistive divider.
Powering the AD590 from the 10 V reference isolates the
AD590 from supply variations while maintaining a reasonable
voltage (~7 V) across it. Capacitor C
1
is often needed to filter
extraneous noise from remote sensors. R
B
is determined by the
β of the power transistor and the current requirements of the
load.
V+
V+
V–
R
H
R
SET
R
L
C
1
AD581
OUT
10V
–
AD590
–15V
Figure 20. DAC Set Point
AD590
+
–
2 –
7
R
B
HEATING
ELEMENTS
The voltage compliance and the reverse blocking characteristic
of the AD590 allows it to be powered directly from 5 V CMOS
logic. This permits easy multiplexing, switching, or pulsing for
minimum internal heat dissipation. In Figure 21, any AD590
connected to a logic high passes a signal current through the
current measuring circuitry, while those connected to a logic
zero pass insignificant current. The outputs used to drive the
AD590s may be employed for other purposes, but the additional
capacitance due to the AD590 should be taken into account.
5V
LM311
3 +
10kΩ
GND
1
4
00533-C-019
+
CMOS
GATES
+
+
–
–
AD590
Figure 19. Simple Temperature Control Circuit
DAC to produce a digitally controlled set point. This particular
circuit operates from 0°C (all inputs high) to 51.0°C (all inputs
low) in 0.2°C steps. The comparator is shown with 1.0°C
hysteresis, which is usually necessary to guard-band for
extraneous noise. Omitting the 5.1 MΩ resistor results in no
hysteresis.
+
–
–
Figure 21. AD590 Driven from CMOS Logic
CMOS analog multiplexers can also be used to switch AD590
current. Due to the AD590’s current mode, the resistance of
such switches is unimportant as long as 4 V is maintained
across the transducer. Figure 22 shows a circuit that combines
the principle demonstrated in Figure 21 with an 8-channel
CMOS multiplexer. The resulting circuit can select 1–80 sensors
over only 18 wires with a 7-bit binary word.
Rev. C | Page 11 of 16
00533-C-021
1kΩ (0.1%)
AD [ ANALOG DEVICES ]
