Data Sheet
AD590
V+
Figure 21 is an example of a current transmitter designed to be
used with 40 V, 1 kΩ systems; it uses its full current range of 4
to 20 mA for a narrow span of measured temperatures. In this
example, the 1 µA/K output of the AD590 is amplified to
1 mA/°C and offset so that 4 mA is equivalent to 17°C and
20 mA is equivalent to 33°C. RT is trimmed for proper reading
at an intermediate reference temperature. With a suitable choice
of resistors, any temperature range within the operating limits
of the AD590 can be chosen.
AD581
V+
V–
OUT
10V
HEATING
ELEMENTS
+
–
R
B
R
H
AD590
7
2
3
–
+
R
SET
AD790
4
1
R
L
C1
10kΩ
V+
GND
Figure 22. Simple Temperature Control Circuit
+
–
AD581
4mA = 17°C
12mA = 25°C
20mA = 33°C
V
The voltage compliance and the reverse blocking characteristic
of the AD590 allow it to be powered directly from 5 V CMOS
logic. This permits easy multiplexing, switching, or pulsing for
minimum internal heat dissipation. In Figure 23, 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 can be employed for other purposes, but the additional
capacitance due to the AD590 should be taken into account.
OUT
35.7kΩ
30pF
R
T
5kΩ
+
–
+
AD590
OP177
–
5kΩ
500Ω
12.7kΩ
10Ω
0.01µF
10kΩ
5V
V–
Figure 21. 4 to 20 mA Current Transmitter
Figure 22 is an example of a variable temperature control circuit
(thermostat) using the AD590. RH and RL are selected to set the
high and low limits for RSET. RSET could be a simple pot, a
calibrated multiturn pot, or a switched resistive divider. Powering
the AD590 from the 10 V reference isolatesthe AD590 from
supply variations while maintaining a reasonable voltage (~7 V)
across it. Capacitor C1 is often needed to filter extraneous noise
from remote sensors. RB is determined by the β of the power
transistor and the current requirements of the load.
+
AD590
–
CMOS
GATES
+
–
+
–
+
–
1kΩ (0.1%)
Figure 23. AD590 Driven from CMOS Logic
Rev. G | Page 11 of 16
ADI [ ADI ]
