AD594/AD595
REV. C
–7–
ALARM CIRCUIT
In all applications of the AD594/AD595 the –ALM connection,
Pin 13, should be constrained so that it is not more positive
than (V+) – 4 V. This can be most easily achieved by connect-
ing Pin 13 to either common at Pin 4 or V– at Pin 7. For most
applications that use the alarm signal, Pin 13 will be grounded
and the signal will be taken from +ALM on Pin 12. A typical
application is shown in Figure 10.
In this configuration the alarm transistor will be off in normal
operation and the 20 k pull up will cause the +ALM output on
Pin 12 to go high. If one or both of the thermocouple leads are
interrupted, the +ALM pin will be driven low. As shown in Fig-
ure 10 this signal is compatible with the input of a TTL gate
which can be used as a buffer and/or inverter.
CONSTANTAN
(ALUMEL)
IRON
(CHROMEL)
+5V
ALARM
TTL GATE
20kV
ALARM OUT
GND
OVERLOAD
DETECT
G
–TC
+TC
1234567
13 12 11 10
AD594/
AD595
14
ICE
POINT
COMP.
+A
98
G
10mV/8C
Figure 10. Using the Alarm to Drive a TTL Gate
(“Grounded’’ Emitter Configuration)
Since the alarm is a high level output it may be used to directly
drive an LED or other indicator as shown in Figure 11.
CONSTANTAN
(ALUMEL)
IRON
(CHROMEL)
COMMON
LED
270V
V+
OVERLOAD
DETECT
G
–TC
+TC
123 4 5
67
13 12 11 10
AD594/
AD595
14
ICE
POINT
COMP.
+A
98
G
10mV/8C
Figure 11. Alarm Directly Drives LED
A 270 Ω series resistor will limit current in the LED to 10 mA,
but may be omitted since the alarm output transistor is current
limited at about 20 mA. The transistor, however, will operate in
a high dissipation mode and the temperature of the circuit will
rise well above ambient. Note that the cold junction compensa-
tion will be affected whenever the alarm circuit is activated. The
time required for the chip to return to ambient temperature will
depend on the power dissipation of the alarm circuit, the nature
of the thermal path to the environment and the alarm duration.
The alarm can be used with both single and dual supplies. It
can be operated above or below ground. The collector and emit-
ter of the output transistor can be used in any normal switch
configuration. As an example a negative referenced load can be
driven from –ALM as shown in Figure 12.
CONSTANTAN
(ALUMEL)
IRON
(CHROMEL)
+10V
GND
ALARM
RELAY
–12V
OVERLOAD
DETECT
G
–TC
+TC
1234567
13 12 11 10
AD594/
AD595
14
ICE
POINT
COMP.
+A
98
G
10mV/8C
Figure 12. –ALM Driving A Negative Referenced Load
The collector (+ALM) should not be allowed to become more
positive than (V–) +36 V, however, it may be permitted to be
more positive than V+. The emitter voltage (–ALM) should be
constrained so that it does not become more positive than 4
volts below the V+ applied to the circuit.
Additionally, the AD594/AD595 can be configured to produce
an extreme upscale or downscale output in applications where
an extra signal line for an alarm is inappropriate. By tying either
of the thermocouple inputs to common most runaway control
conditions can be automatically avoided. A +IN to common
connection creates a downscale output if the thermocouple opens,
while connecting –IN to common provides an upscale output.
CELSIUS THERMOMETER
The AD594/AD595 may be configured as a stand-alone Celsius
thermometer as shown in Figure 13.
+5V TO +15V
OUTPUT
10mV/8C
GND
0 TO –15V
OVERLOAD
DETECT
G
–TC
+TC
123
4567
13 12 11 10
AD594/
AD595
14
ICE
POINT
COMP.
+A
98
G
Figure 13. AD594/AD595 as a Stand-Alone Celsius
Thermometer
Simply omit the thermocouple and connect the inputs (Pins 1
and 14) to common. The output now will reflect the compensa-
tion voltage and hence will indicate the AD594/AD595
temperature with a scale factor of 10 mV/°C. In this three termi-
nal, voltage output, temperature sensing mode, the AD594/
AD595 will operate over the full military –55°C to +125°C tem-
perature range.
