AD594/AD595
CONSTANTAN
(ALUMEL)
14
13
12
11
+5V TO +30V
10
9
8
The printed circuit board layout shown also provides for place-
ment of optional alarm load resistors, recalibration resistors and
a compensation capacitor to limit bandwidth.
To ensure secure bonding the thermocouple wire should be
cleaned to remove oxidation prior to soldering. Noncorrosive
rosin flux is effective with iron, constantan, chromel and alumel
and the following solders: 95% tin-5% antimony, 95% tin-5%
silver or 90% tin-10% lead.
FUNCTIONAL DESCRIPTION
OVERLOAD
DETECT
AD594/
AD595
G
G
+A
ICE
POINT –TC
COMP.
SPAN OF
5V TO 30V
+TC
IRON
(CHROMEL)
1
2
3
4
5
6
7
COMMON
0V TO –25V
Figure 2. Dual Supply Operation
The AD594 behaves like two differential amplifiers. The out-
puts are summed and used to control a high gain amplifier, as
shown in Figure 4.
–IN
14
–ALM
13
+ALM
12
V+
11
COMP
10
VO
9
FB
8
With a negative supply the output can indicate negative tem-
peratures and drive grounded loads or loads returned to positive
voltages. Increasing the positive supply from 5 V to 15 V ex-
tends the output voltage range well beyond the 750°C
temperature limit recommended for type J thermocouples
(AD594) and the 1250°C for type K thermocouples (AD595).
Common-mode voltages on the thermocouple inputs must remain
within the common-mode range of the AD594/AD595, with a
return path provided for the bias currents. If the thermocouple
is not remotely grounded, then the dotted line connections in
Figures 1 and 2 are recommended. A resistor may be needed in
this connection to assure that common-mode voltages induced
in the thermocouple loop are not converted to normal mode.
THERMOCOUPLE CONNECTIONS
OVERLOAD
DETECT
AD594/AD595
+A
G
G
+TC
ICE
POINT
COMP. –TC
1
+IN
2
+C
3
+T
4
COM
5
–T
6
–C
7
V–
Figure 4. AD594/AD595 Block Diagram
The isothermal terminating connections of a pair of thermo-
couple wires forms an effective reference junction. This junction
must be kept at the same temperature as the AD594/AD595 for
the internal cold junction compensation to be effective.
A method that provides for thermal equilibrium is the printed
circuit board connection layout illustrated in Figure 3.
IRON
(CHROMEL)
+T
+C
CONSTANTAN
(ALUMEL)
+IN
1
–IN
14
–ALM
+ALM
In normal operation the main amplifier output, at Pin 9, is con-
nected to the feedback network, at Pin 8. Thermocouple signals
applied to the floating input stage, at Pins 1 and 14, are ampli-
fied by gain G of the differential amplifier and are then further
amplified by gain A in the main amplifier. The output of the
main amplifier is fed back to a second differential stage in an in-
verting connection. The feedback signal is amplified by this
stage and is also applied to the main amplifier input through a
summing circuit. Because of the inversion, the amplifier causes
the feedback to be driven to reduce this difference signal to a
small value. The two differential amplifiers are made to match
and have identical gains, G. As a result, the feedback signal that
must be applied to the right-hand differential amplifier will pre-
cisely match the thermocouple input signal when the difference
signal has been reduced to zero. The feedback network is trim-
med so that the effective gain to the output, at Pins 8 and 9, re-
sults in a voltage of 10 mV/°C of thermocouple excitation.
In addition to the feedback signal, a cold junction compensation
voltage is applied to the right-hand differential amplifier. The
compensation is a differential voltage proportional to the Celsius
temperature of the AD594/AD595. This signal disturbs the dif-
ferential input so that the amplifier output must adjust to restore
the input to equal the applied thermocouple voltage.
COMP
7
8
COMMON
–T
–C
V–
V
OUT
V+
Figure 3. PCB Connections
Here the AD594/AD595 package temperature and circuit board
are thermally contacted in the copper printed circuit board
tracks under Pins 1 and 14. The reference junction is now com-
posed of a copper-constantan (or copper-alumel) connection
and copper-iron (or copper-chromel) connection, both of which
are at the same temperature as the AD594/AD595.
–4–
The compensation is applied through the gain scaling resistors
so that its effect on the main output is also 10 mV/°C. As a
result, the compensation voltage adds to the effect of the ther-
mocouple voltage a signal directly proportional to the difference
between 0°C and the AD594/AD595 temperature. If the thermo-
couple reference junction is maintained at the AD594/AD595
temperature, the output of the AD594/AD595 will correspond
to the reading that would have been obtained from amplification
of a signal from a thermocouple referenced to an ice bath.
REV. C
AD [ ANALOG DEVICES ]
