= 5 V, Type J (AD594), Type K (AD595) Thermocouple,
unless otherwise noted)
Min
AD595A
Typ
Max
36
+V
S
+V
S
–V
S
+ 36
+V
S
+125
Min
AD595C
Typ
Max
36
+V
S
+V
S
–V
S
+ 36
+V
S
+125
Units
Volts
Volts
Volts
Volts
Volts
°C
AD594C
Typ
Max
36
+V
S
+V
S
–V
S
+ 36
+V
S
+125
–V
S
– 0.15
–V
S
–V
S
–V
S
–55
Indefinite
–V
S
– 0.15
–V
S
–V
S
–V
S
–55
Indefinite
3
0.05
1.5
10
193.4
(Temperature in
°C) ×
51.70
µV/°C
0.1
–10
+50
–V
S
– 0.15
–V
S
– 4
10
10
–V
S
+ 2.5
0
±
5
15
0.3
1
+V
S
– 4
20
+V
S
= 5, –V
S
= 0
+V
S
to –V
S
≤
30
160
100
AD594AD
AD594AQ
300
20
+V
S
– 2
+V
S
– 2
1
0.025
0.75
10
193.4
(Temperature in
°C) ×
51.70
µV/°C
0.1
–V
S
– 0.15
–V
S
– 4
10
10
+V
S
– 2
–V
S
– 2
3
0.05
1.5
10
247.3
(Temperature in
°C) ×
40.44
µV/°C
0.1
–10
+50
–V
S
– 0.15
–V
S
– 4
10
10
–V
S
+ 2.5
0
±
5
15
0.3
+V
S
– 2
+V
S
+ 2
1
0.025
0.75
10
247.3
(Temperature in
°C) ×
40.44
µV/°C
0.1
–10
+50
–V
S
– 0.15
–V
S
– 4
10
10
–V
S
+ 2.5
0
±
5
15
0.3
+V
S
– 2
+V
S
– 2
°C
°C/°C
%
mV/°C
µV
µA
mV
Volts
mV/V
mV/V
Volts
Volts
mA
kHz
Volts
µA
max
Volts
mA
Volts
Volts
–V
S
+ 2.5
0
±
5
15
0.3
1
+V
S
– 4
20
+V
S
= 5, –V
S
= 0
+V
S
to –V
S
≤
30
160
100
AD595AD
AD595AQ
1
+V
S
– 4
20
+V
S
= 5, –V
S
= 0
+V
S
to –V
S
≤
30
300
160
100
AD595CD
AD595CQ
1
+V
S
– 4
+V
S
= 5, –V
S
= 0
+V
S
to –V
S
≤
30
160
100
AD594CD
AD594CQ
300
300
µA
µA
NOTES
1
Calibrated for minimum error at +25°C using a thermocouple sensitivity of 51.7
µV/°C.
Since a J type thermocouple deviates from this straight line approximation, the AD594 will normally
read 3.1 mV when the measuring junction is at 0°C. The AD595 will similarly read 2.7 mV at 0°C.
2
Defined as the slope of the line connecting the AD594/AD595 errors measured at 0°C and 50°C ambient temperature.
3
Pin 8 shorted to Pin 9.
4
Current Sink Capability in single supply configuration is limited to current drawn to ground through a 50 kΩ resistor at output voltages below 2.5 V.
5
–V
S
must not exceed –16.5 V.
Specifications shown in
boldface
are tested on all production units at final electrical test. Results from those tests are used to calculate outgoing quality levels. All min and max specifications
are guaranteed, although only those shown in
boldface
are tested on all production units.
Specifications subject to change without notic
e.
INTERPRETING AD594/AD595 OUTPUT VOLTAGES
To achieve a temperature proportional output of 10 mV/°C and
accurately compensate for the reference junction over the rated
operating range of the circuit, the AD594/AD595 is gain trimmed
to match the transfer characteristic of J and K type thermocouples
at 25°C. For a type J output in this temperature range the TC is
51.70
µV/°C,
while for a type K it is 40.44
µV/°C.
The resulting
gain for the AD594 is 193.4 (10 mV/°C divided by 51.7
µV/°C)
and for the AD595 is 247.3 (10 mV/°C divided by 40.44
µV/°C).
In addition, an absolute accuracy trim induces an input offset to
the output amplifier characteristic of 16
µV
for the AD594 and
11
µV
for the AD595. This offset arises because the AD594/
AD595 is trimmed for a 250 mV output while applying a 25°C
thermocouple input.
Because a thermocouple output voltage is nonlinear with respect
to temperature, and the AD594/AD595 linearly amplifies the
–2–
compensated signal, the following transfer functions should be
used to determine the actual output voltages:
AD594 output
= (Type
J
Voltage
+ 16
µV) ×
193.4
AD595 output
=
(Type K Voltage +
11
µV) ×
247.3
or conversely:
Type
J
voltage
=
(AD594 output/193.4) –
16
µV
Type K voltage
=
(AD595 output/247.3) –
11
µV
Table I lists the ideal AD594/AD595 output voltages as a func-
tion of Celsius temperature for type J and K ANSI standard
thermocouples, with the package and reference junction at
25°C. As is normally the case, these outputs are subject to cali-
bration, gain and temperature sensitivity errors. Output values
for intermediate temperatures can be interpolated, or calculated
using the output equations and ANSI thermocouple voltage
tables referred to zero degrees Celsius. Due to a slight variation
in alloy content between ANSI type J and DIN F
E
-C
U
N
I
REV. C
AD594/AD595
Table I. Output Voltage vs. Thermocouple Temperature (Ambient +25°C, V
S
= –5 V, +15 V)
Thermocouple
Temperature
°C
–200
–180
–160
–140
–120
–100
–80
–60
–40
–20
–10
0
10
20
25
30
40
50
60
80
100
120
140
160
180
200
220
240
260
280
300
320
340
360
380
400
420
440
460
480
Type J
Voltage
mV
–7.890
–7.402
–6.821
–6.159
–5.426
–4.632
–3.785
–2.892
–1.960
–.995
–.501
0
.507
1.019
1.277
1.536
2.058
2.585
3.115
4.186
5.268
6.359
7.457
8.560
9.667
10.777
11.887
12.998
14.108
15.217
16.325
17.432
18.537
19.640
20.743
21.846
22.949
24.054
25.161
26.272
AD594
Output
mV
–1523
–1428
–1316
–1188
–1046
–893
–729
–556
–376
–189
–94
3.1
101
200
250
300
401
503
606
813
1022
1233
1445
1659
1873
2087
2302
2517
2732
2946
3160
3374
3588
3801
4015
4228
4441
4655
4869
5084
Type K
Voltage
mV
–5.891
–5.550
–5.141
–4.669
–4.138
–3.553
–2.920
–2.243
–1.527
–.777
–.392
0
.397
.798
1.000
1.203
1.611
2.022
2.436
3.266
4.095
4.919
5.733
6.539
7.338
8.137
8.938
9.745
10.560
11.381
12.207
13.039
13.874
14.712
15.552
16.395
17.241
18.088
18.938
19.788
AD595
Output
mV
–1454
–1370
–1269
–1152
–1021
–876
–719
–552
–375
–189
–94
2.7
101
200
250
300
401
503
605
810
1015
1219
1420
1620
1817
2015
2213
2413
2614
2817
3022
3227
3434
3641
3849
4057
4266
4476
4686
4896
Thermocouple
Temperature
°C
500
520
540
560
580
600
620
640
660
680
700
720
740
750
760
780
800
820
840
860
880
900
920
940
960
980
1000
1020
1040
1060
1080
1100
1120
1140
1160
1180
1200
1220
1240
1250
Type J
Voltage
mV
27.388
28.511
29.642
30.782
31.933
33.096
34.273
35.464
36.671
37.893
39.130
40.382
41.647
42.283
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
AD594
Output
mV
5300
5517
5736
5956
6179
6404
6632
6862
7095
7332
7571
7813
8058
8181
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
–
Type K
Voltage
mV
20.640
21.493
22.346
23.198
24.050
24.902
25.751
26.599
27.445
28.288
29.128
29.965
30.799
31.214
31.629
32.455
33.277
34.095
34.909
35.718
36.524
37.325
38.122
38.915
39.703
40.488
41.269
42.045
42.817
43.585
44.439
45.108
45.863
46.612
47.356
48.095
48.828
49.555
50.276
50.633
AD595
Output
mV
5107
5318
5529
5740
5950
6161
6371
6581
6790
6998
7206
7413
7619
7722
7825
8029
8232
8434
8636
8836
9035
9233
9430
9626
9821
10015
10209
10400
10591
10781
10970
11158
11345
11530
11714
11897
12078
12258
12436
12524
thermocouples Table I should not be used in conjunction with
European standard thermocouples. Instead the transfer function
given previously and a DIN thermocouple table should be used.
ANSI type K and DIN N
I
C
R
-N
I
thermocouples are composed
CONSTANTAN
(ALUMEL)
14
13
SINGLE AND DUAL SUPPLY CONNECTIONS
+5V
12
11
10
9
10mV/ C
8
The AD594/AD595 is a completely self-contained thermocouple
conditioner. Using a single +5 V supply the interconnections
shown in Figure 1 will provide a direct output from a type J
thermocouple (AD594) or type K thermocouple (AD595) mea-
suring from 0°C to +300°C.
Any convenient supply voltage from +5 V to +30 V may be
used, with self-heating errors being minimized at lower supply
levels. In the single supply configuration the +5 V supply con-
nects to Pin 11 with the V– connection at Pin 7 strapped to
power and signal common at Pin 4. The thermocouple wire in-
puts connect to Pins 1 and 14 either directly from the measuring
point or through intervening connections of similar thermo-
couple wire type. When the alarm output at Pin 13 is not used it
should be connected to common or –V. The precalibrated feed-
back network at Pin 8 is tied to the output at Pin 9 to provide a
10 mV/°C nominal temperature transfer characteristic.
By using a wider ranging dual supply, as shown in Figure 2, the
AD594/AD595 can be interfaced to thermocouples measuring
both negative and extended positive temperatures.
–3–
OVERLOAD
DETECT
AD594/
AD595
G
G
+A
ICE
POINT
COMP. –TC
+TC
IRON
(CHROMEL)
1
2
3
4
5
6
7
COMMON
Figure 1. Basic Connection, Single Supply Operation
of identical alloys and exhibit similar behavior. The upper tem-
perature limits in Table I are those recommended for type J and
type K thermocouples by the majority of vendors.
REV. C
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
AD594/AD595
The AD594/AD595 also includes an input open circuit detector
that switches on an alarm transistor. This transistor is actually a
current-limited output buffer, but can be used up to the limit as
a switch transistor for either pull-up or pull-down operation of
external alarms.
The ice point compensation network has voltages available with
positive and negative temperature coefficients. These voltages
may be used with external resistors to modify the ice point com-
pensation and recalibrate the AD594/AD595 as described in the
next column.
The feedback resistor is separately pinned out so that its value
can be padded with a series resistor, or replaced with an external
resistor between Pins 5 and 9. External availability of the feedback
resistor allows gain to be adjusted, and also permits the AD594/
AD595 to operate in a switching mode for setpoint operation.
CAUTIONS:
this terminal can be produced with a resistor between –C and
–T to balance an increase in +T, or a resistor from –T to COM
to offset a decrease in +T.
If the compensation is adjusted substantially to accommodate a
different thermocouple type, its effect on the final output volt-
age will increase or decrease in proportion. To restore the
nominal output to 10 mV/°C the gain may be adjusted to match
the new compensation and thermocouple input characteristics.
When reducing the compensation the resistance between –T
and COM automatically increases the gain to within 0.5% of the
correct value. If a smaller gain is required, however, the nominal
47 kΩ internal feedback resistor can be paralleled or replaced
with an external resistor.
Fine calibration adjustments will require temperature response
measurements of individual devices to assure accuracy. Major
reconfigurations for other thermocouple types can be achieved
without seriously compromising initial calibration accuracy, so
long as the procedure is done at a fixed temperature using the
factory calibration as a reference. It should be noted that inter-
mediate recalibration conditions may require the use of a
negative supply.
EXAMPLE: TYPE E RECALIBRATION—AD594/AD595
The temperature compensation terminals (+C and –C) at Pins 2
and 6 are provided to supply small calibration currents only. The
AD594/AD595 may be permanently damaged if they are
grounded or connected to a low impedance.
The AD594/AD595 is internally frequency compensated for feed-
back ratios (corresponding to normal signal gain) of 75 or more.
If a lower gain is desired, additional frequency compensation
should be added in the form of a 300 pF capacitor from Pin 10
to the output at Pin 9. As shown in Figure 5 an additional 0.01
µF
capacitor between Pins 10 and 11 is recommended.
AD594/
AD595
VO 9
300pF
COMP 10
0.01 F
+V 11
Both the AD594 and AD595 can be configured to condition the
output of a type E (chromel-constantan) thermocouple. Tem-
perature characteristics of type E thermocouples differ less from
type J, than from type K, therefore the AD594 is preferred for
recalibration.
While maintaining the device at a constant temperature follow
the recalibration steps given here. First, measure the device
temperature by tying both inputs to common (or a selected
common-mode potential) and connecting FB to VO. The AD594
is now in the stand alone Celsius thermometer mode. For this
example assume the ambient is 24°C and the initial output VO
is 240 mV. Check the output at VO to verify that it corresponds
to the temperature of the device.
Next, measure the voltage –T at Pin 5 with a high impedance
DVM (capacitance should be isolated by a few thousand ohms
of resistance at the measured terminals). At 24°C the –T voltage
will be about 8.3 mV. To adjust the compensation of an AD594
to a type E thermocouple a resistor, R1, should be connected
between +T and +C, Pins 2 and 3, to raise the voltage at –T by
the ratio of thermocouple sensitivities. The ratio for converting a
type J device to a type E characteristic is:
r
(AD594) =(60.9
µV/°C)/(51.7 µV/°C)=
1.18
Thus, multiply the initial voltage measured at –T by r and ex-
perimentally determine the R1 value required to raise –T to that
level. For the example the new –T voltage should be about 9.8 mV.
The resistance value should be approximately 1.8 kΩ.
The zero differential point must now be shifted back to 0°C.
This is accomplished by multiplying the original output voltage
VO by r and adjusting the measured output voltage to this value
by experimentally adding a resistor, R2, between –C and –T,
Pins 5 and 6. The target output value in this case should be
about 283 mV. The resistance value of R2 should be approxi-
mately 240 kΩ.
Finally, the gain must be recalibrated such that the output VO
indicates the device’s temperature once again. Do this by adding
a third resistor, R3, between FB and –T, Pins 8 and 5. VO should
now be back to the initial 240 mV reading. The resistance value
–5–
Figure 5. Low Gain Frequency Compensation
RECALIBRATION PRINCIPLES AND LIMITATIONS
The ice point compensation network of the AD594/AD595
produces a differential signal which is zero at 0°C and corre-
sponds to the output of an ice referenced thermocouple at the
temperature of the chip. The positive TC output of the circuit is
proportional to Kelvin temperature and appears as a voltage at
+T. It is possible to decrease this signal by loading it with a
resistor from +T to COM, or increase it with a pull-up resistor
from +T to the larger positive TC voltage at +C. Note that
adjustments to +T should be made by measuring the voltage which
tracks it at –T. To avoid destabilizing the feedback amplifier the
measuring instrument should be isolated by a few thousand
ohms in series with the lead connected to –T.
1
+IN
14 –IN
8 FB
9
VO
AD594/
AD595
+T
3
COM 4
–T
5
Figure 6. Decreased Sensitivity Adjustment
Changing the positive TC half of the differential output of the
compensation scheme shifts the zero point away from 0°C. The
zero can be restored by adjusting the current flow into the nega-
tive input of the feedback amplifier, the –T pin. A current into
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