Datasheet

AD594/AD595
REV. B
–6–
should now be back to the initial 240 mV reading. The resis-
tance value of R3 should be approximately 280 kΩ. The final
connection diagram is shown in Figure 7. An approximate veri-
fication of the effectiveness of recalibration is to measure the dif-
ferential gain to the output. For type E it should be 164.2.
AD594/
AD595
–T
+IN
–IN
+T
COM
1
14
4
2
3
–C
5
6
FB
VO
8
9
R3
+C
R1
R2
Figure 7. Type E Recalibration
When implementing a similar recalibration procedure for the
AD595 the values for R1, R2, R3 and r will be approximately
650 Ω, 84 kΩ, 93 kΩ and 1.51, respectively. Power consump-
tion will increase by about 50% when using the AD595 with
type E inputs.
Note that during this procedure it is crucial to maintain the
AD594/AD595 at a stable temperature because it is used as the
temperature reference. Contact with fingers or any tools not at
ambient temperature will quickly produce errors. Radiational
heating from a change in lighting or approach of a soldering iron
must also be guarded against.
USING TYPE T THERMOCOUPLES WITH THE AD595
Because of the similarity of thermal EMFs in the 0°C to +50°C
range between type K and type T thermocouples, the AD595
can be directly used with both types of inputs. Within this ambi-
ent temperature range the AD595 should exhibit no more than
an additional 0.2°C output calibration error when used with
type T inputs. The error arises because the ice point compensa-
tor is trimmed to type K characteristics at 25°C. To calculate
the AD595 output values over the recommended –200°C to
+350°C range for type T thermocouples, simply use the ANSI
thermocouple voltages referred to 0°C and the output equation
given on page 2 for the AD595. Because of the relatively large
nonlinearities associated with type T thermocouples the output
will deviate widely from the nominal 10 mV/°C. However, cold
junction compensation over the rated 0°C to +50°C ambient
will remain accurate.
STABILITY OVER TEMPERATURE
Each AD594/AD595 is tested for error over temperature with
the measuring thermocouple at 0°C. The combined effects of
cold junction compensation error, amplifier offset drift and gain
error determine the stability of the AD594/AD595 output over
the rated ambient temperature range. Figure 8 shows an
AD594/AD595 drift error envelope. The slope of this figure has
units of °C/°C.
TEMPERATURE OF AD594C/AD595C
0
+0.6
o
C
DRIFT ERROR
–0.6
o
C
25
o
C
50
o
C
Figure 8. Drift Error vs. Temperature
THERMAL ENVIRONMENT EFFECTS
The inherent low power dissipation of the AD594/AD595 and
the low thermal resistance of the package make self-heating
errors almost negligible. For example, in still air the chip to am-
bient thermal resistance is about 80°C/watt (for the D package).
At the nominal dissipation of 800 µW the self-heating in free air
is less than 0.065°C. Submerged in fluorinert liquid (unstirred)
the thermal resistance is about 40°C/watt, resulting in a self-
heating error of about 0.032°C.
SETPOINT CONTROLLER
The AD594/AD595 can readily be connected as a setpoint
controller as shown in Figure 9.
CONSTANTAN
(ALUMEL)
IRON
(CHROMEL)
+5V
COMMON
HEATER
20M
(OPTIONAL)
FOR
HYSTERESIS
SETPOINT
VOLTAGE
INPUT
TEMPERATURE
CONTROLLED
REGION
LOW = > T < SETPOINT
HIGH = > T > SETPOINT
TEMPERATURE
COMPARATOR OUT
OVERLOAD
DETECT
G
–TC
+TC
1234
567
13 12 11 10
AD594/
AD595
14
ICE
POINT
COMP.
+A
98
G
HEATER
DRIVER
Figure 9. Setpoint Controller
The thermocouple is used to sense the unknown temperature
and provide a thermal EMF to the input of the AD594/AD595.
The signal is cold junction compensated, amplified to 10 mV/°C
and compared to an external setpoint voltage applied by the
user to the feedback at Pin 8. Table I lists the correspondence
between setpoint voltage and temperature, accounting for the
nonlinearity of the measurement thermocouple. If the setpoint
temperature range is within the operating range (–55°C to
+125°C) of the AD594/AD595, the chip can be used as the
transducer for the circuit by shorting the inputs together and
utilizing the nominal calibration of 10 mV/°C. This is the centi-
grade thermometer configuration as shown in Figure 13.
In operation if the setpoint voltage is above the voltage corre-
sponding to the temperature being measured the output swings
low to approximately zero volts. Conversely, when the tempera-
ture rises above the setpoint voltage the output switches to the
positive limit of about 4 volts with a +5 V supply. Figure 9
shows the setpoint comparator configuration complete with a
heater element driver circuit being controlled by the AD594/
AD595 toggled output. Hysteresis can be introduced by inject-
ing a current into the positive input of the feedback amplifier
when the output is toggled high. With an AD594 about 200 nA
into the +T terminal provides 1°C of hysteresis. When using a
single 5 V supply with an AD594, a 20 MΩ resistor from V
O
to
+T will supply the 200 nA of current when the output is forced
high (about 4 V). To widen the hysteresis band decrease the re-
sistance connected from V
O
to +T.