OP295/OP495
common-mode rejection performance and minimize drift. T his
instrumentation amplifier can operate from a supply voltage as
low as 3 volts.
T o calibrate, immerse the thermocouple measuring junction in a
0°C ice bath, adjust the 500 Ω Zero Adjust pot to zero volts out.
T hen immerse the thermocouple in a 250°C temperature bath
or oven and adjust the Scale Adjust pot for an output voltage of
2.50 V, which is equivalent to 250°C. Within this temperature
range, the K-type thermocouple is quite accurate and produces
a fairly linear transfer characteristic. Accuracy of ±3°C is achiev-
able without linearization.
A Single Supply RTD Ther m om eter Am plifier
T his RT D amplifier takes advantage of the rail-to-rail swing of
the OP295/OP495 to achieve a high bridge voltage in spite of a
low 5 V supply. T he OP295/OP495 amplifier servos a constant
200 µA current to the bridge. T he return current drops across
the parallel resistors 6.19 kΩ and the 2.55 MΩ, developing a
voltage that is servoed to 1.235 V, which is established by the
AD589 bandgap reference. T he 3-wire RT D provides an equal
line resistance drop in both 100 Ω legs of the bridge, thus im-
proving the accuracy.
Even if the battery voltage is allowed to decay to as low as 7 volts,
the rail-to-rail swing allows temperature measurements to
700°C. However, linearization may be necessary for tempera-
tures above 250°C where the thermocouple becomes rather
nonlinear. T he circuit draws just under 500 µA supply current
from a 9 V battery.
T he AMP04 amplifies the differential bridge signal and converts
it to a single-ended output. T he gain is set by the series resis-
tance of the 332 Ω resistor plus the 50 Ω potentiometer. T he
gain scales the output to produce a 4.5 V full scale. T he
0.22 µF capacitor to the output provides a 7 Hz low-pass filter
to keep noise at a minimum.
A 5 V O nly, 12-Bit D AC That Swings 0 V to 4.095 V
Figure 8 shows a complete voltage output DAC with wide out-
put voltage swing operating off a single +5 V supply. T he serial
input 12-bit D/A converter is configured as a voltage output
device with the 1.235 V reference feeding the current output pin
(IOUT ) of the DAC. T he VREF which is normally the input now
becomes the output.
ZERO ADJ
200Ω
50Ω
10-TURNS
+5V
7
T he output voltage from the DAC is the binary weighted volt-
age of the reference, which is gained up by the output amplifier
such that the DAC has a 1 mV per bit transfer function.
332Ω
26.7k
0.5%
26.7k
0.5%
1
3
2
0.22µF
8
VO
AMP04
6
+5V
+5V
4.5V = 450°C
0V = 0°C
5
100Ω
RTD
1
1/2
4
100Ω
0.5%
8
OP295/
OP495
+5V
8
R1
2
1
V
17.8kΩ
DD
R
FB
2
3
D
4096
1.235
37.4k
3
I
3
2
V
=
(4.096V)
V
+5V
O
REF
+1.23V
DAC8043
OUT
6.19k
1%
OP295/
OP495
1
2.55M
1%
AD589
GND CLK SRI LD
4
AD589
4
6
5
7
R4
100kΩ
Figure 6. Low Power RTD Am plifier
R2
41.2k
DIGITAL
A Cold Junction Com pensated, Batter y P ower ed
Ther m ocouple Am plifier
CONTROL
R3
5kΩ
TOTAL POWER DISSIPATION = 1.6mW
T he OP295/OP495’s 150 µA quiescent current per amplifier
consumption makes it useful for battery powered temperature
measuring instruments. T he K-type thermocouple terminates
into an isothermal block where the terminated junctions’ ambi-
ent temperatures can be continuously monitored and corrected
by summing an equal but opposite thermal EMF to the ampli-
fier, thereby canceling the error introduced by the cold junctions.
Figure 8. A 5 Volt 12-Bit DAC with 0 V to +4.095 Output
Swing
4–20 m A Cur r ent Loop Tr ansm itter
Figure 9 shows a self powered 4–20 mA current loop transmit-
ter. T he entire circuit floats up from the single supply (12 V to
36 V) return. T he supply current carries the signal within the 4
to 20 mA range. T hus the 4 mA establishes the baseline
24.9k
1.235V
AD589
ISOTHERMAL
9V
SCALE
BLOCK
NULL ADJ
6
2
24.3k
1%
REF02
GND
4
7.15k
ADJUST
1%
1N914
20k
1.33MΩ
100kΩ
10-TURN
SPAN ADJ
4.99k
1%
1.5M
1%
24.9k
1%
5V
ALUMEL
10kΩ
10-TURN
182k 1.21M
100Ω
8
2
3
1%
1%
+12V
TO
8
4
AL
COLD
JUNCTIONS
OP295/
OP495
4
VIN
0 + 3V
3
2
500Ω
10-TURN
V
1
220Ω
O
+36V
1
0V = 0°C
5V = 500°C
ZERO
ADJUST
CR
1/2
OP295/
OP495
2N1711
CHROMEL
K-TYPE
475Ω
1%
2.1k
1%
4–20mA
RL
100Ω
THERMOCOUPLE
40.7µV/°C
220pF
100k
1%
100Ω
1%
HP
5082-2800
Figure 7. Battery Powered, Cold-J unction Com pensated
Therm ocouple Am plifier
Figure 9. 4–20 m A Current Loop Transm itter
REV. B
–9–
ADI [ ADI ]
