OP295/OP495
common-mode rejection performance and minimize drift. This
instrumentation amplifier can operate from a supply voltage as
low as 3 volts.
A Single Supply RTD Thermometer Amplifier
This RTD 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. The OP295/OP495 amplifier servos a constant
200
µA
current to the bridge. The 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. The 3-wire RTD provides an equal
line resistance drop in both 100
Ω
legs of the bridge, thus im-
proving the accuracy.
The AMP04 amplifies the differential bridge signal and converts
it to a single-ended output. The gain is set by the series resis-
tance of the 332
Ω
resistor plus the 50
Ω
potentiometer. The
gain scales the output to produce a 4.5 V full scale. The
0.22
µF
capacitor to the output provides a 7 Hz low-pass filter
to keep noise at a minimum.
200Ω
10-TURNS
26.7k
0.5%
ZERO ADJ
+5V
26.7k
0.5%
7
3
1
8 0.22µF
AMP04
2
100Ω
RTD
100Ω
0.5%
2
1
To calibrate, immerse the thermocouple measuring junction in a
0°C ice bath, adjust the 500
Ω
Zero Adjust pot to zero volts out.
Then 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.
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. The circuit draws just under 500
µA
supply current
from a 9 V battery.
A 5 V Only, 12-Bit DAC That Swings 0 V to 4.095 V
50Ω
332Ω
Figure 8 shows a complete voltage output DAC with wide out-
put voltage swing operating off a single +5 V supply. The 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
(I
OUT
) of the DAC. The V
REF
which is normally the input now
becomes the output.
The 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.
6
V
O
4.5V = 450°C
0V = 0°C
1/2
OP295/
OP495
3
5
4
+5V
+5V
8
V
DD
R
FB
V
REF
2
1
3
+5V
R1
17.8kΩ
+5V
+1.23V
3
I
OUT
1.235
37.4k
8
2.55M
1%
6.19k
1%
DAC8043
V
O
=
1
AD589
GND CLK SRI LD
AD589
4
7
6
5
OP295/
OP495
2
4
R4
R2
41.2k
R3
5kΩ
D
(4.096V)
4096
Figure 6. Low Power RTD Amplifier
A Cold Junction Compensated, Battery Powered
Thermocouple Amplifier
DIGITAL
CONTROL
TOTAL POWER DISSIPATION = 1.6mW
100kΩ
The OP295/OP495’s 150
µA
quiescent current per amplifier
consumption makes it useful for battery powered temperature
measuring instruments. The 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.
1.235V
AD589
ISOTHERMAL
BLOCK
1N914
ALUMEL
AL
1.5M
1%
24.9k
9V
24.3k
1%
4.99k
1%
2
8
Figure 8. A 5 Volt 12-Bit DAC with 0 V to +4.095 Output
Swing
4–20 mA Current Loop Transmitter
Figure 9 shows a self powered 4–20 mA current loop transmit-
ter. The entire circuit floats up from the single supply (12 V to
36 V) return. The supply current carries the signal within the 4
to 20 mA range. Thus the 4 mA establishes the baseline
NULL ADJ
SPAN ADJ
100kΩ
10-TURN
6 REF02
GND
5V
3
8
220Ω
1
2
4
4
100Ω
+12V
TO
+36V
2
7.15k
1%
24.9k
1%
SCALE
ADJUST
1.33MΩ 20k
10kΩ
182k 1.21M
10-TURN 1%
1%
1
COLD
JUNCTIONS
500Ω
10-TURN
3
OP295/
OP495
4
CR
CHROMEL
K-TYPE
THERMOCOUPLE
40.7µV/°C
475Ω
1%
2.1k
1%
ZERO
ADJUST
V
O
0V = 0°C
5V = 500°C
V
IN
0 + 3V
1/2
OP295/
OP495
2N1711
4–20mA
R
L
100Ω
100Ω
1%
220pF
100k
HP
5082-2800 1%
Figure 7. Battery Powered, Cold-Junction Compensated
Thermocouple Amplifier
Figure 9. 4–20 mA Current Loop Transmitter
REV. B
–9–
AD [ ANALOG DEVICES ]
