Datasheet OP295, OP495 (Analog Devices) - 11

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OP295/OP495 V+ 1/2 COLD JUNCTION COMPENSATED, BATTERY- OP295/ + 5 + 8 OP495 POWERED THERMOCOUPLE AMPLIFIER 7 1/2 VO V
The 150 μA quiescent current per amplifier consumption of the
IN 6 – 4 OP295/ OP495
OP295/OP495 makes them useful for battery-powered temperature
– 3 +
measuring instruments. The K-type thermocouple terminates
1 –
into an isothermal block where the terminated junctions’ ambient
2 R1 R2 R3 R4
temperatures can be continuously monitored and corrected by
100kΩ 20kΩ 20kΩ 100kΩ
summing an equal but opposite thermal EMF to the amplifier,
V
thereby canceling the error introduced by the cold junctions.
REF RG 1.235V 24.9kΩ
22
AD589 V
0
+ O = (5 + 200kΩ) VIN + VREF
1-
R 9V G
33
–
00
ISOTHERMAL SCALE
Figure 24. Single-Supply Instrumentation Amplifier
BLOCK ADJUST 7.15kΩ 24.3kΩ 1N914 1% 1%
Resistor R
20kΩ
G sets the gain of the instrumentation amplifier.
1.5MΩ 24.9kΩ 4.99kΩ
Minimum gain is 6 (with no R
1.33MΩ
G). All resistors should be matched
ALUMEL – 1% 1% 1% 2 8 –
in absolute value as well as temperature coefficient to maximize
AL COLD 500Ω 1 V
common-mode rejection performance and minimize drift. This
JUNCTIONS O 10-TURN 3 + 4
instrumentation amplifier can operate from a supply voltage as
+ ZERO OP295/ CR ADJUST OP495
low as 3 V.
CHROMEL 5V = 500°C 475Ω 2.1kΩ K-TYPE 0V = 0°C 1% 1%
24
SINGLE-SUPPLY RTD THERMOMETER AMPLIFIER
0
THERMOCOUPLE
1-
40.7µV/°C
33 00 This RTD amplifier takes advantage of the rail-to-rail swing of Figure 26. Battery-Powered, Cold-Junction Compensated the OP295/OP495 to achieve a high bridge voltage in spite of a Thermocouple Amplifier low 5 V supply. The OP295/OP495 amplifier servos a constant To calibrate, immerse the thermocouple measuring junction in 200 μA current to the bridge. The return current drops across a 0°C ice bath and adjust the 500 Ω zero-adjust potentiometer the parallel resistors 6.19 kΩ and 2.55 MΩ, developing a voltage to 0 V out. Then immerse the thermocouple in a 250°C tem- that is servoed to 1.235 V, which is established by the AD589 perature bath or oven and adjust the scale-adjust potentiometer band gap reference. The 3-wire RTD provides an equal line for an output voltage of 2.50 V, which is equivalent to 250°C. resistance drop in both 100 Ω legs of the bridge, thus improving Within this temperature range, the K-type thermocouple is the accuracy. quite accurate and produces a fairly linear transfer characteristic. The AMP04 amplifies the differential bridge signal and converts Accuracy of ±3°C is achievable without linearization. it to a single-ended output. The gain is set by the series resis- Even if the battery voltage is allowed to decay to as low as 7 V, tance of the 332 Ω resistor plus the 50 Ω potentiometer. The the rail-to-rail swing allows temperature measurements to 700°C. gain scales the output to produce a 4.5 V full scale. The 0.22 μF However, linearization may be necessary for temperatures above capacitor to the output provides a 7 Hz low-pass filter to keep 250°C, where the thermocouple becomes rather nonlinear. The noise at a minimum. circuit draws just under 500 μA supply current from a 9 V battery.
ZERO ADJ 200Ω 50Ω 10-TURNS 5V 5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V 332Ω 26.7kΩ 26.7kΩ 7 0.5% 0.5%
Figure 27 shows a complete voltage output DAC with wide
3 1 + 8 0.22µF
output voltage swing operating off a single 5 V supply. The
AMP04 V 6 O 100Ω 2
serial input, 12-bit DAC is configured as a voltage output device
5 – RTD 1 1/2 4 100Ω 4.5V = 450°C
with the 1.235 V reference feeding the current output pin (IOUT)
0.5% OP295/ 0V = 0°C – + OP495
of the DAC. The VREF, which is normally the input, now becomes
2 3
the output.
1.235 5V 37.4kΩ 2.55MΩ 6.19kΩ
The output voltage from the DAC is the binary weighted voltage 23
1% 1% AD589
-0 31 of the reference, which is gained up by the output amplifier such 03 0 Figure 25. Low Power RTD Amplifier that the DAC has a 1 mV per bit transfer function. Rev. G | Page 11 of 16 Document Outline FEATURES APPLICATIONS GENERAL DESCRIPTION PIN CONFIGURATIONS TABLE OF CONTENTS REVISION HISTORY SPECIFICATIONS ELECTRICAL CHARACTERISTICS ABSOLUTE MAXIMUM RATINGS THERMAL RESISTANCE ESD CAUTION TYPICAL PERFORMANCE CHARACTERISTICS APPLICATIONS RAIL-TO-RAIL APPLICATION INFORMATION LOW DROP-OUT REFERENCE LOW NOISE, SINGLE-SUPPLY PREAMPLIFIER DRIVING HEAVY LOADS DIRECT ACCESS ARRANGEMENT SINGLE-SUPPLY INSTRUMENTATION AMPLIFIER SINGLE-SUPPLY RTD THERMOMETER AMPLIFIER COLD JUNCTION COMPENSATED, BATTERY-POWERED THERMOCOUPLE AMPLIFIER 5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V 4 mA TO 20 mA CURRENT-LOOP TRANSMITTER 3 V LOW DROPOUT LINEAR VOLTAGE REGULATOR LOW DROPOUT, 500 mA VOLTAGE REGULATOR WITH FOLDBACK CURRENT LIMITING SQUARE WAVE OSCILLATOR SINGLE-SUPPLY DIFFERENTIAL SPEAKER DRIVER HIGH ACCURACY, SINGLE-SUPPLY, LOW POWER COMPARATOR OUTLINE DIMENSIONS ORDERING GUIDE