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OP495GS-REEL View Datasheet(PDF) - Analog Devices

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Description
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OP495GS-REEL Datasheet PDF : 16 Pages
First Prev 11 12 13 14 15 16
+
VIN
R1
100k
1/2
OP295/
OP495
3+
1
2
R2
20k
R3
20k
V+ 1/2
OP295/
5 + 8 OP495
7
VO
64
R4
100k
VREF
RG
( ) VO =
5
+
200k
RG
VIN + VREF
Figure 23. Single-Supply Instrumentation Amplifier
Resistor RG sets the gain of the instrumentation amplifier.
Minimum gain is 6 (with no RG). All resistors should be matched
in absolute value as well as temperature coefficient to maximize
common-mode rejection performance and minimize drift. This
instrumentation amplifier can operate from a supply voltage as
low as 3 V.
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 2.55 MΩ, developing a voltage
that is servoed to 1.235 V, which is established by the AD589
band gap reference. The 3-wire RTD provides an equal line
resistance drop in both 100 Ω legs of the bridge, thus improving
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.
200ZERO ADJ
10-TURNS
26.7k
0.5%
26.7k
0.5%
5V
7
3
1
+
50
332
8 0.22µF
100
RTD
100
0.5%
AMP04
2
5
1
1/2
4
OP295/
– + OP495
6
VO
4.5V = 450°C
0V = 0°C
2.55M
1%
23
1.235
5V
6.19k
37.4k
1% AD589
Figure 24. Low Power RTD Amplifier
OP295/OP495
COLD JUNCTION COMPENSATED, BATTERY-
POWERED THERMOCOUPLE AMPLIFIER
The 150 μA quiescent current per amplifier consumption of the
OP295/OP495 makes them useful for battery-powered temperature
measuring instruments. The K-type thermocouple terminates
into an isothermal block where the terminated junctions’ ambient
temperatures can be continuously monitored and corrected by
summing an equal but opposite thermal EMF to the amplifier,
thereby canceling the error introduced by the cold junctions.
1.235V 24.9k
AD589
+
9V
ISOTHERMAL
BLOCK
7.15k
24.3k
1N914
1%
1%
SCALE
ADJUST
20k
ALUMEL
AL
+ CR
CHROMEL
K-TYPE
THERMOCOUPLE
40.7µV/°C
1.5M24.9k
1%
1%
COLD
JUNCTIONS
475
1%
4.99k
1%
1.33M
28
500
1
VO
10-TURN
ZERO 3 + 4 OP295/
ADJUST
OP495 5V = 500°C
0V = 0°C
2.1k
1%
Figure 25. Battery-Powered, Cold-Junction Compensated Thermocouple
Amplifier
To calibrate, immerse the thermocouple measuring junction in
a 0°C ice bath and adjust the 500 Ω zero-adjust potentiometer
to 0 V out. Then immerse the thermocouple in a 250°C tem-
perature bath or oven and adjust the scale-adjust potentiometer
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 achievable without linearization.
Even if the battery voltage is allowed to decay to as low as 7 V,
the rail-to-rail swing allows temperature measurements to 700°C.
However, linearization may be necessary for temperatures above
250°C, where the thermocouple becomes rather nonlinear. The
circuit draws just under 500 μA supply current from a 9 V
battery.
5 V ONLY, 12-BIT DAC THAT SWINGS 0 V TO 4.095 V
Figure 26 shows a complete voltage output DAC with wide
output voltage swing operating off a single 5 V supply. The
serial input, 12-bit DAC is configured as a voltage output device
with the 1.235 V reference feeding the current output pin (IOUT)
of the DAC. The VREF, which is normally the input, now becomes
the output.
The output voltage from the DAC is the binary weighted voltage
of the reference, which is gained up by the output amplifier such
that the DAC has a 1 mV per bit transfer function.
Rev. E | Page 11 of 16
 

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