OP113 Datasheet, PDF (6/16 Pages) Analog Devices – Low Noise, Low Drift Single-Supply Operational Amplifiers

OP113/OP213/OP413
A High Accuracy Linearized RTD Thermometer Amplifier
Zero suppressing the bridge facilitates simple linearization of the
RTD by feeding back a small amount of the output signal to the
RTD (Resistor Temperature Device). In Figure 3 the left leg of
the bridge is servoed to a virtual ground voltage by amplifier
A1, while the right leg of the bridge is also servoed to zero-volt
by amplifier A2. This eliminates any error resulting from
common-mode voltage change in the amplifier. A three-wire
RTD is used to balance the wire resistance on both legs of the
bridge, thereby reducing temperature mismatch errors. The
5.000 V bridge excitation is derived from the extremely stable
AD588 reference device with 1.5 ppm/°C drift performance.
Linearization of the RTD is done by feeding a fraction of the
output voltage back to the RTD in the form of a current. With
just the right amount of positive feedback, the amplifier output
will be linearly proportional to the temperature of the RTD.
–15V +15V
16
2
11
14
12
15
13 AD588BD
4
1
6
3
7 9 8 10
10␮F
100⍀
RTD
R_W1
R_W2
R_W3
R3
50⍀
R1
8.25k⍀
R_G FULL SCALE ADJUST
R2
R5 R7
8.25k⍀ _{4.02k⍀ 100⍀}
+15V
R4
100⍀
6
8
A2
7
5 _{4 1/2}
OP213
V_OUT (10mV/؇C)
–1.50V = –150؇C
+5.00V = +500؇C
–15V
R9
R8
49.9k⍀
5k⍀
LINEARITY
ADJUST
2
@1/2 F.S.
A1
1
3
1/2
OP213
Figure 3. Ultraprecision RTD Amplifier
To calibrate the circuit, first immerse the RTD in a zero-degree
ice bath or substitute an exact 100 Ω resistor in place of the
RTD. Adjust the ZERO ADJUST potentiometer for a 0.000 V
output, then set R9 LINEARITY ADJUST potentiometer to
the middle of its adjustment range. Substitute a 280.9 Ω resistor
(equivalent to 500°C) in place of the RTD, and adjust the
FULL-SCALE ADJUST potentiometer for a full-scale voltage
of 5.000 V.
To calibrate out the nonlinearity, substitute a 194.07 Ω resistor
(equivalent to 250°C) in place of the RTD, then adjust the
LINEARITY ADJUST potentiometer for a 2.500 V output.
Check and readjust the full-scale and half-scale as needed.
Once calibrated, the amplifier outputs a 10 mV/°C temperature
coefficient with an accuracy better than ± 0.5°C over an RTD
measurement range of –150°C to +500°C. Indeed the amplifier
can be calibrated to a higher temperature range, up to 850°C.
A High Accuracy Thermocouple Amplifier
Figure 4 shows a popular K-type thermocouple amplifier with
cold-junction compensation. Operating from a single +12 volt
supply, the OP113 family’s low noise allows temperature mea-
surement to better than 0.02°C resolution from 0°C to 1000°C
range. The cold-junction error is corrected by using an inexpen-
sive silicon diode as a temperature measuring device. It should
be placed as close to the two terminating junctions as physically
possible. An aluminum block might serve well as an isothermal
system.
+12V
0.1␮F
K-TYPE
THERMOCOUPLE
40.7␮V/؇C
2 REF02EZ 6
4
1N4148
D1
––
++
R4
5.62k⍀
+5.000V
R1
10.7k⍀
R2
2.74k⍀
R6
200⍀
R3
53.6⍀
R5
40.2k⍀
R8
453⍀
R9
124k⍀
+12V
10␮F
+
0.1␮F
2
8
1/2
OP213
3
4
1
0V TO 10.00V
(0؇C TO 1000؇C)
Figure 4. Accurate K-Type Thermocouple Amplifier
R6 should be adjusted for a zero-volt output with the thermo-
couple measuring tip immersed in a zero-degree ice bath. When
calibrating, be sure to adjust R6 initially to cause the output to
swing in the positive direction first. Then back off in the nega-
tive direction until the output just stops changing.
An Ultralow Noise, Single Supply Instrumentation Amplifier
Extremely low noise instrumentation amplifiers can be built
using the OP113 family. Such an amplifier that operates off a
single supply is shown in Figure 5. Resistors R1–R5 should be
of high precision and low drift type to maximize CMRR perfor-
mance. Although the two inputs are capable of operating to zero
volt, the gain of –100 configuration will limit the amplifier input
common mode to not less than 0.33 V.
+5V TO +36V
+
V_IN
–
*R1
10k⍀
1/2
OP213
*R2
10k⍀
*R3
10k⍀
1/2
OP213
*R4
10k⍀
V_OUT
*R_G
(200⍀ + 12.7⍀)
*ALL RESISTORS ؎0.1%, ؎25ppm/؇C
GAIN
=
20k⍀
R_G
+
6
Figure 5. Ultralow Noise, Single Supply Instrumentation
Amplifier
–6–
REV. C