OP213EP AD [Analog Devices], OP213EP Datasheet - Page 6

• Manufacturer Part Number: OP213EP
• Description: Low Noise, Low Drift Single-Supply Operational Amplifiers
• Manufacturer: AD [Analog Devices]

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.
To calibrate the circuit, first immerse the RTD in a zero-degree
ice bath or substitute an exact 100
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
(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
(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.
10 F
100
RTD
12
13
11
4
6
7
–15V
AD588BD
16
9
+15V
8
Figure 3. Ultraprecision RTD Amplifier
2
R
R
R
10
W1
W2
W3
15
14
1
3
R1
8.25k
50
R3
2
3
A1
R2
8.25k
R4
100
OP213
1/2
R
G
1
FULL SCALE ADJUST
4.02k
6
5
R5
A2
resistor in place of the
+15V
–15V
8
4
100
49.9k
R7
OP213
R8
1/2
7
R9
5k
LINEARITY
ADJUST
@1/2 F.S.
V
–1.50V = –150 C
+5.00V = +500 C
OUT
(10mV/ C)
resistor
resistor
–6–
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.
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.
THERMOCOUPLE
Figure 5. Ultralow Noise, Single Supply Instrumentation
Amplifier
Figure 4. Accurate K-Type Thermocouple Amplifier
40.7 V/ C
+12V
+
–
0.1 F
K-TYPE
V
IN
*ALL RESISTORS
10k
*R1
2
+
–
REF02EZ
–
+
5.62k
4
1N4148
D1
OP213
R4
1/2
6
10k
0.1%,
*R2
(200
+5.000V
R1
10.7k
R2
2.74k
R3
53.6
200
*R
25ppm/ C
+ 12.7 )
R6
10k
G
*R3
R5
40.2k
453
R8
+5V TO +36V
OP213
1/2
10k
124k
*R4
GAIN =
2
R9
3
OP213
+12V
1/2
8
4
20k
0.1 F
R
10 F
+
G
V
OUT
1
0V TO 10.00V
(0 C TO 1000 C)
+ 6
REV. C