Datasheet

OP1177/OP2177/OP4177
Rev. D | Page 19 of 24
⎟
⎠
⎞
⎜
⎝
⎛
−
+
+
−δ
δ
=
δ
δ
R2R3R1R4
R2R3R2R4
R2R3R1R4
R1R4
R1R1
CMRR
22
2
22
()
R1R4
R2R3
R1
CMRR
2
2
1
−
=
δ
δ
Assuming that
R1 ≈ R2 ≈ R3 ≈ R4 ≈ R
and
R(1 − δ) < R1, R2, R3, R4 < R(1 + δ)
the worst-case CMRR error arises when
R1 = R4 = R(1 + δ) and R2 = R3 = R(1 − δ)
Plugging these values into Equation 1 yields
δ
≅
2
1
MIN
CMRR
where δ is the tolerance of the resistors.
Lower tolerance value resistors result in higher common-mode
rejection (up to the CMRR of the operational amplifier).
Using 5% tolerance resistors, the highest CMRR that can be
guaranteed is 20 dB. Alternatively, using 0.1% tolerance resistors
results in a common-mode rejection ratio of at least 54 dB
(assuming that the operational amplifier CMRR × 54 dB).
With the CMRR of OPx177 at 120 dB minimum, the resistor
match is the limiting factor in most circuits. A trimming
resistor can be used to further improve resistor matching and
CMRR of the difference amplifier circuit.
A HIGH ACCURACY THERMOCOUPLE AMPLIFIER
A thermocouple consists of two dissimilar metal wires placed in
contact. The dissimilar metals produce a voltage
V
TC
= α(T
J
− T
R
)
where:
T
J
is the temperature at the measurement of the hot junction.
T
R
is the temperature at the cold junction.
α is the Seebeck coefficient specific to the dissimilar metals used
in the thermocouple.
V
TC
is the thermocouple voltage. V
TC
becomes larger with
increasing temperature.
Maximum measurement accuracy requires cold junction
compensation of the thermocouple. To perform the cold
junction compensation, apply a copper wire short across the
terminating junctions (inside the isothermal block) simulating a
0°C point. Adjust the output voltage to zero using the R5
trimming resistor, and remove the copper wire.
The OPx177 is an ideal amplifier for thermocouple circuits
because it has a very low offset voltage, excellent PSRR and
CMRR, and low noise at low frequencies.
It can be used to create a thermocouple circuit with great
linearity. Resistor R1, Resistor R2, and Diode D1, shown in
Figure 64, are mounted in an isothermal block.
V+
7
4
Cu
Cu
TR
TR
D1
D1
ADR293
V
CC
C1
2.2µF
R3
47kΩ
10µF
R2
4.02kΩ
R8
1kΩ
R7
80.6kΩ
R6
50Ω
R9
200kΩ
0.1µF
10µF
0.1µF
10µF
V–
10µF
R4
50Ω
R5
100Ω
R1
50Ω
ISOTHERMAL
BLOCK
V
TC
T
J
(–)
(+)
6
2
3
OP1177
V
OUT
02627-064
Figure 64. Type K Thermocouple Amplifier Circuit
LOW POWER LINEARIZED RTD
A common application for a single element varying bridge is an
RTD thermometer amplifier, as shown in
Figure 65. The
excitation is delivered to the bridge by a 2.5 V reference applied
at the top of the bridge.
RTDs may have thermal resistance as high as 0.5°C to 0.8°C per
mW. In order to minimize errors due to resistor drift, the
current through each leg of the bridge must be kept low. In this
circuit, the amplifier supply current flows through the bridge.
However, at the OPx177 maximum supply current of 600 µA,
the RTD dissipates less than 0.1 mW of power, even at the
highest resistance. Errors due to power dissipation in the bridge
are kept under 0.1°C.
Calibration of the bridge can be made at the minimum value of
temperature to be measured by adjusting R
P
until the output is zero.
To calibrate the output span, set the full-scale and linearity
potentiometers to midpoint and apply a 500°C temperature to
the sensor or substitute the equivalent 500°C RTD resistance.
Adjust the full-scale potentiometer for a 5 V output. Finally,
apply 250°C or the equivalent RTD resistance and adjust the
linearity potentiometer for 2.5 V output. The circuit achieves
better than ±0.5°C accuracy after adjustment.