In order for this circuit to act as a difference amplifier, its output
must be proportional to the differential input signal.
From Figure 13,
Arranging terms and combining the equations above yields:
CMRR = R 4R1 + R 3R 2 + 2R 4R 2
2R 4R1 − 2R 2R 3
The sensitivity of CMRR with respect to the R1 is obtained by
taking the derivative of CMRR, in Equation 1, with respect to R1.
1R 4 − 2R
2R 2R 4 + R 2R 3
2R1R 4 − 2R 2R 3
(2R 2R 3)
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:
where δ is the tolerance of the resistors.
Lower tolerance value resistors result in higher common-mode
rejection (up to the CMRR of the op amp).
Using 5% tolerance resistors, the highest CMRR that can be
guaranteed is 20 dB. On the other hand, using 0.1% tolerance
resistors would result in a common-mode rejection ratio of at
least 54 dB (assuming that the op amp CMRR ϫ 54 dB).
With the CMRR of OP1177 at 120 dB minimum, the resistor
match will be the limiting factor in most circuits. A trimming
resistor can be used to further improve resistor matching and
CMRR of the difference amp circuit.
A High-Accuracy Thermocouple Amplifier
A thermocouple consists of two dissimilar metal wires placed in
contact. The dissimilar metals produce a voltage
( ) VTC = α TJ − TR
where TJ is the temperature at the measurement of the hot junction,
TR is the one at the cold junction, and ␣ is the Seebeck coefficient
specific to the dissimilar metals used in the thermocouple. VTC is the
thermocouple voltage. VTC becomes larger with increasing temperature.
Maximum measurement accuracy requires cold junction compen-
sation of the thermocouple as described below.
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 trimming resistor R5 and then remove the copper wire.
The OP1177 is an ideal amplifier for thermocouple circuits since
it has a very low offset voltage, excellent PSSR and CMRR, and
low noise at low frequencies.
It can be used to create a thermocouple circuit with great linearity.
Resistors R1 and R2 and diode D1 shown in Figure 14 are
mounted in an isothermal block.
3 7 OP1177
Figure 14. 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 15. The excita-
tion 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 OP1177 maximum supply current of 600 µA,
the RTD dissipates less than 0.1 mW of power even at the high-
est 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 RP until the output is zero.
To calibrate the output span, set the full-scale and linearity pots
to midpoint and apply a 500°C temperature to the sensor or
substitute the equivalent 500°C RTD resistance.
Adjust the full-scale pot for a 5 V output. Finally, apply 250°C
or the equivalent RTD resistance and adjust the linearity pot for
2.5 V output.
The circuit achieves better than ±0.5°C accuracy after adjustment.