OP1177/OP2177/OP4177

Rev. E | Page 19 of 24

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

where:

α is the Seebeck coefficient specific to the dissimilar metals used

in the thermocouple.

increasing temperature.

Maximum measurement accuracy requires cold junction compen-

sation of the thermocouple. To perform the cold junction compen-

sation, 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

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

(–)

(+)

6

2

3

OP1177

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 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. 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 resis-

tance. Errors due to power dissipation in the bridge are kept

under 0.1°C.

Calibration of the bridge is made at the minimum value of

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.