OP2177ARM datasheet(14/20 Pages) AD

14 / 20 page

REV. B

OP1177/OP2177/OP4177

–14–

In order for this circuit to act as a difference amplifier, its output

must be proportional to the differential input signal.

From Figure 13,

O =−





































Arranging terms and combining the equations above yields:

CMRR

R R

−

24 2

241 22 3

(1)

The sensitivity of CMRR with respect to the R1 is obtained by

taking the derivative of CMRR, in Equation 1, with respect to R1.

CMRR

R R

214

2 2 3

224

2 3

214

2 2 3

−









CMRR

22 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:

CMRR

MIN ≅

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

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

=−

()

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.

VOUT

200k

15V

+15V

OP1177

(+)

( )

0.1 F

10 F

0.1 F

10 F

100

80.6k

47k

10 F

4.02k

ISOTHERMAL

BLOCK

VTC

ADR293

VCC

2.2 F

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.

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