Data Sheet

ADA4077-2

Rev. 0 | Page 17 of 20

APPLICATIONS INFORMATION

OUTPUT PHASE REVERSAL

Phase reversal is defined as a change of polarity in the amplifier

transfer function. Many operational amplifiers exhibit phase

reversal when the voltage applied to the input is greater than the

maximum common-mode voltage. In some instances, this can

cause permanent damage to the amplifier. In feedback loops, it can

result in system lockups or equipment damage. The ADA4077-2 is

immune to phase reversal problems even at input voltages

beyond the supplies.

Figure 59. No Phase Reversal

LOW POWER LINEARIZED RTD

A common application for a single element varying bridge is an

RTD thermometer amplifier, as shown in Figure 60. The excitation

is delivered to the bridge by a 2.5 V reference applied at the top

of the bridge.

RTDs can have a 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 ADA4077-2 maximum supply current of 500 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 is made at the minimum value of

the temperature to be measured by adjusting RP 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.

Figure 60. Low Power Linearized RTD Circuit

2

CH2 5.00V

M10.0ms

A CH1

300mV

1

200Ω

500Ω

4.37kΩ

100Ω

100Ω

20Ω

4.12kΩ

4.12kΩ

5kΩ

49.9kΩ

ADR4525

+15V

0.1µF

V+

100Ω

RTD

1/2

ADA4077-2

7

6

5

1/2

ADA4077-2

1

8

2

3

4

V–