Data Sheet

ADA4077-1/ADA4077-2/ADA4077-4

APPLICATIONS INFORMATION

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Rev. F | 22 of 25

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 phase reversal can

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

result in system lockups or equipment damage. The ADA4077-1/

ADA4077-2/ADA4077-4 are immune to phase reversal problems

even at input voltages beyond the power supply settings.

Figure 69. 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 70. 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/mW to

0.8°C/mW. To minimize errors due to resistor drift, keep the current

low through each leg of the bridge. In this circuit, the amplifier

supply current flows through the bridge. However, at a maximum

supply current of 500 µA for the ADA4077-2, the RTD dissipates

less than 0.1 mW of power, even at the highest resistance. There-

fore, 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 potenti-

ometers 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 a 2.5 V output. The circuit achieves higher than

±0.5°C accuracy after adjustment.

Figure 70. Low Power Linearized RTD Circuit

PROPER BOARD LAYOUT

The ADA4077-1/ADA4077-2/ADA4077-4 are high precision devi-

ces. To ensure optimum performance at the PCB level, care must

be taken in the design of the board layout.

To avoid leakage currents, maintain a clean and moisture free

board surface. Coating the surface creates a barrier to moisture

accumulation, and reduces parasitic resistance on the board.

Keeping supply traces short and properly bypassing the power

supplies minimizes the power supply disturbances caused by the

output current variation, such as when driving an ac signal into a

heavy load. Connect bypass capacitors as closely as possible to

the device supply pins. Stray capacitances are a concern at the

outputs and the inputs of the amplifier. It is recommended that the

signal traces be kept at least 5 mm from supply lines to minimize

coupling.

A variation in temperature across the PCB can cause a mismatch

in the Seebeck voltages at solder joints and other points where

dissimilar metals are in contact, resulting in thermal voltage errors.

To minimize these thermocouple effects, orient resistors so that

heat sources warm both ends equally. Ensure, where possible, that

input signal paths contain matching numbers and types of compo-

nents, to match the number and type of thermocouple junctions.

For example, dummy components such as zero value resistors can

be used to match real resistors in the opposite input path. Place

matching components in close proximity to each other, and orient

them in the same manner. Ensure that leads are of equal length so

that thermal conduction is in equilibrium. Keep heat sources on the

PCB as far away from amplifier input circuitry as is practical.

The use of a ground plane is highly recommended. A ground plane

reduces electromagnetic interference (EMI) noise and maintains a

constant temperature across the circuit board.

LONG-TERM DRIFT

The stability of a precision signal path over its lifetime or between

calibration procedures is dependent on the long-term stability of the