ADA4077-1/ADA4077-2/ADA4077-4

Data Sheet

Rev. D | Page 22 of 25

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 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, and the 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 a

resistance temperature detector (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 to 0.8°C per

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.

Therefore, 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

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

higher than ±0.5°C accuracy after adjustment.

Figure 70. Low Power Linearized RTD Circuit

PROPER BOARD LAYOUT

The ADA4077-1, ADA4077-2, and ADA4077-4 are high precision

devices. 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

components, 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 EMI noise and maintains a constant temperature

across the circuit board.

2

CH2 5.00V

M10.0ms

A CH1

300mV

1

200Ω

500Ω

FULL-SCALE ADJ

4.37kΩ

100Ω

100Ω

20Ω

ZERO ADJ

4.12kΩ

4.12kΩ

5kΩ

LINEARITY

ADJ

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–

0.1µF