Datasheet

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
CH1
5.00V CH2 5.00V M10.0ms A CH1 300mV
1
T 0.000%
10238-063
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–
V
OUT
V
OUT
10238-064