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Optics Components and Electronic Equipment
Published in Vadim Backman, Adam Wax, Hao F. Zhang, A Laboratory Manual in Biophotonics, 2018
Vadim Backman, Adam Wax, Hao F. Zhang
Most instrument amplifiers are differential amplifiers. A differential amplifier takes two input signals and amplifies the difference between the two inputs. The capability to amplify only the difference can be very beneficial when there is a common value between these two inputs. A differential amplifier can easily be built using a commercial operational amplifier (op-amp). Figure 2.38a shows a schematic of a differential amplifier based on a single op-amp, and more complicated designs of differential amplifier are also available. In this simple schematic, the output voltage is calculated by Equation 2.8, and the gain is controlled by the ratio of the resistors used in the circuit. One thing to keep in mind is that the output voltage cannot go beyond the value of the op-amp power supply. In most situations, however, a low-cost commercial amplifier can easily be obtained. Figure 2.38b is a commonly used wideband low-noise amplifier with a gain of ∼12 dB. The user can simply connect power to the unit, and it is ready to use. Vout=(R1+R2)R4(R3+R4)R1Vin+−R2R1Vin−;whenR2R1=R4R3,Vout=R2R1(Vin+−Vin−)
A computational investigation and smooth-shaped defect synthesis for eddy current testing problems using the subregion finite element method
Published in Research in Nondestructive Evaluation, 2019
Mohammad R. Rawashdeh, Anders Rosell, Lalita Udpa, S. Ratnajeevan H. Hoole, Yiming Deng
The excitation coil that contains the AC input voltage will be the source for the first part of the magnetic flux density at the predetermined measuring points BCoil (T), while the eddy current will contribute the second part as BEddy Current (T). We are dealing with a 2D problem, so we will choose By as the normal value that we are planning to measure using the TMR sensor at these measuring points. The TMR sensor is placed on a scanner that positions the sensor over the surface of the tested sample where it will read the values of the normal magnetic flux densities By (T). The TMR sensor will output a voltage Vy (V) directly proportional to the value of By (T). It is necessary to note that we are planning to have values of By within the linear region of the sensor, so we will have proportional output voltages. If the values of By are high, there is a risk of saturating the sensor. This will be a limitation for the coil size. The voltages Vy (V) are small related to input values, so we need to amplify these values. To do that, we will use an instrument amplifier with a gain Gm. The output of the amplifier then will be Vmy = GmVy (V). This amplified signal will be then input to a lock-in amplifier to measure phasor parameters for Vmy (V). We use the same input waveform as a reference to measure the in-phase and quadrature components of the output signal. There may be a small phase shift in the output voltages due to the existence of the low pass filter in the lock-in amplifier. The output voltages of the lock-in amplifier are related to the magnetic flux density values. This relation is linear, and the resulted signal will be a scaled version for By. We will study two experimental cases to validate the subregion FEM.