Lakhasly

To accurately measure the changes in the sense capacitors, we can apply the synchronous demodulation technique.With the conditioning module located away from the LVDT, it is necessary to have a well-balanced wiring with low distributed capacitance.The synchronous demodulator basically multiplies the amplifier output by the excitation voltage (either Vdrive+ or Vdrive-) to convert the square wave at the amplifier output to a DC voltage that reveals the amount of displacement as well as its direction.When the movable electrode moves closer to one of the fixed electrodes, a larger portion of the excitation voltage from that electrode appears at the amplifier input Vbridge, which means the square 93 wave that appears at the amplifier input is in-phase with the excitation voltage of the closer electrode.A good example is making measurements in harsh environments of radioactive applications where the conditioning circuitry should be placed in safe areas, even up to several hundred meters away from the LVDT.Fig.(5) Synchronous Demodulation In this case, a 1 MHz square wave is used as the AC excitation of the sense capacitors Cs1 and Cs2.

To accurately measure the changes in the sense capacitors, we can apply the synchronous
demodulation technique. Figure (5) shows a simplified version of the signal conditioning
employed in the ADXL family of accelerometers from Analog Devices.
Fig.(5) Synchronous Demodulation
In this case, a 1 MHz square wave is used as the AC excitation of the sense capacitors
Cs1 and Cs2. The square waves applied to the fixed electrodes have the same amplitude
but are 180° out of phase with respect to each other. When the movable electrode is at its
resting position, the voltage at the amplifier input is zero volts. When the movable
electrode moves closer to one of the fixed electrodes, a larger portion of the excitation
voltage from that electrode appears at the amplifier input Vbridge, which means the square
93
wave that appears at the amplifier input is in-phase with the excitation voltage of the
closer electrode. For example, in Figure (5), the amplified output is a square wave in
phase with Vdrive+ because Cs1 is larger than Cs2. The amplitude of Vbridge is a function of
the proof mass displacement; however, we also need to know the phase relation of
Vbridge with respect to Vdrive+ and Vdrive- to determine in which direction the proof mass is
displaced. The synchronous demodulator basically multiplies the amplifier output by the
excitation voltage (either Vdrive+ or Vdrive-) to convert the square wave at the amplifier
output to a DC voltage that reveals the amount of displacement as well as its direction.
LVDT demodulation techniques shown in figure (6) can be used to amplify the voltage
Fig. (6) LVDT demodulation (a) four wires (b) five wires
There are many applications where the conditioning circuitry is located at a long distance
from the sensor. A good example is making measurements in harsh environments of
radioactive applications where the conditioning circuitry should be placed in safe areas,
even up to several hundred meters away from the LVDT. In these cases, it can be
challenging to transmit the two secondary voltages over a long distance through a 5-wire
configuration. With the conditioning module located away from the LVDT, it is necessary
to have a well-balanced wiring with low distributed capacitance. This means a considerable
increase in the cost of wiring.