Mode of operation carrier frequency measuring amplifier

Carrier frequency measuring amplifiers for inductive and resistive sensors in half or full bridge circuits produce an AC voltage for supplying the measuring bridge. They evaluate the voltage picked up on the bridge and provide the measurement value, for example via a standardized interface.

Typical construction of a carrier frequency measuring amplifier:
 

Mode of Operation:
The oscillator generates a generally sine wave form AC voltage with a frequency in the range of a few kilohertz. This is directed to the measuring bridge and simultaneously used as reference for the signal returned from the measuring bridge.
The amplitude of the measurement signal (A) is preamplified and evaluated (ratiometric evaluation) in order to avoid errors due to an altered supply voltage.
The subsequent filter eliminates interference caused by external influences such as thermoelectric voltage, 50 Hz couplings and high-frequency fields.
The filtered measurement signal is scaled for further processing by the output amplifier.

In the case of inductive measuring bridges, a phase shift has to be taken into account which can arise between the supply voltage and the output signal. A difference in phase between the measurement signal and the reference voltage can lead to significant (amplitude) measurement errors. Good carrier frequency measuring amplifiers permit a corresponding phase correction in addition to scaling the output signal.

Note:
When choosing a suitable carrier frequency measuring amplifier for inductive displacement sensors (or resistive sensors in bridge circuitry), the following points should be considered:

    • Is the displacement sensor constructed with differential inductor (half bridge) or differential transformer (LVDT) circuitry?
      Not all measuring amplifiers on the market support full bridges, for example.

    • Which supply voltage and which carrier frequency does the displacement sensor require?

    • Is the dynamic range of the measuring amplifier sufficient if rapid movements are to be acquired?

    • Can the sensitivity of the measuring amplifier to the nominal output be adjusted?

    • Does the inductive displacement sensor have an appreciable phase shift which must be corrected?

    • Which supply voltage is available for operation of the measuring amplifier?

    • What output signal (e.g. voltage or current output) is required?

    • Where is the measuring amplifier to be installed (e.g. in a control cabinet or directly on the machine)?

 

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