Ultrasonics Applications and Solutions for Industry and Research

Remote Module Preamplifiers

The Remote Modules move the critical high gain functions from the SIA-7 out to the transducers’ location.  This provides the best possible performance while maintaining the simplicity of the SIA-7 design concept.  The Remote Modules are fully and completely controlled by the SIA-7 and they require no operator input (except of course connecting the cables in the first place).

 The Remote Modules also offer a built in bias voltage needed for capacitive transducers.  Capacitive transducers do require a bias voltage to operate correctly.  The Remote Modules provide the bias voltage when needed by clicking a button in one of the SIA-7 setup windows.

Although piezoelectric and capacitive devices have very different gain and operating characteristics, electrically they are actually the same.  Both piezoelectric and capacitive transducers operate as simple charge source devices.  The transducers actually produce a charge per unit sound pressure level.  The charge then appears across the total capacitance of the transducer (including any capacitance connected to the transducer such as coax cable).  The voltage that appears across the transducer is just the charge divided by the total capacitance. 

In most cases a length of coax cable is used to connect a transducer to a preamplifier.  Coax cables have some specified capacitance per unit length.  For example RG58 has a capacitance of approximately 30 Pico Farad per foot.  A 6 foot long length of cable has a capacitance of just less than 200 Pico Farad.  A 60 foot length of cable has a capacitance of 2000 Pico Farad.  But how much capacitance does a typical transducer have?  A number of factors determine the capacitance of the device including its diameter and the construction of the radiating element (electrodes, etc.).  Typical air coupled devices have capacitance values ranging from a few hundred Pico Farad to several thousands Pico Farad.

Remember that the transducer sees its own capacitance along with the capacitance of the cable it’s connected too, not to mention the capacitance of whatever device the cable is plugged into at the other end.  Depending on the transducer, the coax cable can easily have a higher capacitance than the transducer.  That has the effect of dividing down the apparent signal.  Ideally a short length of low capacitance coax should be used.  This has another benefit of greatly reducing the noise pickup from external electrical interference.

The best way to deal with ultrasonic transducers is to insert a low noise preamplifier close to the transducer.  A properly designed preamplifier greatly improves the dynamic range and minimizes electrical noise pickup.  Preamplifiers can then be placed an arbitrary distance from the electronics and that makes installing and using transducers much simpler.

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Copyright © 2007 VN Instruments Ltd
Last modified: April 02, 2007