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Selecting an airborne amplifier for flight test application

Question

On your 2680M1-M7 data sheet there are versions M1 through M7 having different gain ranges allowable. How do I select the correct gain based on my accelerometer?

Answer

The output scaling of the airborne charge amplifier (2680M1-M7) or airborne Isotron conditioner (2685M1-M7) take into consideration the accelerometer sensitivity and the full scale range anticipated for the flight test. See the examples below for both a charge accelerometer and a model 2680MX and an Isotron/IEPE accelerometer and a model 2685MX.

PE charge amplifier Model 2680 MX vs Model 2685 MX

Ex. 1)

The 2680MX airborne charge amplifier converts the high impedance charge signal from a single-ended piezoelectric accelerometer to a low impedance voltage output signal. The output sensitivity from the 2680MX is determined by the end users full scale range anticipated, scaled to the maximum undistorted output voltage of the amplifier. See below example:

Model 2221F accelerometer and model 2680M4 airborne charge amplifier for use in a 50g pk FS vibration event. The full scale output from the 2680MX is 2325 mV (2.35Vpk) = 50pk.

    • The 2680M4 has an adjustable gain range from 1.0 to 10.0

This accelerometer (2221F) and airborne amplifier (2680M4), for this particular full scale range test are compatible.

To adjust and verify proper gain on amplifier, input a function generator voltage of 100 mV pk at 100Hz, through a 1000pF capacitor (model 2947C) and adjust gain potentiometer on 2680M4 until output voltage is 100 mV times the gain of 4.65 or 465mV (20% of FS or 10g's pk).

Ex. 2)

The 2685MX airborne Isotron conditioner provides the dc power and constant current for an Isotron/IEPE single-ended piezoelectric accelerometer. The output sensitivity from the 2685MX is determined by the end users full scale range anticipated, scaled to the maximum undistorted output voltage of the conditioner. See below example:

Model 7251A-10 accelerometer and model 2685M4 airborne charge amplifier for use in a 50g pk FS vibration event. The full scale output from the 2685MX is 2325 mV (2.35Vpk) = 50pk.

    • The 2685M4 has an adjustable gain range from 1.0 to 10.0

This accelerometer (7251A-10) and airborne amplifier (2685M4), for this particular full scale range test are compatible.

To adjust and verify proper gain on amplifier, input a function generator voltage, with a + 2.00 V dc offset. The dc offset voltage simulates the dc bias voltage Isotron/IEPE accelerometers provide under normal operation. Input 100 mV pk at 100Hz and adjust gain potentiometer on 2685M4 until output voltage is 100 mV times the gain of 4.65 or 465mV (20% of FS or 10g's pk).

Note: The filter roll-off characteristics are selected per the table on the appropriate data sheets for the 2680MX and 2685MX.