CN-0350 gain tuning

Hi there,

I have the CN-0350 connected to my piezoelectric sensor. The data output is between 0 and 4096, which matches 0 to 2.5 V, as the datasheet explains. The offset is around 1.25 V. The attached picture shows the graph of my data when I press the piezoelectric sensor 4 times, while increasing the pressing force as we go from left to right. It is clear that the peak-to-peak amplitude of the signal is greater than 2.5 V. Can I tune the gain of the amplifier inside the CN-0350 so my signal will not hit the limits of the ADC converter (0 - 2.4 V)? if so, how can I do that?

Best,

Ebrahim



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[edited by: JValeriani at 4:58 PM (GMT 0) on 3 Oct 2019]
  • 0
    •  Analog Employees 
    on Oct 2, 2019 6:22 AM over 1 year ago

    Hi Ebrahim,


    CN0350 eval board allows you to have access in modifying the gain to match that to your sensor requirement.  By referring on page 3 of the circuit note, it gives an example on how to calculate the component value found in CN0350 eval board.  The component you should look at is the capacitor (C2) that defines the voltage output swing at the first stage of the signal conditioning circuit.  Its current value was derived from the property of the specific sensor that is  Kistler type 8002K quartz accelerometer.  But please note that changing values of C2 will effect the minimum frequency measured and the lowest limit of frequency input range which can be compensated by also changing resistor R3.


    Cheers,
    Erbe

  • Hi Erbe,

    Thanks for your reply. will adding more capacitance in the system lead to measurement delays?

    sincerely,

    Ebrahim

  • +1
    •  Analog Employees 
    on Oct 3, 2019 1:03 AM over 1 year ago in reply to Ebrahim

    Hi Ebrahim,

    Base from the schematic design implemented in CN0350, capacitor C2 defines the minimum corner frequency the device could response from.  The minimum corner frequency in CN0350  is currently set at 1.6Hz, neglecting the input resistance of the op amp and insulation resistance of the piezoelectric sensor. 

    If we try to increase the capacitance to 2nF, it will results to changing the minimum corner frequency from 1.6Hz to 0.8Hz. and for the voltage output swing, given the same sensor from Kistler, it would results from reducing the  voltage output swing from +/-1V to +/-0.5V.  we recommend that the value of the capacitor (C2) should be derived from the your sensor specification such as its sensitivity (pC/g), its range of accelerations (g) with a given a desired voltage output swing of amplifier that is  +/-1V.  you can use the following equation to get the value for C2:

    C2 =  Sa*A/dV

    where

    Sa = sensor sensitivity (pC/g)

    A  = range of accelerations (g)

    dV = desired voltage output swing of amplifier

    Cheers,

    Erbe 

  • Thanks Erbe,

    I am trying to use a piezoelectric sensor to measure the force. I do not have the sensor sensitivity (pC/g),

    but I do have the piezoelectric charge constant (400*10-12 C/N). the charge range for the CN0350 is +/- 1000 pC, which means it will be saturated when we try to measure forces around 3 Newtons, as I understand. I need to measure forces of about 50 N, so would you suggest adding capacitors, or getting another amplifier? and if so, would you please recommend one for me?

    Best,

    Ebrahim

  • 0
    •  Analog Employees 
    on Oct 3, 2019 5:54 AM over 1 year ago in reply to Ebrahim

    Hi Ebrahim,

    You could try first increase the cap value for C2 (e.i 2.2nF) as that directly affects the gain at the first stage of the amplifier.

    Cheers,

    Erbe