How to design a charge integrator amplifier?

Hi,

  Now we want to convert the charge acquired by the sensor to a voltage, so how to select a proper amplifier to design a charge integrator amplifier? What parameters are important? Will the Ib, noise, or others affect the performance? Woud you please give me a reference design or an article which describes the details?

 

On the other hand, if the sensor output with DC offset value, the maximum value is about 1V, Is using ac couple to the charge integrator amplifier a good way? Or can you show me other approach to cancel the dc offset?

 

Thanks!

  • 0
    •  Analog Employees 
    on Jan 11, 2021 3:51 AM 1 month ago

    Hi jerrytang,

    Maybe this application Note can help you design your charge to voltage amplifier?

    https://www.analog.com/media/en/reference-design-documentation/reference-designs/CN0350.pdf

    The amplifier bias current must be low (relative to the rate of change of the charge from your sensor) or otherwise you will integrate the input bias current and produce an output error voltage. I think LTspice or pspice can be used reliably to predict the output noise voltage density (or its RMS value) for any amplifier that you choose to use.

    Here is a list of low input bias current amplifiers selected for low input voltage noise < 8nV/RtHz:

    https://www.analog.com/en/parametricsearch/11090#/p4101=2.7n|8n

    As far as the sensor DC offset is concerned: You can AC couple the charge from your sensor. The circuit will respond to the sensor charge regardless.

    Regardless,

    Hooman

  • Hi Hooman,

     

    Thanks for your help. In this case, If using ac couple, how to choose a proper capacitance, especially capacitance value, and what kind of capacitance is better?

     

    Thanks!

  • 0
    •  Analog Employees 
    on Jan 12, 2021 8:37 AM 1 month ago in reply to jerrytang

    Hi jerrytang,

    Frankly, I don't have a clear understanding of your charge sensor which has a built-in DC offset to recommend anything. When I think of a charge sensor, I imagine a high impedance current source (along with some shunt sensor capacitance) and I cannot picture where the 1V DC offset of your sensor is coming from or how to model it schematically? May be if you provide more information on your sensor datasheet, it'd help.

    Regards,

    Hooman

  • 0
    •  Analog Employees 
    on Jan 12, 2021 6:34 PM 1 month ago in reply to Hooman

    Hi jerrytang,

    To answer your question on how to choose the value of the cap you can use to couple your charge sensor to the amplifier, I've done a LTspice simulation assuming some arbitrary operating conditions (since I don't know yours).

    In this simulation, the value of the AC coupling cap (C3) is varied from 100pF to 10uF. I've assumed that your sensor capacitance (C2) is 3.8nF.

    With lower C3 values, the charge-to-voltage gain drops. Beyond 1uF, the gain is no longer impacted for the particular simulation I've dreamed up. So, your situation will be different depending on your charge profile or timing and also the capacitance of your sensor. So, if you chose 1uF or higher, you will be have a gain which is equivalent to the case where you connect your sensor directly to the amplifier inverting node.

    If your sensor capacitance is very small, the AC coupling cap value will not make difference in gain. I think the two input capacitors (C2, and C3) form a current divider and the AC coupling cap has to be large enough to take the larger percentage of the sensor charge, or otherwise your gain will drop.

    Here is the simulation file if you'd like to take a look at and experiment with:

    AD8034 Charge Amplifier AC copuled 1_12_21.asc

    Regards,

    Hooman