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OPA928: OPA928

Part Number: OPA928
Other Parts Discussed in Thread: OPA392, , LMP7721

Tool/software:

Hi TI,

I am making charge amplifier for a piezo disc which will function as hydrophone, which has a capacitance of 11 nF, and DC resistance of 500 K, and impedance varies from 500 KOhms till 100 Ohms at resonance, my piezo disc has a resonance at around 18 KHz, in air and at 2 MHz in water. My frequency of interest is 10 Hz to 20 KHz.

I have to detect very faint charges in range of tens of zepto Coulombs till few hundreds of pico coulombs. Can TI advice me on solution that is very low noise and capable of measuring zepto coulombs. I intend to include variable gain amplifier with attenuator that will attenuate the signal when charge signal is high in case of pico coulombs. However my charge amplifier which is very near to piezo's should be able to measure zepto coulombs to pico coloumbs.

Will OPA928, or OPA392 be able to measure zepto coulombs? Can TI propose me a very low noise solution for my design needs? I have an array of six discs. I look forward to hear back from TI about this. 

  • HI Sumeet,

    We are able to suggest charge amplifier circuits that can measure charge in the range 10s of femto to 100s of pico Coulomb range, but I am not aware of conventional op-amp charge amplifier circuits able to measure at the much smaller range.

    Please see application note: Charge Amplifier Circuit

    Thank you and Best Regards,

    Luis

  • Hi Luis,

    Any particular suggests that will help get that good sensitivity, any unconventional circuit suggestion you can suggest me? In this application which one would be better opamp OPA928 or OPA392?

  • HI Sumeet,

    The lowest input bias current amplifiers we currently offer are the OPA928 and LMP7721 offering ±20fA max input bias current at 25°C. Resolving charge at the femto-Coulombs range at very low frequencies is possible with these amplifiers, with careful circuit and PCB layout design considerations.

    On an integrator op-amp circuit, the charge Q on a capacitor is defined by Q = C*V where C is the feedback capacitance and V is the voltage across the capacitor.  For example, if we use a10pF feedback capacitor (CF) on an integrator, a 0.1V across the 10pF capacitor, corresponds to 1pC. On an integrator, the input current charges the feedback capacitor over time, resulting in a ramping output voltage which represents the total charge accumulation. The current charging a capacitor is given by I = C*dV/dt.  For example, a voltage change of 10mV accumulates across the 10pF feedback capacitor during a measurement period of 1s, where the input current will be in the range of 100fA. The OPA928 or LMP7721 works well in this integrator circuit since the input bias current (Ib) is typically 3fA at 25°C.

    However, charge at the zepto Coulomb level are at the 10^-21 level, this is 8 orders of magnitude smaller than the 100fC level; and the theoretical charge of a single electron is 1.6X10^-19C. The op-amp input bias current, leakage currents present the PCB, stray capacitance on the PCB board, leakage on the capacitor, op-amp input capacitance, the offset and intrinsic noise of the op-amp all result in errors in the measurement. These errors are manageable when attempting to measure charge at the 10s fC level with careful PCB and circuit design considerations. However, measurements of currents (or charge) at much smaller levels is not possible since these errors will dominate. 

    Regards,

    Luis 

  • Hi Luis,

    Thank you for your very good explanation. But I am in fix now, since my application requires me to pick up such a low signals and along with it also detect the frequency of signal. I am not sure, but can you please help me sync up with some one expert in this domain, as my requirement cannot be changed. It would be very grateful.

  • Hi Sumeet,

    The OPA928 and LMP7721 are the ultra-low input bias current, high precision amplifiers available in the precision portfolio achieving femto-ampere current level; these are probably among the lowest bias current amplifiers available in the market. In general, our expertise and support is related to TI amplifiers. Unfortunately, I don't have further suggestions for charge measurements at much lower current (charge) level beyond what can be achieved with these devices.

    Thank you and Regards,

    Luis