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transimpedance gain selector

Yong Quan Tan
Prodigy 20 points
Other Parts Discussed in Thread: IVC102

hi all,

i need help in my circuit design.

currently i am designing a transimpedance to detect pico amp to micro amp. (10pA-1uA)

the gain required for this varies a lot from 1M ohm to 10G ohm

for the 1st stage of amplifier i am using the OPA128 which did quite a good job in getting 20-30pA reading 

but the problem i am facing is that my ADC can only read a range of reading from 1mV to 10V 

therefore, i need to varies the gain to reduce the noise read the current signal.

for having a large range i need to change the gain resistor value to minimize the noise(EMI/RF) and to set the output voltage to a readable range.

is there any ic/or thing that can help me do a gain selector like what have been done in this patient?

http://www.google.com/patents/US6462327

  • 0
    TI__Guru* 93105 points

    Hi Yong,

    Take a look at the data sheet for the IVC102, a precision switched integrator transimpedance amplifier. It is a switched integrator topology that can be used in place of conventional transimedance amplifier circuits and should be well suited for currents down to your 10 pA requirement. An explanation of its operation is provided on data sheet page 6. The output voltage is:

    VO = -(IIN) (TINT) / CINT)

    It is proportional to the integration time, and inversely proportional to the integration capacitance.The effective transimpedance gain is TINT/CINT There are internal capacitors that can be pin strapped to change the total integration capacitance, or the integration time can be varied to change the gain. There are noise reduction advantages to the integration method due to averaging.

    The IVC102 data sheet is here:

    http://www.ti.com/lit/ds/symlink/ivc102.pdf

    Regards, Thomas

    PA - Linear Applications Engineering