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OPA335

Luke Beardslee
Prodigy 60 points
Other Parts Discussed in Thread: OPA335, IVC102

Hi,

             I'm trying to design/test a photodiode integrator circuit for very low light levels. My circuit operates at low frequency. I simulated and am now testing several different op-amps, but I've run into issue with two chips. I've tried using an OPA335 amplifier and it does respond to different levels of light, but I can't get the signal to stop integrating/go to zero. The slope of the integration changes according to light conditions, but it never goes to zero (even under the same conditions that give no signal with other op-amps). The integration in what I think should be complete darkness the signal will still eventually go to saturation over the integration period that I'm using. I'm trying to figure out where this effect may be coming from. The only thing I can think of is some kind of charge injection due to the correction current for the zero offset op-amp. Or, possible some kind of noise effects. Does anyone have any thoughts? Thanks!

  • 0
    TI__Expert 3341 points
    Hey Luke,
    Can you please share the schematic of the circuit,

    Also please share the captured waveform


    Thank you
  • 0 in reply to Shreenidhi Patil
    Prodigy 60 points

    Dear Shreenidhi,

                Thanks for your response. I've provided a rough sketch of the circuit as well as the output waveform that I see. Thanks!

     OPA335 Circuit Rough Sketch.pdf

  • 0 in reply to Luke Beardslee
    TI__Expert 3341 points

    Hey Luke,

    As I understand you are using a OPA335 series of CMOS operational amplifiers which use the  auto-zeroing techniques to  provide very low offset voltage (5µV max.).

    This high accuracy of precision  is accomplished by a switching / chopping proprietary system  inside ,

    This  internal switching might cause some  input current spikes which may  be amplified just as an input current from a current source such as a photodiode. In your circuit it might be that these spikes are getting integrated  along with the valid current signal

    Hence I would not recommend  OPA335 for your solution .

     

    Also the OPA335 has a bias current of 70pA , now as you have configured it as an integrator , this 70pA of bias current will increasing the output till it hits the Vcc rail . given by equation 5V(Vcc) = 1/(710pF)*integral(70pA*dt(switching)              i.e V=1/C .integral .(I.dT)

    The Time taken by the opamp to go  towards complete saturation in total darkness based on the above equation turns out to be  calculated was around 50Sec.  This happens mainly as I bias current > 0 even in total darkness.

     

     

    Please refer to one of our TI design on photodiode integrator

    http://www.ti.com/tool/TIPD176

    and also one of our part

    IVC102

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

    if you see the above part , the input amps in range of femto amps which is very very low as compare to the bias current of opa335