modulated photodetection circuit, revisited

Discussion in 'The Projects Forum' started by sdh314, Oct 6, 2011.

  1. sdh314

    Thread Starter New Member

    Jun 2, 2011
    Continuing the discussion from:

    Thanks for the replies everyone.

    A bit more information:

    This is a portable device that operates in seawater, so 50 & 100Hz flicker isn't going to be an issue. The sunlight and it's interaction with the water surface means the ambient contribution is mostly DC, but maybe up to 20Hz or so.

    I'm looking at squeezing as much optical power from a LED as possible which I plan to run at about 1kHz, and I've got hold of an optical bandpass filter that matches the spectral peak & width of the LED. Regardless, in some cases the ambient will be many orders of magnitude larger than the small modulated LED signal. From what I understand, I've two options:

    1. Transimpedance stage with suitable band pass & high pass filters to remove the 0-20Hz ambient, and let through the 1kHz modulated LED light. The main problem I see is that a simple high pass filter before the TIA is that there is no DC path and the charging on the capacitor will bias the photodiode. I want to run the PD at zero bias. Can anyone suggest any possible circuits that can remove the DC without this happening?

    2. A portable lock-in type circuit, using something like AD630. Uses a TIA signal as an input and the reference TTL signal (one that drives the diode) to dig the 1kHz LED light via phase modulation.

    Being portable I've restrictions on power draw, and would ideally like to run the entire circuit from a single-side supply (9V).

    Any suggests welcome, thanks in advance!