Separating high pass and low pass TIA output bands.

Today I got more specific specifications.

The minimum photodetector size is .3 mm. Larger would be preferred as this would provide a larger target for the laser beam making easier alignment.

The laser's the total irradiant power on the photodetector will range from 10 uW to 5 mW. Linearity must be maintained over the entire range. Of course they would like to be shot noise limited over the entire range, but they recognize this might not be feasible. They are open to having two or more units, each optimized for a narrower range of input power levels. At 40 MHz how practical is it to switch feedback resistors?

The modulation will be roughly 1% of the DC level.

The DC output band will be from 0 Hz to 50 kHz The AC output band will be from 20 kHz to 40 MHz. They do not want bandwidth any higher that 40 MHz for noise suppression reasons. The signal of interest is 40 MHz and below.

I see your point that any source that puts current into the virtual ground to tune out the DC offset will introduce its own noise.

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That's 500:1, which is not horrible. What's the responsivity of the PD?

Assuming it's around 0.25 A/W, which is typical for silicon in the visible, that would be 2.5 uA to 1.25 mA. If you pick a 20k resistor, that gets you to the shot noise + 3 dB at the low end, and 25V at the high end, which you can do with a reasonably normal amplifier if you run it off asymmetric supplies like +28/-4.

At 40 MHz you'll probably need an AC tweak to keep the response flat, and that would differ between ranges.

If you do decide to use range switching, use a relay to switch the whole feedback resistor + AC tweak in and out.

Cheers

Phil Hobbs

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Phil Hobbs

What low noise op amp has sufficient GBW and can operate off a 32V power supply difference?

By tweak do you referring to a subsequent stage that is a filter with a zero in it, or a multipole filter that has some overshoot at the high end of its frequency response?

The photodiode and its TIA is for a laser ultrasonics system's detection laser.

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There are more nowadays. You might look at the ADA4898, which may not be quite fast enough, or (with a sufficiently efficient bootstrap) an LM6171.

Cheers

Phil Hobbs

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