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photodiode transimpedance for quick pulse (20ns) with high DC background light

Pierre-Yves Boulanger
Prodigy 10 points
Other Parts Discussed in Thread: OPA847, THS3001, TINA-TI

Hello,

I'm building a prototype with a single reverse bias (100V) photodiode (photoconductive mode) with a capacitance of 15pF.
I'd like to measure the amplitude (by sampling at around 100Mhz with 10bit ADC) laser pulse of 20ns of length with a quite high DC level (might be up to several mA) due to background light (so should be relatively low freq).

By making some calculation, the current due to the laser pulse could be very different from one setup to another one, typically from 1uA to 20mA.
The repetition rate of the laser pulse is very low, around 1Hz and the triggering of the sampling is done externally.

I thought about using a OPA847, any advice?
How can I cope with the quite high DC component ?

Thanks for your tips

Best regards,

PYB

  • 0
    Genius 5430 points

    Pierre-Yves;

    You have presented an interesting problem-- a very high bandwidth, high dynamic range requirement. It sounds as if you may be using a photomultiplier tube (PMT) or an avalanche photodiode (APD) to get such a high current output. It really does not matter what the source is, though, since you have provided the output current range, 1uA to 20mA, and the device capacitance, 15pF.

    A signal level this high does not require a transimpedance amplifier, only a resistive load for the sensor and a fast, noninverting amplifier. This amplifier must have low noise and a high dynamic range for your application so the OPA847 is not optimum in this case. To provide a very wide dynamic range, a low noise amplifier is required, of course, but it must also be capable of a very high output voltage swing.

    For these reasons, I chose a THS3001 current-feedback op amp which can operate on a 32V power supply. This provides a far higher output swing capability than the more common +5V high-speed op amps. To get the highest output swing, I have operated the THS3001 on a single +30V supply and offset its operating point by +3V so that it is operating within its input common-mode voltage range and so that the output can swing without hitting its negative rail. This amplifier has a very high slew rate so it can provide large amplitude output pulses with good fidelity.

    I have attached a TINA-TI file showing the circuit and the results of a 20ns pulse transient simulation.

     

     

    High Level Photodiode Amplifier 3.TSC