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APD with OPA656 receiver design

Abhishek Singhal
Prodigy 30 points
Other Parts Discussed in Thread: OPA656

Dear Members and Colleagues

I am having some trouble designing the APD receiver in a laser rangefinder. Currently my maximum range is only 100
feet while i need at least 500 feet target range. I feel the problem is in the APD bias circuit/receiver

I have custom designed the APD receiver using First Sensor's AD1500-9 APD and Texas instruments OPA656 transimpedance amplifier. My TIA
receiver has 20 Mhz bandwidth and 20Kohm gain which seems reasonable for rangefinding application.
However, I am not able to operate the APD at maximum gain (M=100). The maximum APD gain i can achieve is
M=40 and i think that is where the problem lies. In the biasing circuit for the APD ( which is just a
series resistor of 660Kohm) on increasing the High voltage output from the DC converter, there is large voltage drop across the
series resistor which results in APD bias of ~ 147 V (M=40) whereas we need 180 V as per the datasheet to
get a gain M=100. Increasing the High Voltage outapd_bias_circuit.pdfput further (upto 300 V) does not increase the APD bias
voltage. Please find the attached diagram for your reference.

Since i am not well versed with electro-optical design i am struggling to figure out how to increase the
range and troubleshoot the APD receiver.Any help/suggestion/comments on this will be greatly appreciated.
Please let me know if you have any questions

Thank You!
Sincerely
Abhishek Singhal

  • 0
    TI__Genius 12631 points
    Abhishek,
    Why do you need the 600k resistor? Can you reduce or eliminate that? The voltage drop across the resistor is due to the dark current of the APD. I am not sure why increasing the dc-dc converter voltage to 300V does not resolve your issue. Perhaps the dark current is increasing linearly with reverse bias voltage? I think you need to take this up with the APD manufacturer.
  • 0 in reply to Samir Cherian
    Prodigy 30 points

    Hi Samir,

    Thanks for your response. I need the bias resistor to limit the maximum current through the APD (PEAK DC current=0.25mA). There is an application circuit in the APD datasheet (please find attached) but they do not specify the bias resistor value. Based on literature, the bias resistor can vary from 100K to 3 Meg. With my experimentation, changing (reducing) the bias resistor value increases the APD bias voltage, however i cannot reduce it below 600K to avoid damaging the APD (which i have done before resulting in a very costly replacement)

    I read a previous post from you that we can connect the APD directly to output of DC-DC convertor but as i had allready damaged my previous APD i was hesitant to connect the new one.

    Also I have used a SMD to DIP adapter to connect the OPA656 in the circuit. I am hoping to design the smd PCB once i get the APD issue resolved. I understand this can put limits on the OPA656 bandwidth. Do you think this can also contribute to the lower range besides the APD bias issue?

    I tried contacting the APD company (first-sensor) but hey did not respond to my request.

    Thanks for the response to the TINA query. I will rework the value of the feedback capacitor.

    Please let me know if you have any questions

    Thanks!

    Abhishek

    ad1500-9-501208_0.pdf

  • 0 in reply to Abhishek Singhal
    TI__Genius 12631 points
    Abhishek, Thanks for the APD datasheet. When I look at the dark current at 180V reverse bias, the dark current is only 1nA which results in a very small voltage drop and shouldnt cause the problem you are seeing. However, that dark current does increase exponentially past 180V which could be the cause of your issue. Unless the OPA656 is damaged it should not be drawing any dc current that could be causing additional voltage drop through the 600k resistor.