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LTC6560 with positive biased APD

Hi together,

at the moment were busy designing a OTDR application. Because of the high trans impedance gain we thought  of LTC6560 which is suitable for LIDAR application.

Datasheet on page 14 states:

So we decided to use a positive biased ADP which anode is connected in direction of TIA input in combination with a capacitor. But up to now there is no chance to get it work. It doesn't matter if we use ac decoupling capacitor or not the resulst stays the same.

When using LTSPICE I recognized the same effect. When changing input current into positive dircetion there is no chance to get the output of LTC6560 below 0,9V.

This leads to the follwing questions:

- Did misunderstand the information in the datasheet quoted above?

- Is there a contradiction within datasheet because later on datasheet explains something about limited ability of sinkung current or is 0,9V the lowest possible voltage at the output?

Is there any hint  how to get setup work without changing APD power supply to negative?

Thanks in advance


  • Hello Christian,

    LTC6560 should work with a positively biased APD using AC coupling on the input as the datasheet states.  Datasheet Figure 3 schematic shows it AC coupled on the input and with AC coupling the APD bias polarity does not matter.

    Are you DC coupling the output? If so, then there should be no limitation due to the output mostly only sourcing current. The comment in the datasheet about output sinking current capability only applies if you're AC coupling the output.

    It'd be good if you share the schematic of the circuit you're having issues with.

    Hope this helps. Let me know if you have any questions.



  • Hello again Christian,

    Keep in mind that with a positively biased APD, the current pulses will try to move the output more negative (towards ground). So, the dark / non-illuminated state needs to be shifted positive (towards Vcc) to allow for that.

    I tried to attach a LTspice simulation file to show you how to do this but EZ is not allowing me to do so.

    To push the "dark" state output towards Vcc for the conditions that you have, you can add a 55k resistor from the input to ground with your APD (positively biased) AC coupled through a 100pF cap (or similar). This way, with no input current, output will be around 3V (RL = 100ohm to ground) and 30uA input current pulses bring the output down to about 1V.

    Hope this helps.



  • Hello Christian,

    The issue with Engineer Zone not being to post images is fixed as of today.

    Here is the schematic of what I tried to describe earlier where I added R3 (55k) to shift the output positive when "dark":

    Here is the LTspice simulation file:

    LTC6560 Positive Bias APD AC Coupled EZ 10_9_20.asc



  • Hello Hooman,

    thank you for your detailed answer. Sorry for the delay of my response but I was on holiday for a week.

    See below an excerpt of our evalution schematic:

    At the moment we use pin Out without internal serial termination. The outbut is DC- coupled to a differential OPamp like ADA4932 or THS4541 form TI.

    I think the major difference is, that we haven't installed the 55k resistance at the input. I tried to vary this in  the LT- Spice simulation an could see big difference it what is the output signal looks like.

    Is there any detailed recommendation concerning the value because I could not find find information about that in the datasheet easily?

    Thanks in advance


  • Hi Christian,

    To be clear: The LTC6560 can be used with a positive biased APD as described below.  However for best performance we recommend biasing the APD with a negative voltage.

    Regarding the shunt resistor (55k) recommendation: This value was chosen based on the typical input bias voltage (1.55V) and the gain of LTC6560 (74kohm) in order to create a positive output shift [(1.55V/55k) * 74kohm = 2.1V]. In LTspice this measures close 3V. You are right that this approach is not shown in the current datasheet as most customers have a negatively biased APD and this would not be needed for them.

    In order to make this output shift more precise and not subject to variations in input bias voltage (part-to-part), you could take advantage of a servo scheme similar to what is shown here to set the output near 3V. The loop is purposely made to be “slow” so that it sets the average output, corresponding to a “dark” APD, to 3V. For this to work, your photo pulses must have a low duty cycle so that the average voltage is maintained.

    Here is the simulation file for reference:

    LTC6560 Positive Bias APD AC Coupled Servo loop EZ 10_19_20.asc