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Original question:

LMH32401: LMH32401 APD bias voltage

LMH32401: with common cathode mode pin diode

Ugur KILIC
Prodigy 50 points
Part Number: LMH32401
Other Parts Discussed in Thread: TINA-TI

Hi Team,

First of all, sorry about my poor English skills and thank you for all of your life and time saving works. TI is the best. :)

I am trying to design a TIA circuit for a given common cathode PIN diode array which has 10 pin diodes (I use BPX61 for testing because its properties are nearly the same with the pin diodes in the array.) and wanted to use LMH32401. I ordered the op-amp and design a pcb to test it but while I'm testing I encountered unstable-ish states appears that you can see last part of the video. Unfortunately, I didn't realize that I didn't check the sinking/sourcing properties before I started to testing the circuit.

You can see the schematic of the circuit. I can send the pcb design if needed.

Internal light cancelation is enabled (It it set to disable at the schematic but I changed on the pcb to see the differance). I am using 20k gain. If I calculated correctly; pin diode is sourcing around 10uA.

After I encountered this problem I dig into the datasheet and forum.

As I can understand that LMH32401 didn't intented to work with pin diodes with the sourcing configuration which explained in the section 7.3.2 of LMH32401 datasheet.

Is it true? If not, what kind of modifications I should make to correct that problem?

Best regards.

  • 0
    TI__Mastermind 28865 points

    Hello Ugur,

      Thank you for providing these details on your application! The LMH32401 can work with both configurations: cathode is tied to the amplifier's input (photodiode sinking current) or when anode is tied to the amplifier's input (photodiode sourcing current). But, you are correct, it is recommended in the sinking current configuration. In the configuration you have currently (sourcing current), you will run into slower recovery, and the ambient light cancellation functionality will not work. 

      As a note: the BPX61 has a capacitance of 10pF above 30V bias, which will correspond to the system bandwidth shown below: (was making sure the input capacitance wasn't too high for this device)

       I would try setting the Vocm to midsupply, you might be running into output voltage range violations. And, if possible, please do send the PCB layout over private message on E2E. 

    Thank you,

    Sima 

  • 0
    Guru 140200 points

    Hi Ugur,

    I would run a TINA-TI sumlation:

    LMH32401 TINA-TI Model (Rev. C)

    How have you wired VOCM and VOD?

    Take care, directly probing the output with the 50R scope input might overload the LMH32401, especially if the output is emitting a DC potential differing from 0V. I would add an isolation (or termination) resistor and probe with the 10M scope input. You could also think about AC coupling the output. And see figure 56 of datasheet.

    Kai

  • 0
    Prodigy 50 points

    Hi

    I checked the bandwidth as you suggest: My bandwidth is around 13MHz. So I believe bandwidth isn't the problem.

    As I mentioned in my private message I was using a series 100nF capacitor the block the DC at the output. You can see the both outputs without output capacitor. And I added a 100ohms as termination between outputs and 25ohms serie resistor as isolation after the Kai's suggestions. You can see the results in attached video below. There is a little improvement but i think it is not enough.

    Hi ,

    I run the simulation and it nearly fits with my experimental results. I added the serie output resistor that i mentioned above and put 100ohms for termination between the outputs. I took measurements with 10M probe and you can see the result in the video below.

    I believe you refer to pcb when you said "...VOCM and VOD..." sent pcb design files to both of you.

    I am waiting your comments.

    Thanks.

  • 0 in reply to Ugur KILIC
    TI__Mastermind 28865 points

    Hello Ugur, 

      Thanks for sharing the scope shots! Correct, bandwidth wouldn't be the issue, but you might still be violating the output range of the amplifier:

      Are you negatively offsetting the scope on the first shot? 

    Thank you,
    Sima 

  • 0 in reply to Ugur KILIC
    TI__Mastermind 28865 points

    Hello Ugur,

      Were you able to fix these issues in the new board revision?

    Thank you,
    Sima