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OPT101: Shall I measure 1MOhm resistance between pins 4 and 2 on bare device ?

Alex Zabrovsky
Prodigy 170 points
Part Number: OPT101

Hi

Have few sensors, before soldring to the board, I guess if I'd measure the resistance between pins 4 and 2 by DVM I should see around 1MOhm of internal feedback

Am I wrong ?

I have several units at hand, one of them showing floating resistance of several hundred kOhm between pin 2 and 4, the other one shows nothing (like disconnection)

Is this normal ?

The units are bare, new, not soldered into PCB

Thanks

Alex

  • 0
    TI__Mastermind 25815 points

    Hi Alex,

    I will need to check on this. I don't think we list pin to pin resistance in the datasheet and I'm not sure if this is spec'd. Is there a reason you are looking into this? Did you see any unexpected behavior?

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 170 points

    Thank you Alex

    I obtained several of these devices and the first one I tried to operate did nothing. The circuit is very simple one - follows the basic configuration of internal 1MOhm feedback, so no issues where expected.

    So I suspected faulty device. Since there should be 1MOhm internal feedback resistor between pin 2 and 4, I thought measuring by DVM there should produce approx 1MOhm resistance between these pins

    Well, the issue is resolved now, this particular device was faulty indeed - there was no internal bonding between silicon to the pins. Inspected the internals through microscope and compared with functional devices, this one exhibited no internal gold wiring of bonding.

    I have another question now

    I’m willing to use the device with very wide light power range, ranging from 2-3mW to probably a few tens of nW on the active area

    I thought to switch the external feedback resistor to 3-4 value ranges from 2-3MOhm to few kOhms

    The data sheet refers mostly to higher value gains, the lowest one mentioned in table 2 is 50kOhm.

    My question is whether there are any limitation to bear in mind if using as low gain resistor as 1-5kOhm ? What value of parallel capacitance is recommended for this ?

    I have no particular interest in wide bandwidth, however would appreciate stable performance in relatively dark environment

    Please advise

    Regards, Alex

  • 0 in reply to Nir Flysher
    TI__Mastermind 25815 points

    Hi Alex,

    Thanks for the patience with my reply here. There is not much I can add in terms of using the lower feedback resistance than the values mentioned in the datasheet as I have not used the device in this type of configuration. Best I would be able to suggest is to see how performance is using the smaller resistance value and we can go from there if there are issues seen.

    Best,

    Alex 

  • 0 in reply to Alex Bhandari-Young
    Prodigy 170 points

    Thank you Alex, I understand

    I changed the gain to 10k resistor and scaled the feeback cap accordingly following the table in the data sheet.

    The feedback cap value for 100k is recommended to be 33pF, so to preserve tau 330pF should fit. However I want ahead and used 1nF instead

    Now, what I see is that the stage is constantly not stable, oscillating at about 240kHz.

    This is weird because larger feedback cap values should provide even greater stabilization margin...(I do not have any particular requirement for bandwidth)

    Is there any reason too much feedback cap value per given gain may cause instability ?

    Thanks

  • 0 in reply to Alex Zabrovsky
    TI__Mastermind 25815 points

    Hi Alex,

    Thanks for testing. With smaller feedback cap values do you see the same instability? Using 10kohm feedback do you see this issue? Also do you have anything connected at the output that is being driven?

    Sorry I cannot provide too much more input for this type of use case.

    Best,

    Alex