• State Resolved
  • Locked Locked
  • Replies 3 replies
  • Subscribers 49 subscribers
  • Views 1192 views
  • Users 0 members are here
Support feedback
Options
Options
Related

This thread has been locked.

If you have a related question, please click the "Ask a related question" button in the top right corner. The newly created question will be automatically linked to this question.

LMV7219: Photodetector using a fast comparator.

Norm Bain
Prodigy 50 points

Part Number: LMV7219
Other Parts Discussed in Thread: TLV3501, , LMH6629

I am designing a pair of fast photodetectors that will direct the output of a 24MHz oscillator to a counter in HCMOS. Linearity is not at all necessary, just a binary output.

The datasheet for the LMV7219 comparator shows a photodetector circuit that is very appealing for its simplicity. I cannot quite believe that the task can be done with so few components. I understand the need to tweak the resistor values to suit the photodiode and ambient light, but is there any more to a practical circuit than that? Can a TLV3501 be used instead of the LMV7219?

  • 0
    Guru 140200 points
    Hi Norm,

    of course, this neat circuit isn't able to handle widely varying ambient light or IR signal levels. But if the amount of ambient light is stable and the IR signal amplitude is almost constant, this circuit can work!

    Kai
  • 0
    TI__Intellectual 1740 points
    Hi Norm,

    The comparator spec to keep in mind is the maximum toggle frequency. This can be calculated by adding rise time, fall time, propagation delay from high to low, and propagation delay from low to high and taking the reciprocal of the result. The maximum toggle frequency represents the maximum frequency the outputs can toggle between VOH and VOL and therefore limit the maximum frequency of the input signals applied to the comparator. With a 24Mhz frequency both the LMV7219 and TLV3501 should be fine.

    Regards,
    Jaskaran
  • 0 in reply to Jaskaran Atwal
    Prodigy 50 points
    Thank you Kai and Jaskaran. The ambient light level will vary only a small amount and the IR (from a collimated laser diode, extending into the visible region) will be steady as a rock until the total occlusions that I want to detect. So I now have enough positive expectation to make up a pair and see how they go. I can build a more complicated transimpedance amplifier (using LMH6629 fast op-amps) if the comparator circuit is inadequate.