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OPT101: Driving opt101 using sine wave of kilo hertz

Uttam Pal
Prodigy 80 points
Part Number: OPT101


Dear Sir/Madam,

We are trying to drive the OPT 101 with sine wave in order of kilohertz. As per the datasheet, the -3dB cutoff of OPT 101 is 14 kHz, however, even at the operating frequency of 10 [Hz], we are unable to observe the sine waveform. The observed waveform is a tapered square wave.

As per the datasheet we have tried bot the circuits mentioned in section 8.2 and Fig. 18, however, we fail to observe a sine wave.

Can you please guide us in choosing the correct circuit in order fo detect a sine wave in order of tens of kilohertz?

Regards,

Uttam Pal

  • 0
    TI__Mastermind 25895 points

    Hi Uttam,

    What do you mean by driving with a sine wave? The light source is driven with a sine wave?

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 80 points

    Dear Alex,

    Thanks for your reply. We are driving the LED which is frequeuency modulated from 1 Hz  to 10Hz, with a DC Bias. We are able to see visually the visible light is very smoothly increasing and decreasing.

    However once we observe output of OPT 101, it is more or less square wave. Have some one tried to do this kind of application?

    Regards,

    Uttam

  • 0 in reply to Uttam Pal
    TI__Mastermind 25895 points

    Hi Uttam,

    Please make sure that LED is driven by a smooth sine wave current source rather than PWM type signal as PWM could make LED look to be changing smoothly to the eye but actually be strobing.

    Please also check that given your circuit configuration for the device that the bandwidth is not lower than you require. Also please check that no clipping is happening and you power supplies are the correct level for the expected voltage output to be smaller than this.

    Best,

    Alex

  • 0 in reply to Alex Bhandari-Young
    Prodigy 80 points

    Dear Alex, 

    Thanks indeed for your reply. We are driving the LED using the Function generator. I will check with a voltage sine wave of 1[Hz], do we get a constant current source. 

    Else, we will build the sine wave current source circuit and revert back to you. Please keep this case open.  

    Regards,

    Uttam Pal

  • 0 in reply to Uttam Pal
    Prodigy 80 points

    Dear Alex, 

    Yes, we built a current sine wave generator using the circuit in https://www.electrical4u.com/voltage-to-current-converter/ (i.e., using Floating Load Voltage to Current Converter circuit). Now we are able to detect reasonably good the sine wave till about 5 [kHz]. Following are the observations. 

    When we drive the source LED with 10[Hz] and place the LED too close (<1[cm]) to the OPT 101, we are getting the square wave in output. This may be because the OPT 101 is getting saturated with the intensity of the LED. However, if we place the LED somewhat far away (> 2[cm]) we are able to get the required sine wave. This observation is true for frequencies from 1[Hz] to 5[kHz], after which the amplitude started to reduce. 

    Thanks again for your help Alex, appreciate it. We can close this case. 

    Regards/Uttam