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TLV7011: IR Receiver analog front end

GACEM Youcef
Prodigy 50 points
Part Number: TLV7011

Can somebody explain me the mathematics involved in the IR Receiver Analog Front End? It is mentioned in the figure 42, section 8.2.2 in the TLV7011 data sheet. I’d like to understand this in order to adapt to my IR PIN photodiode and to change the baud rate.

 

  • 0
    Guru 140200 points
    Hi Gacem,

    IR LED and R1 create an AC voltage signal (Vin). R2 and C1 generate an average value from this AC voltage signal to provide a threshold value (Vref) for the comparator. This threshold voltage is applied to the +input of comparator. (Assume R3=0 and R4=infinite for understanding the comparator.) Vin is applied to the -input of comparator. When the Vin swings arround Vref, the comparator output is toggling. R3 and R4 provide a hysteresis which stabilizes the toggling and makes the circuit more immune against noise.

    To adapt the circuit to your needs, R1 must be properly chosen to get a high enough AC voltage signal. Then R2 and C1 must be properly chosen in accordance to the baud rate: Time constant must be high enough to be able to generate an avarage value without too much ripple. R3 and R4 might not need a change. The same is probably true for R2. R1 and C1 must be matched to your needs.

    Kai
  • 0 in reply to kai klaas69
    TI__Mastermind 23405 points

    Gacem

    In addition to Kai's comments, there is a Technical Note on this topic as well.  See One additional point to consider is the RC time constant created by the diode reverse bias capacitance and the value of R1.  this can also limit the response of the photodiode.  A simplified model of the photodiode can be a current source in parallel with the diode capacitance.

    Chuck

  • 0 in reply to kai klaas69
    Prodigy 50 points
    Thank you very much. I think that we can speak about AC voltage when using IR for remote control. Generally it is pulses modulated at 38 KHz. In my case it is IrDA. So I wonder if I can speak about AC voltage ?
  • 0 in reply to GACEM Youcef
    Guru 140200 points
    Hi Gacem,

    wouldn't you need a 38kHz bandpass and an automatic gain control in the receiver?

    Kai
  • 0 in reply to kai klaas69
    Prodigy 50 points

    Hi Kai Klaas
    I don't need a 38 KHz band pass and automatic gain control. I managed to implement the receiver using a voltage divider to make the Vref on the inverter input. The PIN photodiode is connected to a resistor that is tuned according to the current I measure in presence/absence of IR light. This resistor is connected to the non-inverter input. That’s why I wondered why such assembly.

  • 0 in reply to GACEM Youcef
    Guru 140200 points
    Hi Gacem,

    the only difference is that the circuit of datasheet additionally provides a hysteresis which can make the output toggling of OPAmp more stable.

    Kai