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LM2901: Hysteresis for differential sine input signal

Nam, Dino
Expert 5775 points
Part Number: LM2901
Other Parts Discussed in Thread: TLV7022, TLV7044

Dear team, 

Below schematic is optical encoder input schematic. The customer said below schematic is general concept of optical Encoder.  

 

- Input signal : differential sine wave Vp-p 50mV from photo diode module

- Output : Comparator output (0V to 5V)

The customer want to change hysteresis period as changing R7(feedback resister) value.Is it possible? 

Can you recognize hysteresis period of this concept? If it is available, please let me know the way of calculating hysteresis period.   

As changing R7 value, Can Hysteresis period change?. At this time, we can't change schematic concept. 

Please let me know your opinion. 

Thank you.  

  • 0
    TI__Mastermind 23425 points

    Dino

    If the photo diode module has a low impedance output, a resistor needs to be added between the photo diode module output and this circuit.

    Otherwise, the feedback resistor will not create hysteresis.  Another option would be to use a comparator that has internal (integrated) hysteresis.  I am not sure what the speed of the signal is, so I am not sure what devices to recommend.  For example, look at the TLV7022 which has integrated hysteresis.  This may help solve your problem as well.  Unfortunately it is only available as a dual and single.  The TLV7044 is a QUAD but it is slower and may not meet the speed requirements of the application.

    Chuck

  • 0
    Guru 140200 points

    Hi Nam,

    can you give some details on the photo diode modul? Do you have a datasheet or a link?

    Kai

  • 0 in reply to Chuck Sins
    TI__Expert 5775 points

    Hello Chuck, 

    Thank you for your reply

    If the photo diode module has a high impedance output, As changing R7 value, Hysteresis period can change. Is it right? 

    I found several document regarding comparator with hysteresis, but there is some different with this application. As mentioned before, the ref voltage is not fixed due to differential input. 

    I think that Hysteresis period(Vh-Vl) decrease as increasing R7 value, Is it right? please let me know your opinion. If there is equation, let me know.

    Thank you.  

  • 0 in reply to Nam, Dino
    Guru 140200 points

    Hi Nam,

    can you specify the output impedance of your photo diode modul?

    Kai

  • 0 in reply to kai klaas69
    TI__Expert 5775 points

    Hello Kai, 

    Photo diode module is not catalog device, custom device. so the customer said that they can't open specific data.

    So I assume the output impedance, Rx and I calculated threshold voltage Vl and Vh, As a result , Vh-Vl (Hysteresis period) has a value inversely proportional to R7 value(feedback resister). Please let me know your opinion whether this result is right or not. 

    The customer want to know whether  Vh-Vl (Hysteresis period) is proportional to or inversely proportional to R7 value(feedback resister)

    Thank you

     

  • 0 in reply to Nam, Dino
    TI__Mastermind 23425 points

    hysteresis is inversely proportional to R7; larger R7 creates smaller hysteresis, while smaller R7 creates larger hysteresis

    Chuck