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THVD1500: The function of resistive divider on Receiver

Naizeng Wang
Expert 5430 points
Part Number: THVD1500

Hi team,

How to analyze following circuit? 

（1）This allows the comparator to evaluate the differential voltage between A and B without the need for a connection between the driver ground and receiver ground.

（2）R2 and R3 attenuate the voltages appearing at the A and B terminal, to ensure that the comparator inputs are not saturated.

Why？ Could you please explain it using some formula? Thanks

  • 0
    TI__Guru 62920 points

    Hi Naizeng,

    This circuit both attenuates the input voltage and rebiases them to voltages within the range supported by the comparator. This is useful in supporting a wide input voltage range (the RS-485 standard specifies an operating common mode voltage range extending from -7 V to 12 V). To analyze the circuit, you may want to R2 and R3 to a Thevenin equivalent circuit. The attenuation factor would be R1 / (R1 + R2||R3).

    Regards,

    Max

  • 0 in reply to Miguel Robertson
    TI__Expert 5430 points

    Max, 

    I get following result using Thevenin equivalent circuit. Do you mean (VbQ1-VbQ2) / (VA-VB)=  R1 / (R1 + R2||R3)? I can't derive the result.

  • 0 in reply to Naizeng Wang
    TI__Mastermind 25705 points

    Naizeng,

    Your calculation about Vbase is correct. For small signal analysis, you can consider Vcc as 0V (DC source). Therefore Vbase is equal to VA is divided by R1 and (R2||R3). If you replace Vcc by 0V, you can get the same result from your equation. Some typical resistor values are shared in this post.

    e2e.ti.com/.../779327

    The divider ratio is about 1:10. You can run some calculations to see that it can convert -7V to 12V input into around 1.5V to 2.5V range.  

    Please let us know if you have more questions.

    Regards,

    Hao

  • 0 in reply to Hao L
    TI__Expert 5430 points

    Hao,

    According to your explanation,  it can convert -7V to 12V input into around 1.5V to 2.5V range.  So the actual divider ratio is (2.5-1.5)/(12+7)=1:19

    Is there any problem in my calculation?

    And then I calculate Vbase according to the resistance you provided, I get the same divider ratio.

     

  • 0 in reply to Naizeng Wang
    TI__Mastermind 25705 points

    Naizeng,

    You're right. The ratio in this example is about 20 (175+(16||20))/(16||20). Please let me know if you have more questions.

    Regards,

    Hao