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About TL712

bingtong
Prodigy 105 points
Other Parts Discussed in Thread: TL712

Hello, sir/madame
I am sorry to interrupt you.
I have some doubts about TL712.I want to use TL712 to generate TTL LEVE.Now,I can not understand some instructions of the datasheet.

Some questions about TL712:
Q1: Is label 1 connected to label 2 ?
Q2: the resistance in label 3 is R-load?
What is the value of the resistance?
Q3: What is the use of diodes in label 4 and label 5 ?

  • 0
    TI__Genius 12631 points

    Hello,

    Q1 - Yes label 2 is connected to label 1. Note that the load circuit is used for testing the switching characteristics of the comparator. (See page 3 "Switching Characteristics" on the datasheet).

    Q2 - Yes that is the resistive load. I was unable to find any information on what the actual load is, however I expect it to be > 1kOhm.

    Q3 - The diodes in the circuit perform a clamping function. It is difficult to perform high-speed transition measurements when the output is swinging several volts. To enable easier measurements a clamping circuit such as the one shown is used. When the output is HIGH (say 5V), then diode 4 is reverse biased and the final output (between 2kOhm and label 5) is clamped at 2.1V (3 diode drops). When the output transitions from HIGH to LOW (5V -> 0V, for example), diode 4 will forward bias. The final output will move from 2.1V to 0.7V. Diode 5 will be OFF under this condition. In this manner we only have to measure a 2.1V -> 0.7V transition (1.4V delta), vs a full 5V transition.

    -Samir

  • 0 in reply to Samir Cherian
    Prodigy 105 points

    Hello,

    This is my application circuit,but my out waveform is wrong.I don't know what is the problem.

    And this is my output waveform,

  • 0 in reply to bingtong
    TI__Genius 12631 points
    Hello,
    Usually the threshold voltage is set somewhere close to mid-supply (2.5V in your case). You are currently connecting the threshold voltage to 0V (through pin 2, Vin-).

    Is your sine-wave going between +/-250mV? If your sine wave is going between +/-250mV and Vin- is set to GND, then you should see a rectified waveform. Is the output the green signal or the yellow signal? Also, can you switch your scope to 1MOhm input impedance and measure?

    -Samir
  • 0 in reply to bingtong
    TI__Guru**** 152966 points
    Bington,

    The TL712 expects a TLL load. 2k to ground is not a good match. 2k to 5V would be OK.
    I see CH1 using a 50 ohm scope setting, Do you have an active (powered) probe connected?

    For your original questions.
    A1: Label 1 can connect to an external circuit that is labeled 2
    A2: That resistor seems out of place, at least 10k if you use it. It should be like Equivalent enable input schematic (middle picture).
    A3 those diode approximate the transistors in a TLL input stage.