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Unknown second peak on the output signal

Karsten Pflug
Prodigy 70 points
Part Number: TLV1704

Hello everyone,

i have a problem with the TLV1704. I use it as window comparator and to generator a TTL-similar signal.

Her you can see my simplified circuit:

And i made some screenshots with the scope at different supply voltages.

Purple is the Input, Yellow is the line between the comparators and green is my unknown signal output (OUT6).

Question: Do anybody has an idea, how i can get rid of this second peak?

1.) 30 V

2.) 20V

3.) 17V

  • 0
    Prodigy 70 points

    2. Try with the pictures:

  • 0 in reply to Karsten Pflug
    Guru 243820 points

    I'd guess the problem is that the second comparator (U2) does not have any positive feedback. Try monitoring the supply voltage.

    Anyway, this circuit is not a window comparator. What is the actual problem you're trying to solve?

  • 0 in reply to Clemens Ladisch
    Prodigy 70 points
    Sorry i meant U1 is a comparator with hystereses (like www.ermicro.com/.../comp_12.jpg) and this part is working quite well. (yellow signal)
    Only the U2 is creating a weird output signal. (green) This supply and reference voltage are stable.
  • 0 in reply to Karsten Pflug
    Guru 140200 points

    Hi Karsten,

    your input signal is always smaller than the supply voltage?

    What is OP1? Why no direct connection between the voltage divider and -input of U2?

    Is this a breadboard circuit? Have you proper signal ground connections and supply voltage filtering?

    Why not providing a hysteresis at U2?

    Kai

  • 0 in reply to kai klaas69
    Prodigy 70 points
    Hey Kai,

    1.) yes my input signal is always smaller Vcc (10-30V), but is placed symmetrically around Vcc/2 (5-15V).

    2.) OP1 is a Buffer to generate Vcc/2. This source is also used for other parts of system which need low input resistance.

    3.) Breadboard circuit: yes and no, i have the problems on my final design and on the bread board. My Ground Connections are good and the supply voltages are well filtered.

    4.) Because sometimes my input voltage is only 10Vpp around the 15V DC, then i would get no output signal, if i would work with the 5V line.
    And I can't work on the 5V line because if have only a few mA from the 5v-source and the main part of the energy is used by my sensor chip.

    I don't have much space and energy left in my sensor.

    Karsten
  • 0 in reply to Karsten Pflug
    Guru 140200 points
    Hi Karsten,

    hmm, if the supply voltage is dynamically changing, then there's a certain chance that the input voltage of U6 exceeds the supply voltage. I would introduce a current limiting resistor to the -input of U6. Take a 4k7 resistor. Do the same with the +input of U2.

    Again, what is OP1? How is it powered?

    Kai
  • 0 in reply to Karsten Pflug
    TI__Mastermind 49946 points

    Hi Karsten,

    How "simplified" is your schematic?

    The rise times seem very slow - are there more circuits you are not showing? Can you share the full schematic?

    Is your pull-up resistor really 270k? The rule-of-thumb is that the pull-up resistor should be at least 10x less than the hysteresis feedback resistor (15k), otherwise the feedback resistors and the pull-up resistor form a voltage divider.

  • 0 in reply to Paul Grohe
    Prodigy 70 points
    Hi Kai and Paul,

    @ Kai: OP1 is powered like U2 and U4 with 10 to 30V and is used to create half of Vcc as virtual ground. I can't change my values of the pull-up because this change influences my threshold values.

    @Paul: The only simplification is that i left away the parts before and after this part of the schematics.I can reproduce this "error" in a stand-alone breadboard system. Yes i'm curious about this very slow rise time, so i reduced the resistor values and the rise time is better than.
    As i mentioned for Kai: I can't change my values of the pull-up because this change influences my threshold values. I'm using the voltage divider to generate the threshold values i want.

    Regards,
    Karsten Pflug
  • 0 in reply to Karsten Pflug
    Guru 140200 points
    Hi Karsten,

    does a current limiting resistor at the -input of U6 and at the +input of U2 help? I recommended about 4k7.

    Kai
  • 0 in reply to kai klaas69
    Prodigy 70 points
    Hi Kai,

    i tried to a high 100kOhm resistor at the input+ of the U2. And this worked quite well. i don't a resistor at the input of u6 because there the signal is correct.

    I'll implement this in my layout. The curious thing is, that this error isn't shown at every assemble PCB. Only some show this effect. :-/
    I'll ignore that fact and hope it is working stable with the addition resistor.

    Thank you for your help!

    Regards,
    Karsten