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TLV3601: TLV3601 rise time

Part Number: TLV3601
Other Parts Discussed in Thread: BUF634A

Hi TI teams,

I use a 555 timer to generate a clock of 1kHz, the amplitude is 4.5V. 

The clock signal has 250ns of rise time.

And I feed this clock into TLV3601 comparator. The -VIN set to 2.5V or 1V.

I measured the output signal of TLV3601, the rise time is 2.5ns.

But according to datasheet AC Characteristics, the rise time should be 0.75ns.

How could I achieve 0.75ns of rise time output by  TLV3601?

Thank you.

SK.

  • additional info the TLV3601 applied Vcc = 5V, Vee = 0V.

  • Hi Sk,

    in this frequency range parasitic impedances of board and probe can play a major role having the potential of fully ruining your measurement. Your circuit must be designed like a HF circuit with a solid ground plane and proper supply voltage decoupling. Then the test conditions given in section 6.5 of datasheet (also see at the top of the tables there) must exactly be fullfilled.

    Then there are the restrictions of your scope probe and oscilloscope. In any case use such a scope probe with a ground spring as shown below:

    With a pig tail ground connection showing lots of unwanted inductance instead, you will merely see the limited rise time ruined by the unwanted scope prope resonance.

    Then, is your scope fast enough, at all? Touch with the scope probe a properly bypassed (!) DC voltage while having the scope set to DC level triggering. By touching the properly bypassed DC voltage you are able to introduce an instantaneous voltage step. What you see now on the scope screen is the intrinsic rise time of your scope.

    Kai

  • Thanks Kai, always great support.

    SK

    Just to follow up on Kai's response, devices with edges as fast as TLV3601, board layout and probing technique becomes critical.  If you download our EVM user's guide, you will see our board layout where we try to minimize stray capacitance on the output trace.  If you wish, you can upload a snapshot of the schematic surrounding the comparator and we can review.  Mainly I would be interested in what is connected to the comparator output.  I was actually just looking at this device on our EVM yesterday using the same technique that Kai described above and was seeing trise and tfall very close to the datasheet typical specs.

    Chuck

  • Thanks, Chuck Relaxed

  • Hi Chuck and Kai,

    Thanks for your great support.

    English is not my native language, I will try to not to make misunderstanding in English.

    I have considered if my experiment tool is suitable for my requirement. Now I learned that is pretty critical.

    Below is the measurement result.

    I used a oscilloscope, Agilent MSO7054A, which is only bandwidth of 500MHz.

    The measurement may not precise for signal < 2ns. 

    And also attached the schematic in my application as below:

    The circuit is to generate a narrow pulse for radar platform. You will see a pair of diode which is step recovery diode MMD835 manufactured by MACOM.

    Additional question: Can I use a RF cable to feed to output signal of TLV3601 to a high speed oscilloscope in stead of using a high speed FET probe?

    I will follow your suggestions to do more experiment.

    Thanks a lot.

    SK.

  • Hi Sk,

    I think that your input signal is too slow. The edge of input signal is not steep enough. What do you see when you give an instantaneous voltage step directly to the input of TLV3601?

    To speed up the edge at the output of TLV3601 you could try to add some hysteresis. See section 8.2.1 of datasheet.

    Additional question: Can I use a RF cable to feed to output signal of TLV3601 to a high speed oscilloscope in stead of using a high speed FET probe?

    This would mean that the TLV3601 has to drive a 50R input? Even with a 50R series termination resistor at the output of TLV3601 the comparator would have to drive a current of 5V / 100R=50mA which is way too much. Keep in mind that the maximum output current the TLV3601 can deliver is the short circuit current and the short circuit current of TLV3601 is only 30mA.

    Kai

  • Hello Kai,

    Thank you for highlighting the short circuit current of the TLV3601 and providing your suggestions.

    Sk,

    Please consider implementing a faster edge on your input signal to improve the response time of the TLV3601. External hysteresis is another option as Kai pointed out.

    Best Regards,

    Joe

    Applications Engineer

    Linear Amplifiers Business Unit | Comparators Product Line (CMPS)

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml)

  • Hi Kai,

    Thanks for the hint.

    But I don't understand why using external hysteresis can improve the edge of input signal?

    below is the measured on TLV3601 output. The  oscilloscope  is set 50ohm input impedance.

    I can see the rise edge is 300ps. But I didn't use hysteresis circuit.

    Is it correct?

    Besides, I used a high speed buffer BUF634A at the output of TLV3601.

    SK.

  • Hi Sk,

    so the FET-probe was the wrongdoer "increasing" the rise time?

    But I don't understand why using external hysteresis can improve the edge of input signal?

    Adding hysteresis can help when the input signal is too slow. Then the positive feedback of hysteresis makes the edge of the input signal at the +input steeper. The signal goes faster through the threshold region and the output of comparator can also toggle faster. At the same time unwanted oscillation is prevented.

    Kai

  • SK,

    Kai is definitely correct when he said: 

    Adding hysteresis can help when the input signal is too slow. Then the positive feedback of hysteresis makes the edge of the input signal at the +input steeper. The signal goes faster through the threshold region and the output of comparator can also toggle faster. At the same time unwanted oscillation is prevented.

    Best Regards,

    Joe

    Applications Engineer

    Linear Amplifiers Business Unit | Comparators Product Line (CMPS)

    All information in this correspondence and in any related correspondence is provided “as is” and “with all faults”, and is subject to TI’s Important Notice (http://www.ti.com/corp/docs/legal/important-notice.shtml)