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converting 40MHz sine wave to square wave using a comparator

Sailesh Vempaty
Prodigy 165 points
Other Parts Discussed in Thread: TL712, TLV3501

I am looking for a high speed (40MHz or greater switching), high current output (need to supply ~65mA drive current to a 50 ohm input impedance laser diode load) comparator IC that will convert a 5Vpp sinewave (0 to 5V) into a 3.3Vpp square wave (0 to 3.3V) at 40MHz. The 40MHz, 5Vpp sinewave is being generated using a function generator.

Can someone suggest a suitable comparator chip and a possible circuit layout to achieve this? The comparator needs to be a DIP package and not a surface mount device.

  • 0
    TI__Guru* 93105 points

    Hi Sailesh,

    We don't have a comparator in our Precision Analog product line-up that meets all of your requirements. Product availability in DIP packages is on the decline because its use and demand continues to drop. And the high output current expected is higher than most of the comparators can sink and source. Those requirements limit what can be suggested.

    Although we don't have an exact product fit there are a couple of comparators that you may want to review and consider. As an example the TL712 high-speed comparator is available in a 8-pin DIP package. It might be just fast enough for 50 MHz use, but note its "typical" maximum operating frequency is 50 MHz, so it could be lower. You can view the TL712 datasheet here:

    An faster comparator is the TLV3501. It should be able to keep up with a 50 MHz input. There is even a datasheet graph, Fig. 10, showing its output driving a 50 Ohm load at 100 MHz. The issue would be that it is only available in surface mount packages. TLV3501 datasheet Page 16 provides PC board layout examples for the two different surface mount packages that it comes in.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 165 points

    Thank you for your response Thomas. I am now considering a new approach. I was thinking I may generate my 40MHz or greater 3.3Vpp square wave by using an astable multivibrator similar to the one I have attached. I would need to choose appropriate circuit components based on the given formula to obtain 40MHz. I would also need to find a high speed (bandwidth atleast 500MHz), high current output (>65mA) op-amp which was capable of operating on a single supply of (V- = GND, V+ = +3.3V).

    Do you think this approach would work better? Also, are you able to suggest any such op-amp in a DIP package that can meet my needs?


  • 0 in reply to Sailesh Vempaty
    TI__Guru* 93105 points

    Hi Sailesh,

    It does seem that you should be able to use a digital approach to generate a 40 MHz square wave; be it a fast multivibrator, or crystal based oscillator. Actually, the crystal based oscillator would be more precise and all of the guesswork is removed. They are available with sine, or digital output signals.

    I looked online to see what the availability of 40 MHz crystal oscillators having a digital output is from a distributor such as DigiKey. Here's the page I found online:

    www.digikey.com/.../852334

    A limitation for either approach is they don't provide the high output current level you need for your application. We don't have any op amps in our Precision Analog product line that provide the speed and current needed. However, TI's High Speed Amplifiers product line likely does. I am not too familiar with what they have to offer, but one of their Applications Engineers should surely be able to assist you in selecting the best amplifier. They have an E2E forum where you can ask pose your question:

    I hope this helps.

    Regards, Thomas

    PA - Linear Applications Engineering

  • 0 in reply to Thomas Kuehl
    Prodigy 165 points
    Thanks for that Thomas. I will take a look through the oscillator range.