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Delay circuit using Ti IC LMH6628

Abhay Negi
Prodigy 40 points
Other Parts Discussed in Thread: LMH6628, LM7121, LMH6601

Hi,

I'm trying to use delay circuit as given in datasheet of LMH6628 (shown below) . I'm using a input pulse of 5V amplitude. R and C are 41.2 ohm and 33 pF respectively. + Vcc is 9V and -Vcc is 0V for both LM7121 and LMH6628. The output of LM7121 is fine. However, the amplitude of output of LMH6628 at pin 3 and pin 7 is around 200-250 mV.  The output of delay circuit is connected to a inverter which in turn is driving an optocoupler. Could anyone advise what  might be wrong?

Thanks

  • 0
    TI__Mastermind 22825 points

    Hello,
    I think you need to use dual supplies (Vs= +/-4.5V) in order to not violate the input CM voltage range. For the LMH6628, the input CM range (CMIR) is 1.3V above V-, and 1.3V below V+ (or +/-3.7V when operated from Vs= +/-5V).
    Alternative is to "shift" your signal to within the CMIR of each device.

    Regards,
    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 40 points

    Hi Hooman,

    Thanks for your inputs. 

    However, on page 1 of the datsheet it is stated that "  The LMH6628's low noise and very low harmonic distortion combine to form a wide dynamic range op amp that operates from a single (5V to 12V) or dual (±5V) power supply. "  

    And as per page 2, the common mode range should also be fine.

    So using  " + Vcc as 9V and -Vcc as 0V " should be fine for a pulse input of 5V.

    Thanks

    Abhay

  • 0 in reply to Abhay Negi
    TI__Mastermind 22825 points

    Hi Abhay,

    I understand where you might have gotten the impression about the LMH6628 input CM voltage range includes the V- rail. I think the page 1 sentence that you've quoted is a "stretch" . I believe it implies that both single and dual supplies "can be accommodated", since this is a common question with many customers. Some devices, have issues with allowing both power supply conditions due to the specifics of their design. An example being a device like the LMH6601 which would need special care when going from dual supplies to single supply (namely because the Shutdown pin level needs to be shifted). However, I agree with you that the conventional / historical wording for "single supply" is when the input CM voltage range includes the V- rail!

    The following LMH6628 Electrical Characteristic table entry shows what I had noted about CMIR being a voltage range within the V- and V+ potentials and which does not extend to the V- rail (or the V+ rail):

    Hopefully you can find a way to use dual supplies or otherwise shift the signal to circumvent this issue.

    Regards,

    Hooman

     

  • 0 in reply to Hooman Hashemi
    TI__Mastermind 22825 points

    Hi Abhay,

    I forgot to mention.

    The Absolute Maximum Ratings table you have shown should not be used to make a determination about operation of the part and the voltage ranges! The entries in that table "indicate limits beyond which damage to the device may occur" as Note 1 reads.

    Regards,

    Hooman