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LMV793 Vin Differential spec

Kazuya Nakai59
Guru 20140 points
Other Parts Discussed in Thread: LMV793, LMV791

 Hello guys,

 My customer is checking LMV793 datasheet to adopt this device for their products.

 Then they found one strange parameter value in the absolute maximum rating table. The datasheet says that Vin differential voltage is +/-0.3V as the absolute maximum. What does ;/-0.3V differential voltage mean? When the voltage difference between +IN and -IN terminal is more than 0.3V, is this device damaged?

 Could you please give me your advice?

 Thank you and best regards,

 Kazuya Nakai. 

  • 0
    TI__Genius 13935 points

    Hi Kazuya,

    I think you are correct. 

    The abs max ratings are to prevent damage to the part.  In normal operation there should be no differential voltage in the input pins.  So, this amplifier should not have more than 0.3V of potential between the two inputs (either positive voltage or negative).

    This part is low voltage CMOS input amplifier, so the 0.3V limit is not surprising.  The best way to protect this amplifier is to make sure that the input signal stays within the input common mode range listed in the EC tables. In addition, make sure to use a feedback resistor with values shown in the sample schematics in the datasheet. Don't try to use the part without a feedback resistor (comparator configuration). 

    Regards,

    Loren

  • 0 in reply to Loren Siebert 1
    Guru 20140 points
    Hello Loren,
    Thank you for your reply. I understood.

    Best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    Guru 20140 points
    Hello Loren,

    Please arrow me to ask you a few additional questions about LMV793.

    Q1. When customers use LMV793, are external compensation parts always needed?

    Q2. Which case doesn't need compensation parts?

    Q3. How much LMV793 gain setting is needed at least for no external compensation parts?

    Your reply would be appreciated.

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    Guru 20140 points
    Hello Loren,

    Could you please reply to the following questions I sent two days ago?

    Q1. When customers use LMV793, are external compensation parts always needed?

    Q2. Which case doesn't need compensation parts?

    Q3. How much LMV793 gain setting is needed at least for no external compensation parts?

    Your reply would be appreciated.

    Thank you anf best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    Guru 20140 points
    Hello Loren,

    I know you are busy but I need your help.

    Could you please reply to my questions?

    Thank you and best regards,
    Kazuya Nakai.
  • 0 in reply to Kazuya Nakai59
    TI__Genius 13935 points
    Hi Kazuya,

    Sorry for the delay. I don't normally support this part.

    Q1. Not necessarily. It depends on the gain and the application.

    Q2. Page 12 of the datasheet says gains of 10V/V or more are stable. So for a gain of 10V/V or more no external compensation is required.


    Regards,
    Loren
  • 0 in reply to Kazuya Nakai59
    TI__Guru 50016 points
    Hello Kazuya,

    As Lorne stated, the inputs are differentially clamped to prevent breakdown of the input stage. This is very common for precision and high voltage (>5V) parts. Under normal feedback conditions, the inputs should be nearly equal to each other due to feedback action and the clamps stay out of the circuit. Large differential voltages can occur during slewing events (where the input moves faster than the output can move), so the diodes keep the differential voltage at safe levels. Devices with input clamps should never be used as a comparator...for obvious reasons...

    As to your questions:

    Q1: External compensation is required only when the closed loop gain is less than 10 V/V. There are ways to 'burn" closed loop gain to make it stable at lower signal gains, as explained in the datasheet Applications Section.

    Q2: If the closed loop gain is greater than or equal to 10 (at all frequencies), then no external components are required. You can drop-in replace a compensated device (LMV791) and immediately gain the improved BW and slew (at the same supply current).

    Q3: The noise gain of the circuit needs to be 10 or greater. Do not place any caps in feedback path (across RF) as the cap can cause the gain to drop at higher frequencies, causing instability. If the existing circuit has a large compensation cap across the feedback resistor, it may need to be removed (why would you use a faster device and purposely slow it down??). Care must be taken with filters, since the gains change over frequency.

    Regards,
  • 0 in reply to Paul Grohe
    Guru 20140 points
    Hello Paul,

    Thank you very much for your reply.

    Could I ask you a few additional question?

    Q1. 10pF capacitance is attache when small signal and large signal transient response are taken as Figure 34 and 35 on the device datasheet. The 10pF is mandatory to keep the ringing leveI. If the input capacitance of a device connected to LMV793 output is less than 10pF, do we need to add some capacitors to make up for 10pF?

    Q2. I'm sorry I coundn't understand the mean of the sentence "There are ways to 'burn" closed loop gain ".
    Could you please tell me the mean using other word?

    Thank you again and best regards,
    Kazuya Nakai.