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LM7171QML-SP: Questiion regarding the Diff Input Voltage and Common mode input voltage ranges w/r to supply rails

Richard Lajoie
Prodigy 10 points
Part Number: LM7171QML-SP
Other Parts Discussed in Thread: LM7171, OPA690

I have a technical questions regarding your LM7171QML-SP (5962F9553602VXA). We have 25 of these parts on our board.

 

This is more precisely regarding the Differential input voltage as well as the Common Mode Input Voltage, both with respect to the supply voltages.

 

In one of our more problematic case, we have Power Suply Rails of +30Volts on (pin7: V+) and -5Volts on (pin4: V-).

 

When we read the datasheet, we may conclude that the diff. input voltage is limited to 5volts below rails (ie: +/- 10V when supplied at +/- 15V). But since there is only one specification at +/-15V, it is not clear whether or not this is proportional to the supply rails…

 

So it would be very appreciated, if you could help us in this particular application, with +30 and -5Volts of supply rails. Can we expect to have a maximum of +25V/0V (25V) for the differential input voltage?

 

Also, for the common mode input voltage, could we expect to have a limitation of around 2Volts, from each supplies, which would mean around +28V/-3V for the common mode range with +30V and -5Volts of supply rails?

 

I am looking forward to read you.

 

Best Regards. Richard. richard.a.lajoie@ca.abb.com

  • 0
    TI__Genius 15555 points

    Hello Richard,

    The differential voltage is irrespective of the supplies. This spec specifies the max differential voltage between the inverting and non-inverting pin before the device is damaged, in your case this can be +25V and +15V applied to each input. There shouldn't be a differential input voltage across the device under regular operation.

    In terms of the common-mode input voltage your assumption is correct. You can expect the range to be around 2V from the supplies.

    Best,

    Hasan Babiker

  • 0
    Guru 48395 points

    Hey Richard, 

    Having done well over 100 high speed amplifier datasheets, I find myself uncertain that you want to depend on that 10V max diff that shows up in the -SP datasheet. 

    Something that high is very unusual and might be a misunderstanding in the early TI HSP days. 

    So this is a unity gain stable VFA using the high transconductance input stage - two buffers driving an R element - you can tell that by the slew rate and high voltage noise. So that diagram on page 2 of the -SP datasheet the "A" buffer is the same as the 4 transistor buffer on the IN+ side. What is good I think, is those buffers will not show a VEBO breakdown which causes a permanent degradation in noise. 

    Normal operation will not build up higher V across the inputs if the loop is closed. 

    IF we force a high delta V with power one, the two input buffers will just try to follow those seperate inputs putting a very high current into the RE element on page 2 I think. Not sure why you need this spec, but what is missing is that RE value which then gets mirrored to the high gain current mirrors increasing that total power supply current - could get pretty large - which is what I think might be sitting behind the 10V spec. 

    If you really want to know, I would suggest you get a commercial grade version and test it looking at the total supply current. These parts also get a big increase in supply current as you push into any higher LSBW type condition.

  • 0
    Guru 140200 points

    Hi Richard,

    why do you need such a huge differential voltage between the inputs? Do you want to use the LM7171 as a comparator?

    Kai

  • 0 in reply to Michael Steffes
    Guru 48395 points

    For instance Richard, 

    The OPA690 is the same kind of topology, where I put 1.2V max diff in that abs max table,