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TLV9301: Input common mode voltage range of TLV930x

tobias kosub
Prodigy 150 points
Part Number: TLV9301
Other Parts Discussed in Thread: OPA1678,

The part is *not* listed as a rail-to-rail input and the stated input common mode voltage range is listed as extending to (V+) - 2V in section 6.7. Electrical Specs.

However, the block diagram shows a complementary input stage. Also the plots of offset and input bias current vs common mode voltage extend all the way to the positive supply. So judging on the available info in the datasheet, it pretty much is a rail-to-rail input. The spice model, too, seems to work up until the positive supply on the input.

What is wrong then ? The input common mode voltage range under 6.7. or all of the subsequent plots ?

Thank you very much.

  • 0
    Guru 243600 points

    The complementary input stage just prevents phase reversal. When you go above the specified common-mode range, the electrical characteristics are no longer guaranteed, and in fact become much worse.

  • 0
    Prodigy 150 points

    Dear Clemens,

    obviously the offset and noise specs will deteriorate close to the positive rail, when the NMOS input transistors take over. This is the situation with most complemetary input opamps. But anyway other MOSFET amps with non-complementary inputs also don't have phase reversal, e.g. OPA1678. So the complementary input transistor stage is not needed to prevent phase reversal.

    The question is: will the TLV9301 still regulate (albeit with higher noise and larger offset/bias current) , when its input common mode voltage is within a few 100 mV of V+ ?

  • +1 in reply to tobias kosub
    Guru 140200 points

    Hi Tobias,

    you should read section 7.3.6 of datasheet and compare the specifications falling into the recommended common mode input voltage range and falling outside:

    Common mode rejection: 75dB versus 110dB

    Open loop gain: 75dB versus 130dB

    Gain-bandwidth product: 0.7MHz versus 1MHz

    As you can see the common mode rejection and the open loop gain are degraded by a factor of 56 and 560!

    This is not the behaviour of a good input rail-to-rail OPAmp.

    It's your decision Relaxed

    Kai

  • 0
    Prodigy 150 points

    Thank you Kai, excellent information!