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LMC6042: input common-mode range

Ue
Genius 16565 points
Part Number: LMC6042
Other Parts Discussed in Thread: LPV521, TLV521

Hi all

My customer is looking at using the LMC6042 as a buffer amplifier with a gain =1. The input signal range is +/-2.5V and they consider using a -2.5V/+4.0V supply.

Is this ok as far as input common mode range is concerned ?

Can you suggest another suitable device for this application ?

Main specs of interest are low input bias current / low power consumption max. 20uA

Best regards

Ueli

  • 0
    TI__Genius 15410 points

    Hi Ueli,

    The input common mode voltage range for the LMC6042 is from (V-)-0.1V to (V+)-1.9V (typ). On a -2.5V/+4.0V supply, the input common mode voltage is from -2.6V to 2.1V. Therefore, having an input voltage up to 2.5V will violate the device specifications.

    Do you need the input bias current to be 2fA (typ) like the LMC6042 for a comparable device? Very few devices offer this low level of an input bias current. 

    Thanks,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Genius 16565 points

    Tim

    Thank you for the quick reply and confirmation.

    I will have to check with the customer as to the exact value for the max. input bias current.

    Best regards

    Ueli

  • 0 in reply to Ue
    Prodigy 10 points

    Hello Ueli and Tom,

    first off all thank you for addressing my issue.

    you mentioned:

    <<gain =1. The input signal range is  +/- 2.5V and they consider using a -2.5V/+4.0V supply.

    but correct is:

    gain =1.0  The input signal range is  +/- 2.048V and they consider using a -2.5V/+4.0V supply.

    We use the LMC6042 as a impedance converter on a sensor with an output resistance up to 10Gohm. Therefore we need a low input bias current (at least 50 to 100fA).

    What about the zero crossing? Do I get a problem with zero crossing?

    regards and thank you for your reply in advance...

    André

  • 0 in reply to user778511
    TI__Genius 15410 points

    Hi Andre,

    If the input signal is +/-2.048V then there should not be a common mode voltage violation using -2.5V/+4.0V supplies.

    When you say "Do I get a problem with zero crossing?", are you asking about the input stage cross over region? If not, can you please provide more information about what you mean by zero crossing?

    Here is an Analog Design Journal explaining the input stage cross over region in amplifiers.

    Op amps with complementary-pair input stages: What are the design trade-offs?

    Thank you,

    Tim Claycomb

  • 0 in reply to Tim Claycomb
    TI__Genius 16565 points

    Hi Tim

    Thank you for pointing to the Application-Note.

    ------------------------------------

    Hi Andre

    Referring to our conversation earlier today let me summarize:

    Crossover distortion is normally a problem encountered for OpAmps labelled as RRI (Rail-to-Rail Input). These OpAmps have two input-pairs. The offset of these two pairs have some mismatch, this leads to the mentioned crossover distortion. Thats what you found with the LPV521 and TLV521.

    The LMC6042 does not have this problem as it has only one input pair.

    Best regards

    Ueli