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LMH6642: Input Offset Average Drift

BO
Genius 12760 points
Part Number: LMH6642
Other Parts Discussed in Thread: LMP7731

I just ran into a problem that I can't quite figure out the cause of.
The LMH6642 is wired as shown below. The primary function of this circuit is to buffer a reference voltage, the secondary function has AC characteristics.
R110, R111 and C83 serve as external compensation. The OpAmp is stable.
VBRGP = 2.5V DC.



The LMH6642 datasheet gives an input offset average drift of typ. +/- 5uV/°C.
However, measurements I made myself in the oven give values up to 100 times the specification (500uV/°C).

The exact same circuit with the LMP7731 behaves correctly.

What have I overlooked in the design with this OpAmp or is this behavior known?

Regards Bernd

  • 0
    TI__Genius 16251 points

    Hello Bernd,

    I will take a longer look at your circuit and investigate the input offset drift.  Thank you for including your circuit, setup, and that the LMP7731 operates within spec with your configuration.

    Best

    Alec

  • 0 in reply to Alec Saebeler
    TI__Genius 16251 points

    Hello again Bernd,

    I did some investigating on the LMH6642 (and the LMP7731).  The LMH6642 has a higher bias current than the LMP7731 by an order of magnitude.  The uA bias current of the LMH6642 enables the performance difference you are seeing at relatively large feedback resistor resistance values.  Your circuit having a 3.3kOhm Rf.  The LMP7731 has a much lower bias current, on the order of nA.  

    In order to test this out or explore this cause, I would suggest using an feedback network with a lower feedback resistance value.  You may find the input offset drift to be closer to spec with this change.  When Rf is large, the bias current can have a greater effect on voltage offset (and drift).  Is there a design constraint requiring Rf to be 3.3kOhm?

    I did look into prior test and data files, and the LMH6642 did meet spec and perform as expected under test conditions.  There are not any known performance factors related to input offset drift with this device in particular.  I would recommend attempting to use a smaller feedback resistor to account for the higher bias current.

    Please continue to reach out with any updates or questions.

    Best,

    Alec Saebeler

  • 0
    Guru 140200 points

    Hi Bernd,

    you violate the common mode input voltage range of LMH6642. According to the datasheet you should stay 1.4V below the positive supply voltage which is +1.9V for a +3.3V supply voltage. +2.5V input voltage is way too high:

    bernd_lmh6642.TSC

    Kai

  • 0 in reply to Alec Saebeler
    TI__Genius 12760 points

    Hi Alec

    thank you for your offline comments. Come back on that asap.

    Regards Bernd

  • 0 in reply to BO
    TI__Genius 16251 points

    Hi Bernd,

    I will look to reconnect with you offline.

    Best,

    Alec