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Offset error for LMH6554

Jianxiong Zou
Prodigy 150 points

Other Parts Discussed in Thread: LMH6554

I am using the LMH6554 as the ADC driver to convert single-end input signal to differential style. Because the ADC input range is from -250mV to 250mV. I want to add some shift to the differential output signal to use the full input range of the ADC. 

So, I input the signal to the positive input of LMH6554 and add some voltage offset to the negative input pin to shift the differential output signal. But after I do this, the output of LMH6554 have some kind of low frequency noise about 2~3mV. Why can this happen? Does the asymmetry cause this? How can I solve this?

thanks a lot.

  • 0
    TI__Mastermind 22825 points

    Hello,

    I wished  you had attached a schematic. Is it possible to do so?

    Regards,

    Hooman

  • 0 in reply to Hooman Hashemi
    Prodigy 150 points

    The TINA simulation schematic is attached. I add some offset voltage to the negative pin through R6(150Ohm) to shift the differential output. Then the problem occurs.

  • 0 in reply to Jianxiong Zou
    TI__Mastermind 22825 points

    Hello,

    The asymmetry you've introduced (R8 =150ohm instead of 50ohm and possibly also C1=1nF instead of open) means that any "noise" / ripple that your Vcc may have, coupled through the 300ohm pull-up resistors, gets a larger gain to the device output. According to simulation, the gain increase due to this asymmetry is 76dB!! The path for the Vcc noise coupling to the output is through the 300ohm resistors (R9, and R11) that you have added to try and center the CM voltage on the inputs pins.

    So, it is essential to maintain symmetry or find another method to introduce the offset.

    If the Vcc noise were high frequency, you could have blocked it by using inductors in series with your 300ohm resistors, but you mentioned it is low frequency. Better filtering of your Vcc might also help.

    Hope this helped.

    Regards,

    Hooman