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OPA171 input range

Other Parts Discussed in Thread: OPA171


I would like to ask you about the inpur voltage of OPA171.

There describes the specification as below, ”(V-)-0.1V, (V+)-2V”.

My understanding is that the CMRR performance is secured if the input signal is in this range.

Is this correct? Or are there any other performance down happened if the input is out of this range?

And if the input is out of this range, could you tell me how much CMRR is degraded if you have the data?

Thank you very much in advance.

Best Regards,  

  • Suzuki-san,

    Products are most often not characterized beyond their specified operating regions. That is the case for the OPA171 CMRR. The OFFSET VOLTAGE vs COMMON-MODE VOLTAGE gaph shown on OPA171 data sheet page 7 shows how the offset voltage changes when the maximum VCM of [(V+) - 2 V] is exceeded.

    The graph is set up for +/-18 V supplies. Thus, the maximum positive VCM is +16 V for linear operation. The VCM increases from +16 V, to the +18 V supply level. Notice how the VOS remains flat until about 16.5, or 16.6 V, undicating that the CMRR remains unchanged. Then, at about 16.7 V the VOS begins to change very quickly. This is a region where the CMRR range becomes non-linear and it degrades very quickly. If we equate the VOS change with the VCM change the CMRR degardes from its former high > 100 dB, it falls to something around 30 dB.

    Beyond approximately +17 V, the VOS again appears constant with VCM indicating that the CMRR has increased again. However, this is outside the linear operating region for the amplifier. The operational amplifier may still function in that region, but all of its electrical performances are tremendously degraded. It is not a region that one should operate the amplifier.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Thomas-san,

    I'm sorry for my late response.

    Thank you very much for your kind reply!

    I understood well.

    But I do not understand why "(V+)-2V" is upper range? Why 2V?

    Best Regards,

  • Hello Suzuki-san,

    The common-mode voltage (CMV) range is dictated by the amplifier's input stage design. The input stage active devices must operate within their linear range in order to maintain high common-mode rejection (CMRR). If one attempts to apply an input voltage that exceeds the CMV range such as (V+) - 2V for the OPA171, some sections of the input amplifier will be driven outside of their linear operating range. 

    When that happens internal nodes get pushed to the point where they can no longer move and further increases in the CMV forces them into a non-linear operating region. Once there, the balance in the input stage is lost and the CMRR degrades. The OPA171 needs at least 2 V between the CMV input level and the V+ supply rail to keep everything operating linear.

    Regards, Thomas

    PA - Linear Applications Engineering

  • Hi Thomas-san,

    I understood, thank you very much for your explanation!

    Best Regards,