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CMVR(Common Mode Voltage Rage) for LMH6644

Other Parts Discussed in Thread: LMH6644, LMH6618

Hello

I would like to know the value of the CMVR of LMH6644 as below specification.

The specification shows CMVR which are Min Typ Max in yellow color. The values are scattered from Min:1.8, 1.6, Typ: -0.5, 2.0, Max:-0.2, -0.1. I am very confused because which one is considers for designing.

1. Which Values is considered for design factor?

2. If the typical CMVR is -0.5V, is the input voltage limited as Vs-0.5V(V+=3V, V-=0V, Gain=1)?

  Does the means that Violation VCM is 2.5V?

 http://e2e.ti.com/cfs-file.ashx/__key/communityserver-discussions-components-files/14/6661.lmh6644.pdf

 

 

 

 

  • Hello Jin,

    The numbers are actual input voltage as a single supply application. Figure 60 confirms this.

    The upper common mode limit is about 1.2V below V+, which is typical for this series.

    Typicals are the average at room temperature and are NOT guaranteed.

    The numbers in the Min and Max columns are guaranteed at 25°C.

    The BOLD numbers are guaranteed over the entire operating temperature range, and are much tighter than the min/max as they must accommodate expected variations over temperature.

    The typical valid input range is -0.5 to 2V on a 3V supply.

    The guaranteed limit is -0.2V to 1.8V at 25°C on a 3V supply.

    The guaranteed limit over temperature is -0.1 to 1.6V on a 3V supply.

    So as a follower (Gain=+1), at 3V supply, with an input of 2.5V would violate the input common mode range. I would expect the output hover around 2.3V under this condition.

    You will need a rail-to-rail input device if you need a 2.5V operating point, or increase the supply voltage to over 4V.

    The LMH6618/19 is the R-R input version of the LMH664x series. Also see the OPAx354 series.

    Regards,

  • Hello Paul

    First of all, your answer is very helpful for me,

    I would like ask you a new one as below.

    My customer was testing several number of LMH6644 under over CMVR input range and each value of output are different.

    Question number 1: The customer wants to know why the output values are different?

    Question number 2: Is it because that the OP amps give randomized outputs values due to over CMVR?

    ====================== Test Condition and Results ===================================

    1. Configuration: Follower(G=1) / V+ = 3.3V, V-=0V / Input Signal: PWM signal 4KHz(Duty: 50%) Amplitude: 3V

    2. LMH6644 Output values

       1) DUT1 : 2.6V

        2) DUT2: 3.0V

    3) DUT3: 3.2V

    ==============================================================================

    Please let me know your opinion on above.

    Thanks and Best Regards

  • Hello Jin,

    Their measured values are different because they are violating the common mode specification.

    At a 3.3V supply, the maximum guaranteed input is 1.9V!

    The CMR cutoff point is not immediate. There will be a "gray" zone of a few hundred mV above the limit where the amp seems to still work, but the specifications are degrading (offset, gain, etc).

    So, this is what I would expect at the conditions above for the given input voltage:

    0 to 1.9V = Proper guaranteed operation over temperature.

    1.9 to 2.3V = Proper operation at room temperature

    2.3V to 2.5V = Seems to be working, but AC and DC specifications are quickly degrading. Linearity errors.

    >2.5V = Clipping or distortion occurs, possible oscillations, or output rails.

    At some point, the input stage will fail and send a random signal to the output stage. At that point, the amplifier is like a chicken with it's head cut off - no telling what direction it will go. There are no guarantees on when and how the output will fail. This will vary lot-to-lot, device to device. Supply voltage, voltages on the inputs and temperature all have an effect on the failure mode.

    High speed devices tend to have lower CMR limits because they need more headroom for their BJT input stages and they need to maintain more stringent AC specifications across the input range.

    The simple fix is to NOT violate the common mode range! Or use a R-R input device.

    See Bruce Trump's Blog  on input and output range for a little more info.

    Regards,