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Need clarification to understand the common mode input voltage range for LMC660

Jon Gilreath
Prodigy 20 points
Other Parts Discussed in Thread: LMC660, LMC6484

I'm using the LMC660 with a 5V single supply as a voltage follower.   The + input is at 2.0vdc as a buffer for a voltage reference.

The data sheet lists Vicmr (max) as 0V and Vicmr(min) as Vcc+-2.5V, so Vicmr(min) is 5V-2.5V = 2.5V?

That was confusing because I had expected the min-to-max range to include the center-voltage (2.5V) +/- about a volt  (i.e. 1.5Vmin to 3.5Vmax).  The data sheet appears to state that the acceptable common mode input range for this configuration would be 2.5V to 5.0V, but that seems weird. 

Question:  How to correctly interpret this range?  Is the acceptable common mode input 0V to 2.5V or from 2.5V to 5.0V or something different?

 

 

  • 0
    TI__Guru 53826 points

    Hi Jon,

    You are correct. At 5V, the common mode limit is at the mid supply range (2.5V).

    It is just as it states, "V+ - 2.5V", or 2.5V below V+ - regardless of supply voltage.

    So on a 5V single supply, the legal input range is -0.1V to 2.5V. On a +12V supply, it would be -0.1 to 9.5V.

    The LMC660 was our (well..Nationals) first CMOS opamp. The process did not allow for much headroom, and the focus was really getting the common mode smooth down to V- (which was not common in the mid 80's).

    Newer devices go about a volt below V+.If operating as a buffer at a low supply voltage, you may want to consider a R-R input device.

    The LMC6484 is the R-R input cousin, if you need the full input and output range. Otherwise, there are literally dozens of LMV/TLV/OPA devices that will give you a little wider input range.

    Regards,

  • 0 in reply to Paul Grohe
    Prodigy 20 points
    Thanks for the answer. The range makes sense because Vcc can be 5V to 15V, so Vcm is basically the lower supply to "near" the upper supply. The limited Vcm range is limited because 5V supply is relatively low. Another (related) question. Since the Vcm range is 0V to 2.5V, then why does the datasheet list 0V as Vicmr(max) and 2.5V (i.e. Vcc-2.5) as Vicmr(min). Shouldn't that be reversed? 0V=min & 2.5V=max? Not critical, just curious.