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Original question:

LM321: LM321 output voltage

LM321: Output Voltage Differs Between Boards

Don Pitchford
Prodigy 190 points
Part Number: LM321
Other Parts Discussed in Thread: LMV321, LMV321A

I am using the LM321 as a front end to DAC on my MCU.  I am looking for gain of 1  [ Vout = Vinput * (1+R31/R30) ].     My R31 is 0 ohms, R30 is not installed.   Gain should equal 1.  So what is in on + should equal that on the output.   Or I thought:   I have produced 10 of these boards as a prototype and found that 5 of the boards deliver 5V on the op-amp output with the input of 5V.  (perfect)!!!    However, the other half are consistent on producing maximum of 4.5V on the output (pin 4).     Both boards behave the same when the input voltage < 4.5V.     So my question is what is causing this and how do I fix this? 

I have played with my R31 & R30.  Added 0 ohm & 1K resistor on R30 with no difference.   I don't want to have to adjust the gain because in production, I need consistent behavior and cannot afford to tune each of the boards.    Am I doing something wrong with my design or do I have a bad batch of Op-Amps?

  • 0
    TI__Guru*** 139956 points

    Hello Don,

    I believe you have the LMV321 and not LM321. This observation on LMV321 is very normal. The 5V input is above upper input common mode range of the LMV321. The cutoff input stage may cause the output to try to go low or high. The ones that try high, do so and output 5V. The ones that try low, bring the inverting input back into the valid common mode range.

    See section 1 of the application note. The solution is also in the application note, the LMV321A alternative

  • 0 in reply to Ron Michallick
    TI__Guru*** 139956 points

    Don,

    If LMV321 is in the DBV package then the LMV321A is not a correct pinout alternative, so use TLV9001UIDBVR which is the correct pin out.

    The LMV321A in DCK package is correct pinout to replace LMV321 in DCK.