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LMV981-N: Application problem in design

Paul Yeh
Genius 14205 points
Part Number: LMV981-N
Other Parts Discussed in Thread: LM358

Hi team,

My customer designed this device and met application problem need BU's support. Thanks

1. 
When the input voltage of LMV981_+IN is below 20mv, the response time of output LMV981_OUT becomes longer.
8us@20mv, 24us@6mv, 40us@3mv.
Is this situation caused by the characteristics of the IC itself?
Any suggestions for improvement of this kind of abnormality?

2.
The given Ton from shutdown in the datasheet is typ 12.5us@2.7v. ).
Please also provide the maximum and minimum values ​​of Ton from shutdown@3.3v

3.
When LMV981_+IN input voltage ≤ VOS max (4.0mV),
Does it affect LMV981_IN- output accuracy and NOISE?
If it is affected, what are the suggestions for improvement?

 
 
  • 0
    TI__Guru*** 143716 points

    Hello Paul,

    1) This is normal for all op amps. Small input voltage doesn't fully engage the first stage of the op amp so things can be slower.

    2) Sorry, I do not have this data. My guess is half to double.

    3) Vos directly affects DC accuracy and nothing else.

    I expect a lot of variability to occur. Vos is an always present input (unless shutdown). Output current is (VIN+VOS)/10 ohms.  +/-4mV is a lot error compared to input signal of 20mV or less.  Do you have enough headroom to make sense resistor 100 ohms and make the input 10X bigger?

    I expect a lot a variability in zero current output. with VIN = 0mV, the output might be off or it might be on a little. Off current (not shutdown) will be Vos/10 ohms. For Vos = +4mV , IOUT is  400uA.  For Vos = -4mV, IOUT = 0uA

    I expect a lot a lot a variability in response time for the same input (if zero input was previously present). For an op amp with a positive Vos, the zero input creates a small IOUT and output voltage is at VGS of FET. So the response will be fast. For an op amp with a negative Vos, the zero input creates zero IOUT and output voltage is at 0V (VOL). So the response will be slow because the op amp needs to recover from VOL state. This time is called overload recovery time (Tor typical specified in some data sheets). The Tor duration is inversely proportional to input signal level less than 100mV. 

    Chart from LM358/LM2324 app note. Should apply to LMV981-N also 

  • 0 in reply to Ron Michallick
    TI__Mastermind 18737 points

    Hey Paul, 

    Was this issue resolved? 

    I will mark Ron's response as "TI Thinks Resolved," please let us know if you require additional help. 

    All the best,
    Carolina