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Original question:

OPA140: series resistor in a voltage follower: is it useful ?

OPA140: Voltage track circuit ,output has noise

Haiwen Huang
Intellectual 1740 points
Part Number: OPA140
Other Parts Discussed in Thread: OPA320

Dears:

Customer use OPA140 to do voltage track circuit. They found abnormal noise in output voltage.

Green waveform is input sine voltage. Blue waveform is output sine voltage. 

1- the noise overlap sine waveform no matter frequency is.

2-when change input sine waveform amplitude around 2.2V , the noise is coming. you can refer to picture 2.

3-other input amplitude such as 2V or 2.3-4.8V ,don't observer this noise.

4-customer tried to cut off R281, but noise is still there when amplitude around 2.2V.

Could you please help check what happened ? Thanks .

  • 0
    TI__Mastermind 21217 points

    Haiwen,

    The problem is common mode range.  With a 5V supply, the common mode range is Vcm(max) = 5V - 3.5V = 1.5V.  Thus, for this circuit the largest signal that can be applied with out distortion is 1.5V.  The OPA140 is not a good choice for a buffer with a 5V supply.  You  should consider using a device such as OPA320 (rail-to-rail input / output CMOS) as an alternative that will have the full 5V common mode range.  The topic of common mode range is covered in https://www.ti.com/lit/wp/sboa583/sboa583.pdf for your reference.

    I hope this information helped.  Best regards, Art