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OPA2365-Q1: What's the meaning of this parameter of the opamp? Voltage output swing from rail

Jankel
Intellectual 731 points

Part Number: OPA2365-Q1
Other Parts Discussed in Thread: OPA365

Voltage output swing from rail, what's the meaning of this parameter?

and where is the corresponding parameter which I think should be the voltage input swing from rail?

The opamp is OPA2365-Q1.

  • 0
    TI__Guru 70831 points

    Jankel,

    Even in op amps called rail-to-rail output,  its output can never reach either rail. Thus, the output swing from the rail means how close to supply rails the output can get.

    Therefore, in your example above, with single supply of 5.5V and max output swing of 20mV from the rail means that the output can get as low as 20 mV and as high as 5.48V

  • 0 in reply to Marek Lis
    Intellectual 731 points

    Hi Marek,

    Thanks for your reply.

    How about the rail-to-rail input ?

    I know the rail-to-rail output means that we can use the opamp to output a signal very closed to the power rail,

    but what's the detailed meaning of the rail-to-rail input ? Does it mean that, in the 5V 0V power rail system, the dc operating point of my opamp circuit can be at the power rail, like at 0V? Confused...

    Could you give me an example for understanding this parameter?

    In addition, I haven't found any electrical characteristics of this parameter in the datasheet of this opamp.

  • 0 in reply to Jankel
    TI__Guru 70831 points

    Input voltage range is specified under the term of the input common-mode voltage range and in case of OPA365, unlike the output, it actually extends 100mV beyond the rails.  This means that for a single 5V supply, the linear input voltage range is from -0.1V to 5.1V -  see condition column of CMRR parameter.  

    It is easily possible for p-channel transistor to operate linearly below ground but it requires the input to stay a minimum of Vgs (~1.2V) plus Vsat (~0.5V) voltage below positive rail (a total of ~1.7V) or the transistor will be turn off.  However, because of an addition of charge pump to the OPA365 input differential pair, which raises the internal positive rail by 1.8V, the input can operate linearly up to 100mV (1.8V-1.7V) above positive rail - see page 11 for functional block diagram.

  • 0 in reply to Marek Lis
    Intellectual 731 points

    Hi Marek,

    Thanks for your reply. It's very helpful.

    Another question is:

    If there'll be some electrical characteristics degraded to a lower value with a single 5V supply than with a dual ±2.5V supply?

  • 0 in reply to Jankel
    TI__Guru 70831 points
    Since OPA365 does not have an internal ground, it does not differentiate between 5V single supply and 2.5V dual supplies. Thus, as long as the input/output signal is the same with respect to its rails, OPA365 performance in both cases will be identical.
  • 0 in reply to Marek Lis
    Intellectual 731 points

    Hi Marek,

    What types of opamp have an internal ground?

    Could you give me some corresponding opamp device examples for better understanding?

    Thanks.

  • 0 in reply to Jankel
    TI__Guru 70831 points

    Jankel,

    No modern op amp has an internal ground and for this reason as long as the total supply, (V+)-(V-), is between its specified  minimum and maximum value, the op amp performance will be the same.  Therefore,  +/-2.5V, 0->5V, 100->105V, 1000->1005V are all the same as long as the input and output voltages are within these rails.