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  1. #1
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    STM32F3 built-in opertion amplifier

    I look into datasheet STM32F303 page 122 6.3.21 Operational amplifier characteristics and I'm a little confused.

    ILOAD Drive current max 500uA
    RLOAD Resistive load min 4kOhm

    Amplifier is Rail to Rail and Analog supply voltage 2,4 - 3,6
    already at output voltage 2,5V into 4k Load need current 625uA.
    I don't understand the ST documentation or do st make fun of me?

    Last but not least
    Why ST does not specify typical voltage offset of inputs?
    Why ST does not specify typical input current of inputs?

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    Re: STM32F3 built-in opertion amplifier

    Those are two separate specs. There’s nothing that says you can drive 2.5 volts into 4K. It says you can have a minimum load of 4K, but your application has to limit the current to 500uA. If you use Ohm’s law, you’ll find that the maximum voltage you can drive across 4K is 2V. There’s nothing wrong with the spec.

    The data sheet DOES specify maximum offset voltage and bias current. Why do you need typical? If you design for typical conditions only, you’re asking for trouble.



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    Re: STM32F3 built-in opertion amplifier

    With this approach, the min Rload information is slightly obscured.
    Why is Rload min 4K for 2V and not 2k for 1V?
    Why do you need typical?
    Look at any datashet even for the cheapest amplifiers for example LM358
    manufacturers give typical value, it is useful, for example, when sorting components, when evaluating the accuracy of the whole device, software calibration, etc.



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    Re: STM32F3 built-in opertion amplifier

    I pretty much disagree with everything you said. The minimum load resistance IS NOT a function of voltage. It probably has something to do with compensation.

    If you're evaluating accuracy, calibration, etc using TYPICAL performance then youre doing it wrong.
    ;



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    Re: STM32F3 built-in opertion amplifier

    Quote Originally Posted by barry View Post
    The minimum load resistance IS NOT a function of voltage. It probably has something to do with compensation.
    ;
    The minimum load resistance IS NOT a function of voltage. Yes this is primary function of current and for full resistive load is current function of voltage.
    Because this entry does not contain any defined conditions such as frequency, etc. is your consideration of the internal compensation of a purely purposeful fantasy.
    Exactly what should not be in the datasheet.
    Amplifier can be modifed as buffer, PGA or general amplifier with external coupling,. can works for full DC until 8MHz (G=1).
    For DC is Rload 4k Ohm on 2V the same as 2k on 1V.
    By the way ILOAD Drive current ,Operational amplifier output should be fully bidirectional and current +- 500uA an not +500uA as how in in the datasheet.

    Finally
    In OPAMP specification ST write
    TS_OPAMP_VOUT ADC sampling time when reading the OPAMP output = min 400ns.
    ADC max Rain in for 400ns in Slow channels is 2,9kOhm
    ADC max Rain in for 400ns in Fast channels is 3,9kOhm
    ADC max Rain in for 400ns in other channels is 2,2kOhm
    all this value is under Rload min 4kOhm
    so actually ADC cannot be used as a buffer for ADCs running at 2.5MSPS even though the datasheet claims it



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    Re: STM32F3 built-in opertion amplifier

    Finally
    (…)
    so actually ADC cannot be used as a buffer for ADCs running at 2.5MSPS even though the datasheet claims it
    You're simply confusing source and load impedances. Rain is the impedance of the source driving the ADC, minimal OP load impedance has nothing to do with it. Reconsider.

    As for your general complaints about incomplete OP specs, an OP implemented inside a digital chip will undergo certain performance restrictions. Same thing as with on-chip voltage references or high resolution ADC. If you need full analog performance, use a separate analog components.



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    Re: STM32F3 built-in opertion amplifier

    I was looking at the data of the built-in amplifier.
    can someone explain to me what it is
    VOHSAT High saturation voltage min Vdda-100mV
    VOLSAT Low saturation voltage max 100mV

    Somehow I do not understand what the poet means?
    The amplifier is marked as Rail-to-rail input/output



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    Re: STM32F3 built-in opertion amplifier

    HI,

    Indeed even Rail to Rail Opamps don't go to the rails.
    There always is a dropout voltage ... in this case 100mV to each rail.

    Klaus
    Please don´t contact me via PM, because there is no time to respond to them. No friend requests. Thank you.



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