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LMP8358: How to connect this amplifier...

Ricardo Sis Moreira
Prodigy 40 points
Part Number: LMP8358

Hello. Reading the LMP8358 datasheet, I can't figure how to connect this op amp in a circuit to amplify voltage. I have my doubts relate to pins FB, REFF and REFS. Can anybody explain me their functions?

  • 0
    TI__Genius 11135 points
    Ricardo

    The FB pin is used to set a gain value other than the steps available via the digital programming. REFF and REFS are used to bias the device output to a reference value (commonly mid-supply).
    If you could give us some specifics on what you're trying to do we should be able to help you with your design.

    Regards
    Dennis
  • 0 in reply to Dennis Goeke
    Prodigy 40 points
    Hello Dennis. Thanks for your reply.

    Basically I want to amplify a voltage signal, varying the Gain digitally form a MCU.
    So, if I want to used only the gains available digitally, I just leave FB "not connected"?
    And if I just don´t want to Bias the output, I just set REFF and REFS to GND?
    Was the purpose of this BIAS input, to eliminate Output Voltage offset? In that case, I could connect a Trimmer to manually eliminate this value. How do I connect this?
  • 0 in reply to Ricardo Sis Moreira
    TI__Genius 11135 points
    Yes you can leave FB connected and use the digital gain.
    The REFF and REFS are not used for offset trim. This part actually has internal offset correction.
    The REF inputs are meant to level shift the output to keep it in its linear range. For example with a single supply of 5v, if the Ref pins are grounded an input of 0V would actually give an output of about 12 to 60 mV depending on the load.

    What kind of input voltage range are you using? Is it from a bridge or other type of sensor?

    Dennis
  • 0 in reply to Dennis Goeke
    Prodigy 40 points

    Very simple, as follows. At photodiode, I have a pulsed signal (AC).

    What I want is a lowest DC offset possivel at output. I want Vout=0V for Vin=0V. How to I get that using Refs pins? I'm also very interested in linearity.

    I'm actually using the ADA4350ARZ from Analog. Can you compare this chip with yours? Can you convince me that I should use yours istead, for other reasons besides the price?

    Please, feel free to write me to my email <ricardo.moreira(at)techchrom.com.br>. Send me some connection circuits if you like.

  • 0 in reply to Ricardo Sis Moreira
    TI__Expert 6275 points

    Based on your given information ADA4350 is the better choice. If you want 0V to 5V out you will need to use id with asymmetrical supplies like +6V, -5V.  You need to look at output swing characteristics. ADA4350 is better suited for photodiode application since you can use large external feedback resitors and external compensation.