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Need Help with OPA 2314

Hi, all--

I'm not sure if I'm posting in the right section, so let me know if I have to move this to somewhere else! I need help with OPA 2314. I'm trying to design a preamp for Knowles mics ( NR23158). I've been using LF347 the results is acceptable. I'm trying to use OPA 2314 now, but I'm getting a very high oscillation from the circuit with the gain of 50. Basically, even if I don't connect my mic to the circuit there's this constant noise in the circuit. The bypass capacitor helped but it's still not acceptable. I'm pretty sure there's something I'm missing. There should be a difference between LF347 and OPA 2314. I checked the datasheet and they seems to have an almost same characteristics for my preamp. Also, does it matter if I used like 50k and 1k to get the gain of 50 and maybe 500 and 10 ohms to get the same gain? Should I use different values for my capacitor and resistor? I don't think 50 gain is too high, where is this Oscillation coming from. I'm very new to this, I'd really appreciate it if someone can help me out here.

Thanks!

Ramin

  • In reply to Collin Wells:

    Thanks Collin, I will read more carefully and come back with more questions!

    I mean the capacitor (2,22pf) on the circuit that you suggested on this post, the one in the output of the opamp, What's that for?

    Thanks again for your help!

    Regards,

    Ramin

  • In reply to ramin anushiravani:

    Hi Ramin,

    That capacitor in combination with the resistor in series with the output forms a low-pass filter with cutoff = 1/(2*pi*R*C), it could be useful to attenuate out higher frequency noise signals.

    Regards,
    Collin Wells

    Regards,
    Collin Wells
    General Purpose Amplifier Applications

  • In reply to Collin Wells:

    Hi Collin, 

    Could you please take a look at the following circuit. For some reasons I'm getting a very high oscillation in my output. It seems drop if I disconnect the voltage dividers from the non inverting inputs, but of course the amplitude drops too. 

    Also, How can I connect the coupling/decoupling capacitor in this circuit for the microphone and the output of the opamp?  

    1401.opa newest.TSC

    Thanks,

    Regards,

    Ramin

  • In reply to ramin anushiravani:

    I just wanted to give some more info here, even if I don't connect the input signal oscillation is there. I tried different values for the voltage divider, oscillation is still there. The output from the first stage with out connecting it to the second stage is also oscillatated. 

  • In reply to ramin anushiravani:

    Hi Ramin,

    Your circuit looks okay, I'm surprised it oscillates.  However the OPA140 is not an appropriate choice for a +5V application because of the limited input range in this configuration.  With a +5V supply, the input range is only 0.1V - 1.5V as stated in the input common-mode voltage range spec.  Let's close out this thread and move the rest of the conversation to the post where you specified your design requirements.  I'll put together a circuit with the OPA376 that you should hopefully be able to modify the rest of the way to your needs.

    Regards,
    Collin Wells

    Regards,
    Collin Wells
    General Purpose Amplifier Applications

  • In reply to Collin Wells:

    Hi Ramin,

    I realized that you actually had the OPA134 in the circuit not the OPA140.  The OPA134 has a swing of (V- +2.5) - (V+ -2.5V) so with V- = 0V and V+ =5V you basically have no usable input range. 

    I'll continue on the other thread as mentioned before.

    Regards,
    Collin

    Regards,
    Collin Wells
    General Purpose Amplifier Applications

  • In reply to Collin Wells:

    Hi Collins, 

    I figured that the microphone I've been using doesn't have a good flat freq response and not a good 8 figure so I need to use NR23158 knowles microphone Please find the link for the datasheet. http://www.knowles.com/search/prods_pdf/NR-23158-000.pdf

    This microphone is not working with the current design of the OPA2317, any suggestions? 

    Thanks,

    Regards,

    Ramin

  • In reply to ramin anushiravani:

    I have another question, this knowles mic has a high impedance how does that affect my choice of the opamp? I used INA217 before and it didn't work, the IC gets too hot. 

  • In reply to ramin anushiravani:

    Hi Ramin,

    The INA217 won't get hot unless you've connected something improperly.   Regarding your previous question, I assume you mean the OPA314 not OPA317.  The circuit I provided using the recommended OPA376 which is a low-noise OPA should also work with the OPA314 although the results may not be as good.

    Regarding the voltage dividers:  if you look at the circuits, when you connect the same voltage divider to both places you're effectively shorting out the first OPA because the positive input of both amplifiers get tied to the same net and the input signal likely finds some strange path around the first amp causing issues with the second.  It wouldn't be an issue if we weren't building a non-inverting system where the input signal is summed with the common-mode voltage at the non-inverting input.  But since this system is non-inverting we need two dividers.

    Regards,
    Collin Wells

    Regards,
    Collin Wells
    General Purpose Amplifier Applications

  • In reply to Collin Wells:

    Hi Collin 

    I see your point, that's gonna be a lot of voltage dividers then. I think positive and negative supply would be a better choice since I have to amplify four different inputs at the same time. 

    Thanks.

    Regards

    Ramin