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OPA211: Improving SNR in IF circuit - low noise amplifiers

Bart
Expert 6460 points
Part Number: OPA211
Other Parts Discussed in Thread: OPA1612, THS4541, THS4520, LMH5401

Hello team,

I would need some advice/help from someone with analog amplifiers / IF amplifiers expertise.

It is about improving the sensitivity of the intermediate frequency circuit as part of an RF system. 

I can share offline the current schematic of the analog IF amplifier that is connected to the RF mixer on one side and ADC on other side.

From the measurements we have found that main problem is noise of the circuit in baseband. Interested frequency range of baseband is 1 MHz to 10 MHz (if we can extend this to 14 MHz even better). We believe we can find some optimization with a lower noise amplifier.

We did test with various amplifiers in simulation (OPA1612, AD8397) and circuit but not much luck in improving SNR. 

Thanks.

 

  • 0
    Guru 48395 points

    this is one of my areas if you want to send details to msteffes1@comcast.net will need, 

    1. input match?(impedance)

    2. gain

    3. single or diff. out? 

    4. ADC input requirements? (vcm, Vpp)

    5. Any HD concerns? 

    Straight 50ohm terminated op amps run about 9 to 12dB NF best case, with one layer of tricks, can get that below 6dB, with another, down below 4dB. 

  • 0 in reply to Michael Steffes
    TI__Expert 6460 points

    Hi Michael,

    thanks for your great support offline. Your advice were really spot-on and in the simulation we can already observe better results. 

    Thanks a lot! 

  • 0 in reply to Bart
    Guru 48395 points

    You bet Bart, 

    So from the input of that stage (output pin of that last mixer) the gain is 30dB right now. I would only need that next stage gain target to the ADC inputs to work out a THS4541 based 3rd order filter in about 10minutes and send that out for the single to diff stage, 

    Any gain target for that stage - 1V/V is fine if that is needed. 

  • 0 in reply to Michael Steffes
    TI__Expert 6460 points

    Hi Michael,

    we are using gain of additional 32 dB on THS4520 single to diff converter. Signal that we have to catch are extremely low power so very high gain is required in the system. This amplifier is working with self bias DC level and it is AC coupled with 100nF capacitors to the ADC stage.

     Power supply of whole amplifier system is single ended +5V.

  • 0 in reply to Bart
    Guru 48395 points

    Ok, thanks Bart, 

    That is too much gain in that stage for an active filter implementation with cutoffs in the 15Mhz region. You probably need to go passive RLC interstage to the ADC. 

    You can certainly use the THS4520 - it is one of the higher GBP FDA's in the VFA line - the LMH5401 is quite a lot faster, but you might not need it, here is a partial list from my MFB design file reporting PDS vs TINA model GBP, the THS4520 model is a bit off, 

    That 1st column of PDS Bandwidth might not be GBP but min gain SSBW.