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Question of noise analysis on OPA132

BRIAN PENG
Intellectual 310 points
Other Parts Discussed in Thread: OPA132

Dear sir:

     I got problem when getting the noise density plot on OPA132 with PSPICE simulation. The circuit set up and noise plot is as attached.

There's a rise on 35~40Mhz (OPA132_CIRCUIT.JPG and OPA132_NOISE_PLOT.JPG) . Is that because of the inherent behavior of the OPAmp? It is hard to remove it even I build it to be sallen key filter (cut-off ~100Khz) as attached (OPA132_100K_FILTER_CIRCUIT.JPG and OPA132_100K_FILTER_NOISE_PLOT.JPG).

If I use that for my ADC pre driver, will the noise rise on high frequency band mirrorred back to my baseband if my sample rate is in low frequency (200KSPS)? 

 

Thank you!

Brian 

  • 0
    TI__Genius 12325 points

    Brian,

    This behavior is common with many op amps, although it always seems to come as a surprise! The input stage of an operational amplifier is not the only source of noise in the amplifier, the voltage amplification stage and the output stage will also produce some amount of noise. Within the bandwidth of the amplifier (specifically the bandwidth of the input stage) the noise of the second and third stages are divided by the gain of the input stage (40 to 60 dB) which is a benefit of negative feedback. However, as the input stage runs out of bandwidth (and its gain falls off) the noise of the additional stages is no longer reduced and begins to dominate the noise measured at the output. Eventually this noise is also attenuated by the bandwidth of the additional stages as well. This is one of the causes of the noise "bump" which is commonly seen above the unity gain bandwidth of the part.

    The Sallen-Key topology was not effective at removing this noise because it is above the bandwidth of the part. An RC lowpass filter is usually recommended at the output of an ADC driver for other reasons (capacitance provides a charge reservoir for the ADC, resistor maintains amplifier stability) and a side-effect of this filter is attenuation of this noise provided its corner frequency is low enough.

  • 0 in reply to John Caldwell
    Intellectual 310 points

    Dear John:

    Thanks for your replying! From the picture comparison, it seems the "bump" of sallen-key output is slightly lower than unity gain buffer, however, it is not so effective, because it adds more in band noise because of extra resistors. How can I reduce the overall noise density for more order low pass filter, not just for ADC driver application. That's really confuse me from my simulation, I got larger noise density when implementing higher order sallen-key low pass filter because of resistor thermal noise. May I have to use LC ladder filter in place of sallen-key topology to reduce the effect of resistors? Is that help? But I don't think it's work on low frequency. Isn't it?  

    Thank you!

    Brian