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Sine wave amplification

KTC ...
Prodigy 150 points
Other Parts Discussed in Thread: TLC081

In my application, I have a low amplitude (uV level) sine waves with varying frequencies. (From 1KHz to 8Khz). I will need to build an active filter with gain. What is a good opamp for this type of a task from the TI's options. I only have 5V available so it needs to be a single supply opamp, something with very low input noise. 

  • 0
    TI__Guru 62700 points

    KTC,

    Since the effective bandwidth of an amplifier is a function of a close-loop gain, could you please specify what gain you plan on using?   Also,  what is the input common-mode voltage and amplitude of the input signal?

  • 0 in reply to Marek Lis
    Prodigy 150 points

    Marek,

    Thanks for the response. I need to amplify this signal in the order of 100dB or so. My current approach is to amplify using a High Pass active filter (60dB) and than further amplify using a low pass filter (40dB) in the next stage. Two 4th order filters back to back. I have put together some preliminary design but somehow, when combined they didn't work together. 

    See the attached TINA simulation. I have chosen TLC081 as the primary focus due to its low noise. (I will be amplifying low uV signals). 

    I have a few questions:

    - In my current system, I may have high amplitude low frequency signals, I am worried that they may saturate the HP filter. How can I avoid this? Shall I use a passive LC filter first to condition the signal before amplifying filter?

    - Why the existing simulation is not working? (If you look at the AC transfer, something is wrong between HP and LP filter. LP is not working, I cannot see why)

    - Is TLC081 good for this type of conditioning?

    HP_LP_G10000 - Copy.TSC

  • 0 in reply to KTC ...
    TI__Guru* 93105 points

    KTC,

    The reason the LP output response appears incorrect is because the op amp offset voltages from each of the high gain stages causes the output of U4 to be saturated. TINA shows the U4 output is at +4.36 V, up against its positive swing rail. The op amp isn't operating in a linear mode and the gain/filter function collapses. This could be a real problem in an actual circuit. You would need to employ op amps with low voltage offset, or introduce a voltage offset that counteracts their offset and keeps the outputs operating in the linear region.

    Also, you are requiring too much voltage gain from the Sallen-Key high-pass and low-pass filter stages. They are set up for a gain of about 32 V/V, which is likely too high. Texts on filter design usually recommend keeping the maximum gain to 10 V/V for the Sallen-Key filter stage for accurate response. Since you are requiring a very high overall gain of 100,000 V/V, set up the low-pass and high-pass for a gain of 10 V/V each and their combined gain will be 100 V/V. That leaves a gain of 1000 V/V to be divided up among U3 and U4. You could set them up for 31.6 V/V each. That should be doable providing the op amp selected has sufficient gain-bandwidth (GBW) at the maximum frequency of use. Otherwise, you may need a 3rd gain stage. If you are using FilterPro, or the Webench Filter Design programs for your filters, they will tell you what the minimum GBW needs to be for the filter stages.

    The op amp you select for this applicaiton will need to have low voltage offset, and sufficent GBW to support the filter responses and provide enough gain at the pas-band frequencies. You will have to assess the TLC081 relative to those requirements.

    Regards, Thomas

    PA - Linear Applications Engineering