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Do I need an RC on the front end of buffer

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redsox123

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With a simple active low pass filter like in the image it is easy to see that the filter frequency is set by the RC. If however, the buffer bandwidth is the same as the RC cutoff frequency is the RC necessary? Does the RC just give you double the attenuation?

2.-Active-Low-Pass-filter-circuit.jpg
 

Hi,
I think if the bandwidth is same then you can leave RC filter at the input of this buffer but there will be change in the phase in both the cases,i.e the output will be delayed. And about the attenuation,Yes it will square the attenuation to the noise in the signal.
Please correct me if i am wrong.
Thanks
 

Yes in theory this forms a simple two pole system. However the discrete RC has a few extra benefits.

First the cutoff of the amplifier will vary over temperature. So if you care about the cutoff it's better to set it with discrete components. Second it can be wise to 'protect' the amplifier from frequency components above its bandwidth because at high frequencies the opamp may cease to be linear (for example: RFI rectification).

So in an ideal world you'd specify the RC at the cutoff you want and choose an amplifier with maybe 2X-10X higher bandwidth.

On the other hand your application may not warrant any of this concern.
 
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