Hello,

I am laying out another op amp and have a couple questions regarding the BW equations (C & R sizing) and the exact effect they have on the design. The constellation in question is in the attached image. Basically I have a 10kHz signal that is +/-300mV and I would like to amplify it (15x) and feed that into an ADC that is 0-5V. I figured that the design below could work well for me. The signal will be amplified and referenced to Vs/2 (in my case, 2.5V).

There were equations in the reference design that I found describing the various bandwidths:

BW1 = 1 / (2*pi*0.5*Ra*C2)
BW2 = 1 / (2*pi*Rin*Cin)
BW3 = 1 / (2*pi*R1*C1)

I assume that BW2 should be sized for the 10kHz input signal. BW1 is with regard to the supply voltage frequency bandwidth? I am not sure what to do with BW3.

I would appreciate any clarification and advice on sizing these components correctly.

 

Those aren't bandwidths, they are the corner (-3dB) frequencies for the filters, whether high-pass or low-pass.

Corner1 (decoupling filter) is a function of C2 and the parallel value of Ra and Rb along with Rin and Cin. Typically you would make C2 large to provide a virtual AC ground (Xc small) at the lowest frequency of interest at Vin and whatever the lowest power supply ripple frequency is.

Corner2 (high-pass) is determined by Cin and Rin as long as C2's impedance is very low at the lowest frequency. You normally make this corner much lower than the lowest frequency so there's little rolloff at that frequency (Xcin much less than Rin).

Corner3 (high-pass) also affects the lowest frequency of interest. (See Corner 2)