I am trying to find the correct component values for a leg of an amplifier. There will be non-inverting amplifiers attached to From_First_Stage_Out and To_Second_Stage_In. This part of the circuits needs to:
- block the DC offset from the first op amp
- shift the center of the signal from 0V to 0.625V so that the signal range changes from [-0.3V,0.3V] to [0V,1.25V].
Here is the diagram:
I know that the filter comprised of C3 and R4 blocks noise from the +9V supply to R5. I am told that the "ideal" low pass cutoff frequency of this filter is 60Hz, so, knowing the value of C3, I calculated R4 to be\$\ R4 = \frac{1}{2(60Hz)(1μF)}\$ ≈ 2.652 kΩ.
R4 and R5 make a voltage divider. I am also told that since we're blocking the DC offset coming from the first op amp, the only offset remaining will come from the second op amp, which will have a gain of 100. Specifically, my lab manual says, "The voltage divider consisting of R4 and R5 must, when followed by a gain of approximately × 100, yield the required DC output of 0.625 V; i.e. the divider should yield a voltage of approximately 6.25 mV." So \$\ \frac{R4}{R4+R5}\ = \frac{1}{100}\$, which means R5 ≈ 263 kΩ.
Apparently, R4 ≈ 700Ω and R5 ≈ 1 MΩ. I realize that \$\ \frac{R4}{R4+R5}\ = \frac{1}{100}\$ is likely incorrect. I want Vout = 6.25mV so that in the second amplifier the output is 0.625V. I do not know, however, what Vin is. Since the max input before the level shifting is 0.3V, should I use this value? Or do I use 1.25V, the max input after the shifting? This comes down to me not knowing exactly which part of this circuit is going the shifting.
Clearly, I have something totally wrong here. What am I missing?
Thank you in advance.
