9.2.2.2 Calculation of the Cutoff Frequencies and Gain of Stage B

As shown in Figure 32, amplifier B is an inverting bandpass filter and gain stage. Capacitor C4 creates an AC coupled path between the A and the B stages. Thus the signal level must be shifted at the input of the amplifier B. This is done by connecting a midpoint voltage of the reference voltage dividers comprising R10, R9, R8 and R7 to the non-inverting input of amplifier B, biasing the input signal to the mid-rail (1.65 V). A very large feedback resistor R3 is chosen to minimize the dynamic current due to presence of peak-to-peak noise voltage at the output of this stage.

The low cutoff frequency of the filter of the stage B is:

Equation 5.

Choosing R4 = 68.1 kΩ and C4 = 3.3 μF, the low cutoff frequency is f_Clow = 0.71 Hz. The high cutoff frequency of the filter is:

Equation 6.

For R3 = 15 MΩ and C3 = 1000 pF, the high cutoff frequency is f_Chigh= 10.6 Hz. The gain of the stage is:

Equation 7.

For R3 = 15 MΩ and R4 = 68.1 kΩ, the gain is calculated |G| = 220.26 V/V (46.9 dB).