OP Amp circuit - LP Differential Offset amplifier/filter?

[hitech95]

Hi, I'm doing the reverse engineering of a cheap Chinese Bluetooth board ... I found that it use (partially) this scheme, the values are not the same. the gain (Rf / R) is about 2.

I do not understand the configuration of this circuit, should be differential but there is also a LP filter on the feedback and an offset of Vcc / 2.
I do not understand why there are the capacitors: "C3" and "C12".

Thanks, hitech95.

[SebG]

HI.
The circuit basically is a differential amplifier with high frequency filtering (C9, C11, C3 and C12). The half voltage (Vcc/2) is necessary since the input goes through a dc blocking capacitor (on all 4 input signals) so you get both positive and negative voltages at IN+ and IN- and the Op-Amp is just powered from 5V to GND. The resistors divide the voltage and the 4.7uF (relatively large) capacitor acts as a voltage source so the op-amp output would be half Vcc for no (0V differential) input. The small (33pF) capacitors allow high frequencies to be effectively grounded before reaching the Op-amps inputs (either to the Vcc/2 source or Op-amp output which acts as a V source). Then the output is also dc decoupled.

Without the Vcc/2 bias the op-amp would cut off the negative part of the signal like a diode passing an AC signal.

[hitech95]

HI.
The circuit basically is a differential amplifier with high frequency filtering (C9, C11, C3 and C12). The half voltage (Vcc/2) is necessary since the input goes through a dc blocking capacitor (on all 4 input signals) so you get both positive and negative voltages at IN+ and IN- and the Op-Amp is just powered from 5V to GND. The resistors divide the voltage and the 4.7uF (relatively large) capacitor acts as a voltage source so the op-amp output would be half Vcc for no (0V differential) input. The small (33pF) capacitors allow high frequencies to be effectively grounded before reaching the Op-amps inputs (either to the Vcc/2 source or Op-amp output which acts as a V source). Then the output is also dc decoupled.

Without the Vcc/2 bias the op-amp would cut off the negative part of the signal like a diode passing an AC signal.

Then I understood right. I did not know that a differential amplifier could also be a filter. (In school I always made two step)
Apparently the capacitor of the filter must be on both "feedbacks".

The speech about Vcc / 2 I had guessed, I had asked such a thing a while ago.

Thank you!