This op amp circuit is used to offset stepped-down AC voltage by 2.5V(approx), so that the RMS of the voltage can be calculated. I can not change the configuration of the circuit, so that is ruled out.
This looks like a differential configuration, but i have not been able to understand certain decisions:
Why the 2.5V DC offset is fed into using a 10Kohm resistor and why there is a 1Kohm following it.
Why the + and - input of the op am p has this 1Kohm across it.
Why go to the trouble of adding the resistor network consisting of 4.7K and 100K when the AC is stepped down to 5V RMS.
Whether the AC source must be connected to ground or not. (Connecting and disconnecting gives different results).
UPDATE: I noticed something fishy today. When i ramp up the frequency to 50Hz, the mains frequency in India, i see that the Voltage at both the points are equal in magnitude and in DIRECTION! Isn't that 0V across the VSINE then? This happens only when the op amp is connected with the DC offset. When i remove the op amp and DC offset, the voltages are equal in magnitude and opposite in polarity. Is this because of the negative feedback; removing the negative feedback also gives similar results but lower magnitude voltages. See picture.