Can some one explain me the circuit diagram of this Low Pass filter.

 

That's not a low-pass filter, the two amps together act like a high-pass filter.
The 8513 is an integrator that feeds back to the REF input of the 8220 which rolls off frequencies below 1/(2πRC) and keeps its output dc level at 0V average.

 

That's not a low-pass filter, the two amps together act like a high-pass filter.
The 8513 is an integrator that feeds back to the REF input of the 8220 which rolls off frequencies below 1/(2πRC) and keeps its output dc level at 0V average.

Can you elaborate the REF pin working.

 

Can you elaborate the REF pin working.

The REF pin controls the DC output level.
Thus the 8220 output voltage will follow the output voltage from the integrator.
Since that forms a negative feedback loop, the average value of the output will stay at 0V, due to the integrator.

 

The REF pin controls the DC output level.
Thus the 8220 output voltage will follow the output voltage from the integrator.
Since that forms a negative feedback loop, the average value of the output will stay at 0V, due to the integrator.

Output of Integrator for DC input will be zero?

 

Output of Integrator for DC input will be zero?

If you mean for a DC of zero to the 8220 differential input, yes (neglecting any amplifier offset voltages).