hi D,
With 15Vdc excitation, the unloaded load cell should have common mode output of +7.5v.
With the Vref pin at 0V, the Vout of the AD620 should be close to 0V.
If you set set the Gain, assuming a load cell output of 15mV at full load, to give a Vout max of 5V [ suitable for a PIC's analog input]
The Gain would be 5/0.015 = 333, a Rg of ~ 150 ohms would be required. [ I would suggest a 100R fixed and a 100R pot in series]
If you are concerned about over driving the PICs ADC input, ie: greater than 5V, use a series resistor and a 5.6v clamp zener diode on the Vout connection.
E
I would recommend a simple noise/RF filter between the load out and the input to the AD620.
Say a 10K in series with both the Inv and Non Inv inputs and a 10nF across the AD620 input pins,