Well, to reduce the TC of a bandgap, you have to design your resistor and BJT in the ouput branch well.
If you want to reduce the TC to bare minimum, you have to do second order curvature correction. Please refer to the papers attached....
Dear Vamsi Mocherla
Can you tell me ,I want to use first order bandgap ,and Can the miniaum TC below 10ppm.I don't want to use second order ,if I can have the TC I want to.
There is a paper by Dr Leung wher he has reported TC of the order 15ppm. He has used a certain combination of resistors.
Look for it in IEEE Study paper, if it is not their I shall upload it
I think it is realy difficult to get a TC below 10ppm using first order compensation. First order compensation has a theoratical limit and the commonly value is 10~20ppm if well compensated. I think you can pay attention to the TC of the resistor in your circuit, maybe helpful.
To calculate the PSRR in a bandgap reference. Place a \[v_{ac}\] in series with \[V_{DD}\] and measure the gain, \[v_{ref}/v_{ac}\]. Next do the same with \[V_{SS}\].