MSP430AFE253: SD24 ADC sampleing issue.

Hello friend:

when I used msp430afe253 to measure current(50hz,sample 120 points),but result is unstable,looks like adc's switch time isn't fast enough?or some suggestion?thx.

system is looks like following:

1)none extended crystal oscillator is used.(so default:dco is 1M,and aclk = vlo = 12k).

2)SD24_init is as following:(SD24OSR_1024 is better,sampling data is more stable.but when we finished a sample cycle(20ms),it looks like more than 300ms,then these sampling data shouldn't be right.)

3)we set TA timer as 50*120 hz.(as following)

4):TA's ISR  is STARTING sd24 ADC  covert.

5)SD24 ISR is like following:(we just get high 16bit,ignore low 16bit)

so,my question is :

how can i setting/init,then i can sample 120 points in 20ms?(none any extended crystal oscillator).

any errors/understanding is wrong in above code?