ADuC7024 ADC0

I'm using the ADuC7024 ADC ch 0.  Code is similar to what's in the sample code

ADCON = 0x7E3;

while( !ADCSTA) {  }

y1 = ADCDAT >> 16;

I found ADC output for a 2V input to be about correct; but for 1V input, I only get half the digital counts (see below)

Vref is assumed to be internal 2.5V

2V input     ADC output =  (about) 3260   this is about correct

1.5V input  ADC output = (about) 1950     this is low, should be about 2450

1V input     ADC output = (about) 700     this is only half, should be about 1600

Any info on this non-linearity?  Is my setup incorrect ?