Lpc2000

Has anyone used lpc's to operate AD7677s (using lpc's) before?

- 16-bit True differential input (-2.5V to +2.5V input range)
- 1 MSPS conversion rate

Basically the ADC's set up such that:
3V                                ---> Vref pin
Analog input (always +ve Voltage) ---> Vin+ pin
-90mV reference                   ---> Vin- pin

I have a little problem with the readings that I'm currently getting:

Input = 0V (GND)
The lpc reads 0x8000 from the converter. I assume this is the mid-
point.

Input = 330mV
Reading: 0x8000....strange
Input = 660mV
Reading = 0x8050...stranger
Input = 1.32V
Reading = 0xB8F0. OK (the calculation's legit)

The A/D seems to be reading correct but only for a specific range. 
1.32V and above will read correctly. But For some reason the lower 
voltages aren't coming out right. Any ideas? Could the hardware 
setup for Vin+ and Vin- be causing this problem?

thanks,
Leighton