about LTC1864 interfacing

This looks like you are using a very noisy power source. Another possibility is that you do not have a solid ground plane. Still another possibility is that the bypass capacitors are not located right next to the pins being bypassed. The LTC1864 requires a clean DC power source and a clean reference voltage for proper operation.

This ckt works for SMPS 12v 1A but when i am using Adapter of 12V 1A lots of glitches get introduce in ADC count. I have not separated the Analog and DC ground plane , how can i now try to minimize the noise ?

I would recommend keeping the ground plane solid for the ADC. Make sure the bypass caps are close to the pins being bypassed. I would also recommend a linear regulator to directly power the ADC with the power adapter powering the regulator. If the power adapter is particularly noisy, the ADC may need to be shielded.

i did not understand by "keeping the ground plane solid for the ADC" , actually my PCB is finalized and i cant make any changes like separating analog and digital gnd planes,can you pleasetell me how i can shield my ADC ? it will be helpful. or should i just add a copper plate as ground plane?

Keeping the ground plane solid means not splitting the ground plane into separate analog and digital sections.

Shielding the ADC is done by placing a metal shield around the ADC and any analog driver circuitry. This will help reduce noise caused by radiated signals.

Keeping the ground plane solid means not splitting the ground plane into separate analog and digital sections.

Shielding the ADC is done by placing a metal shield around the ADC and any analog driver circuitry. This will help reduce noise caused by radiated signals.