LTC2400

Question

Hello, 
 We are using LTC2400 interfaced with NXP LPC1768 microcontroller and manually controlling SCK and reading SDO lines. 
 As mentioned in the datasheet, we are following the sequence as shown below, 
 
 CS-> Low 
 Check SDO pin value, 
 if == 1, CS=High , Return 
 else Clock SCK 32 times , read the pin value of SDO 
 CS=High 
 Remove last 4 bits of SDO data and mask first 4 bits of data , use the remaining 
 But the converted data is continously flactuating even when analog input line is connected to ground. 
 Where are we going wrong?

ghoover · Answer

With a 24-bit ADC it is not possible to ever have a stable code from conversion to conversion. Looking at the noise histogram on page 6 of the LTC2400 data sheet, you will see that the output code varies +/-1.2ppm. This is +/-10LSBs. This can only be achieved with a very low noise source. You say you are connecting the input to ground but if you are using a long wire to do this, it will act as an antenna picking up lots of noise. 
 Does your PCB have a solid ground plane and do you have the suggested bypass in place with very short leads between pins and ground plane?