ADC connection problem(help pls urgent)

[kingkong83]

Hello.. i hv a problem about the connection of ADC.. i am using the MCP3021 (12 bit ADC).. This ADC is control by the PIC.. But when i connect the ADC that is shown in figure 2 it hv some noise and i cannot receive the correct binary value... Figure 1(this it the diagram from 1 site).. Can anyone tell me how to receive the accurate ADC value??

 

[kingkong83]

Any difference between Figure 1 and 2?? But i think it is same right??

 

[Roff]

Using a digital supply for Vref will generally cause at least a few LSBs of uncertainty, which can look bewildering in the digital domain. Do you have this connected to a D/A converter?

 

[Oznog]

Describe the noise? Too high, too low, or just changing randomly? How much is it changing? A few mV of noise or all over the scale?

Fig 2 needs a ceramic or tantalum cap from Vdd to Vss, and it must be as close to the ADC as possible.

Be aware the MCP3201 loses accuracy if CLK is below 10kHz.

And of course review your code and verify the timing is being met. Be aware that the PIC samples pins in a different part of its instruction cycle than when it updates outputs, so a port write for the clk immediately followed by a port read is not one instruction cycle apart, timewise. When all else fails, add some NOPs in there.