# Find settling time of ADC

In an assignment, I need to find the settling Time of the ADC Input Circuit time to reach 1/4 of the LSB.

The ADC has 12 bits. I also know that C=12 pF and R=10 kohm. I found a formula that i used which worked:

t=-ln(1/(2^N*4))*τ where the 4 symbolizes the 4 in 1/4. N is the wordlength N=12 τ=RC=10*10^3*12*10^-12=1.2*10^-7. => t=1164.448 ns

Now the task is to use the 6 most significant bits, still with 1/4 of the LSB, C and R is the same. My original idea was to use the same formula but now with N=6. t=-ln(1/(2^6*4))*1.2*10^-7=665.421 ns

According to a correcting system this was wrong, so i don't have access to any correct answers.

What am i doing wrong here? How can i do right to find the settling time to reach 1/4 of the LSB when only using 6 most significant bits of a 12 bit ADC?

I see that the ADC is still a 12 bit system, so maybe that is where i am going wrong when i do N=6, changing the ADC to a 6 bit system. But still, i don't know how to specify the time calculation to just find the 6 most significant bits out of the 12.

• Are you changing any settings that might change the capacitive value presented by the ADC circuitry? How did your number "turn out wrong"? Where did you find the formula? – Asmyldof Jul 14 '15 at 9:53
• This is an assignment so that is how i know that it turned out wrong, so therefor no settings is changed its just fictional. The formula was found derived here:silabs.com/Support%20Documents/TechnicalDocs/an119.pdf – elmaggan Jul 14 '15 at 9:56
• In which case, you should show some of your working and the result using the formula and what result your assignment says it should be, else we cannot (literally and ethically) help you. If we don't know what you typed in and why and what result you got, we can't see, nor explain what went wrong and just throwing out a number at you will not help you understand. – Asmyldof Jul 14 '15 at 10:20
• I tried to specify a bit now on my way through the assingment. – elmaggan Jul 14 '15 at 10:33
• I'm assuming you mean a fully automated digital correcting system? Did the same correcting system approve the 1164.448ns and how does it work with number precision? As a point or certain possible relevance. – Asmyldof Jul 14 '15 at 10:39