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I have some basic doubts in ADC conversion. I am using AD7798 ADC. I am reading ADC values using AT90USB1287 micro controller using SPI communication. I am able to read ADC values perfectly but i have some doubts.

I had gone through the datasheet several times. I am setting Configuration register as "0x0230" and mode register as "0x0009". It means update rate as 16.7Hz and gain from "4 to 128".

When i try to read ADC value with 16.7Hz update rate it is taking more time but accurate values.

I would like to get values more fast than present time so i changed update rate from 16.7 Hz to 232Hz. Now ADC reading values more faster but not accurate.

How can i read ADC values more accurate and faster? Changing gain will effect ADC update rate?

Could anyone give me some explanation regarding Gain Vs Update rate vs accuracy?

From the datasheet i see that gain will effect on resolution and update rate will effect on signal to noise ration.

AD7798datasheet

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  • \$\begingroup\$ At 16.7Hz sampling the internal filters will reject 50Hz and 60Hz. Above this the internal filters don't operate so maybe you are picking up AC power frequencies. Try averaging a bunch of readings that are sampled at a multiple of 16.7Hz to see if the numbers come good. Then try averaging a number of samples that is half way between multiples of 16.7Hz to see if the numbers are again bad. \$\endgroup\$ – Andy aka Sep 25 '13 at 21:15
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    \$\begingroup\$ You've demonstrated that speed and accuracy are generally inversely correlated. What makes you think this isn't an intrinsic feature of most ADCs (hint- it is)? \$\endgroup\$ – Connor Wolf Sep 26 '13 at 6:16
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"How can i read ADC values more accurate AND faster?" You can not. They are inversely correlated in delta-sigma ADCs. Read how a delta-sigma ADC works here

But you can take some measures to improve SNR characteristic of your circuit by: 1- Following the design guidelines in your ADC data sheet, in particular Grounding and Layout section. 2- Add appropriate filtering in your input stage. 3- Some general guidelines, like using linear power supplies rather than switching if possible, having separate analog and digital ground planes on your PCB, etc.

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