Electronic – Gain vs ADC performance vs ADC settling time

adcanaloggainmicrocontroller

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

Best Answer

"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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Electronic – ADC registers setup using spi communication

While this datasheet is making my eyes bleed with the complexity of doing a simple conversion/read, you need to make some simple changes.

I'm not sure why people are making this harder than it really is. You pull the chip select line low, and the target IC should simply read whatever you give it until you pull it high again. Just because your method takes an 8-bit value doesn't mean you can't call it twice to pass a 16-bit value to your IC. There is no need to bit-bang anything. That is nonsense.

Call your SPI method to tell the ADC you're looking to write to the configuration register:

spi(0x10);

As I understand from the datasheet, you can immediately write your 16-bit value to the configuration register after that, so you'd do:

spi(0x07);
spi(0x10);

I forget which way that'll end up being assembled on the target IC side, so you could simply reverse them if things don't work right. There's no need to bit shift any values at all. Worst case scenario is you have to pull the CS line back high before pulling it low again to actually send the data to write to the configuration register.

Otherwise, this is super simple. Remember, if the target SPI device expects more than one byte, i.e. a 16-bit value, chances are good that sending the bytes in the order they'd otherwise appear normally (so if you want to send 0x1234, you'd have 0x12 and 0x34) will work fine and you won't need to shift anything.

How to check SPI signals

I have changed the functions to read 8 bit and 16 bit registers then it worked.

unsigned int Bit_16(unsigned char cmd, unsigned int value) 
{
    unsigned int data; 
    SPCR = 0x5D;        // mode #3 F_CPU/16 
    ADC_CS = 0;                 // enable 
    spi(cmd);                   //Place instruction in communication register 
    data = spi(value >> 8);     // read hi-byte 
    data = (data << 8) | spi(value);    // and lo-byte. 
    ADC_CS = 1;                 // disable 
    return data; 
}

for write

AD7798_16(0x08, 0x000A);

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Electronic – ADC registers setup using spi communication

How to check SPI signals

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