ADC Understanding help for sampling rate

Hello Friends,

I am reading a ADC Tutorial where i understand that the resolution..

Now when i enter in to the Sampling rate where i am getting confused in Nyquist Frequency of signal and adc sampling frequency and its relationship (NF=2XSANPLING Frequency)

Now this is confusing me.

If i am having 50hz AC SIGNAL then what would be the Nyquist frequency and ADC Sampling frequency..??

NF=10HZ..??

and how it will help for the signal convertion.

Any example or Some web material if you pls Post..

Marx

The Nyquist frequency is twice of the resolution of interest, which is more related to the sample rate, I mean, if you want to take n samples per cycle of a 50Hz signal, it is 2*n*50Hz

Does it like..

If i am using 10 bit of adc with vref=3.3
so resolution would be 0.00322mV/bit

I want to take 20 sample
Then nyquest frequency=2*20(Sample)*50=2000HZ

is this correct..???

Apart from calculation, I think that 20 samples per complete sin cycle would give 5 samples per 1/4 waveform (0-90 deg) which sounds being a quite poor resolution.

can you pls take an example of sinewave having peak peak to amplitude of 3.3V

Samples you deside..??

Let me which which adc we will select..?

And what will be nyquiest frequency..??

Just taking and example..

Also if do no follow nyquist thereom what would be problem..??

Hi,

Nyquist says that the sample rate must be higher than twice the frequency of interest.

2 x 50Hz = 100 Hz is too low.

any sampling frequency above this nyquist limit is OK. But if you are very close to the limit you need to take a lot of samples to get the correct information:
example: signal 50Hz, sampling frequency 101Hz, then you need to take 100 samples.
Here a picture of a 50Hz sine sampled with 101Hz. (Quick excel calculation)


****
Additionally:
* if you are interested in overtones of the 50Hz signal, then you need to calculate sampling frequency from overtone frequency.
* if you are not interested in overtones of 50Hz, then you need to suppress them on the analog side, else you will see alias frequencies.

****

The Nyquist frequency is twice of the resolution of interest,

Click to expand...

To avoid confusion: Sampling frequency has nothing to do with ADC resolution.
I´d replace "resolution" with "frequency" in andre´s statement.

****
There are countless documents in the web. And a lot of discussions here in this forum.

Klaus

- - - Updated - - -

Hi,

Apart from calculation, I think that 20 samples per complete sin cycle would give 5 samples per 1/4 waveform (0-90 deg) which sounds being a quite poor resolution.

Click to expand...

True, for a scope.

But if he wants to calcluate RMS value of a sine, then this is plenty of information.

Nyquist says: 2 samples per full wave is not enough. So let´s try three samples per full wave. This means 150 Hz sampling frequency on a 50 Hz sine.
Now calculate the RMS value of random 3 values (immediate samples after each other) and get the perfect RMS value. Even if both frequencies are not synchronous to each other (very small difference in exact frequencies).

The RMS calculation is exact for 3, 4, 5, ... 20 ...values per sine. but you need close frequency matching.
For european mains frequency and an a sample clock generated from an XTAL this is good. Expect low deviations.

But for a motor generator with less frequency stability it won´t work good.
Also it won´t work good when the waveform deviates form sine.

Klaus

Hi Klauss

I understand your explanation.

Now my doubt would be I want to select a ADC

What criterion i will look out in the ADC Chips.

Hi,

For an ADC beginner I´d choose a SAR type ADC. Easy to use, fast, good performance, good available.

Use any ADC selection guide.
--> Go to a ADC manufacturer web site or a distributor´s web site.

They will have a lot of parameters to select. Power supply, speed, interface, resolution, channel count....and all this stuff.
Select those items you know. Use a wildcard for the others.
Maybe hundreds of ADCs will show up.

Then choose the cheapest one, or the one you have heard of, or choose the one that is good available for you.
Then before you buy one ... read the datasheet carefully. There will be a lot of information. If you don´t like these informations, then choose another ADC from the list.

Klaus

Now my doubt would be I want to select a ADC

Click to expand...

For mains voltage reading, most microcontroller's built in adc suffice, although you did not mentioned the purpose of that measurement.

KlausST said:

Hi,

Nyquist says that the sample rate must be higher than twice the frequency of interest.

2 x 50Hz = 100 Hz is too low.


Hi,


True, for a scope.

But if he wants to calcluate RMS value of a sine, then this is plenty of information.

Nyquist says: 2 samples per full wave is not enough. So let´s try three samples per full wave. This means 150 Hz sampling frequency on a 50 Hz sine.
Now calculate the RMS value of random 3 values (immediate samples after each other) and get the perfect RMS value. Even if both frequencies are not synchronous to each other (very small difference in exact frequencies).

The RMS calculation is exact for 3, 4, 5, ... 20 ...values per sine. but you need close frequency matching.
For european mains frequency and an a sample clock generated from an XTAL this is good. Expect low deviations.

But for a motor generator with less frequency stability it won´t work good.
Also it won´t work good when the waveform deviates form sine.

Klaus

Click to expand...

Understand..
Now in ADC Datasheet..

1) I will check for the ADC Type :SAR
2) No of bits=for now i will go for 8 bit ADC
3) Sampling frequency of that particular ADC ie more than 150hz

Are any other criterion missing..??

Hi,

Are any other criterion missing..??

Click to expand...

Yes, countless.

1) I will check for the ADC Type :SAR
2) No of bits=for now i will go for 8 bit ADC
3) Sampling frequency of that particular ADC ie more than 150hz

Click to expand...

This is lowest end performance. Why don´t you use microcontroller_internal ADC? They have better performance than you need - for almost nothing.

Klaus

Yes..

I am just reading the basics and it just a Example of Understanding the parameters

Thanks For Help

Do you Have good Explanation on SAR and Signa Delta adc Type..?

Hi,

Do you Have good Explanation on SAR and Signa Delta adc Type..?

Click to expand...

Yes: use wikipedia or google or any other search engine. You will find a lot of good documents.

Again my hint: simply use SAR and don´t waste much time now. (maybe later)

Klaus

KlausST said:

Hi,


Yes, countless.


This is lowest end performance. Why don´t you use microcontroller_internal ADC? They have better performance than you need - for almost nothing.

Klaus

Click to expand...

What else criteria is missing in the above example..

Hi,

What else criteria is missing in the above example..

Click to expand...

* Read post #8 (there are a couple)
* go through a selection guide (there are numerous specifications)
* read a datasheet (there are many specifications)

Klaus

Hello Guys..
I read the article from analog devices of seven steps to choose ADC..

My signal is 10khz so I am selecting a 20ksps ADC..
Now what abt the acquisition time..?
How it can be taken into consideration..
Could anyone give me a simple example with wich all the parameter previously calculated explained nicely..
Again I am just reading and understanding the adc.. so I dont hv any live example..

Hi,

You didn't read post#6.

Use a live example.

Klaus

i am asking about the acquisition time and converstion time.

Which we have not discussed yet.

Hi,

for SAR ADC speaking:

acquisition_time + conversion_time < ( 1 / sample_rate)
* acquisition time: is mainly important if you do analog value multiplexing
* conversion time: is just the time that the ADC needs to convert the analog value into the digital value.

My signal is 10khz so I am selecting a 20ksps ADC..

Click to expand...

--> won´t work.

Klaus