Anti-aliasing filter which is going to
be used for remove a higher frequency noise signal before to be taken a
sampling processing at that time. Generally the sampling frequency should meet
with Nyquist theorem .But in this case, the Aliasing signal doesn’t satisfy with
Nyquist theorem. So that, your ADC digital values may be fluctuated that means
aliasing happened.
Nyquist Theorem:
(ADC)
fs >= 2 x fc
The maximum frequency of the input signal is
less than half or equal to sampling rate (fs).
For
example, if you are application having the frequency of the sensor signal is
1KHz, you can choose your cut off frequency of anti-aliasing filter is greater
than 1KHz and it should be less than sampling rate of your ADC. Here my
sampling rate for my ADC is 10KHz so that I have designed my cut off frequency
of my filter is 5KHz.
Here
I have chosen cut-off frequency (fc) (input signal frequency) is
half of my ADC sampling rate (fs).
The
following online design tool may helpful for you.
Recently
I have used ST electronics android application. It’s really very fun.
You
can go through the following link to work for OpAmp circuit and filter design,
Digital filter:
This is one of the algorithm technique
.Generally we are displaying ADC digital value for every sampling processing.
But here we can display the average of 20 or 30 (whatever..) digital values. So
we can avoid fluctuated data.
The
algorithm looks like this,
Total=ADCValue=FinalVlaue=0;
for(int
i =0;i<=20;i++)
{
ADCValue=ADCRead(channel);
Total=ADCValue+Total;
}
FinalValue=Total/20;
SAC Vs Delta Sigma:
Here
I don’t discuss about SAC and Delta Sigma architecture. So we will see again in
my future tutorial. But right now I will try to explain about differences and
few valid points for our application. Last one year I was using Delta Sigma
Architecture ADC. Because it suitable for my following application.
1.
It is very accuracy .We don’t feel about Anti-aliasing filter at all. But
depends on your application you may need filter circuit.
2.
Compare with SAC architecture, it have slow speed. But it does almost reach enough
speed.
Both
AFD type ADC chips have I2C and SPI protocol communications.
1.
ADS1231:
2.
MX7705:
3.
MCP3421:
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