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Aliasing in sigma-delta ADC output?

Question asked by Alexxx on Jan 22, 2014
Latest reply on Feb 2, 2014 by JohnnyG

Hello,

 

for a precision weigh scale application I got into data sheets of 24 bit ADC (I am testing AD7195).

What I don't understand:

When I look data sheet AD7195, page 35, Figure 29, Figure 30 and Table 30:

1.) FS= 80 => output rate 15 Hz; Figure 29 shows approx. -3dB @ 15Hz

and almost no supression at nyquist frequency (of output rate)!

2.) FS= 480 => output rate 2.5 Hz; first filter-zero is at 10 Hz; almost no supression at 1,25 Hz.

 

Doesn't this behaviour create aliasing (Fin >= 1/2 Outputrate)??

Or is there a misunderstanding?

Why there is no modern ADC with a REAL digital filter (FIR), that have > -80dB at 1/2 output rate?

Do I need an anti-aliasing filter to cut off Fin <<1/2 output rate?

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