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What's the best anti-aliasing filter for a 250 kS/s 16-bit ADC DAQ Board?

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Programmer.C

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Hi guys,
I have an Ethernet 16-bit analog to digital DAQ board with 250 kS/s

This's a link for the board

https://www.mccdaq.com/ethernet-data-acquisition/E-1608-Series

How do i know the best anti-aliasing filter before the analog input?
I'm really confused about this issue i have read a lot of topics & searched google thousand of times about anti-aliasing filters & DAQ systems but still confused about this point exactly.

Please advice
 

Hi,

As so often, asking for the "best" is almost useless.
It my be the
* low cost
* most space saving
* best availability
* least phase shift
* least pass band ripple
* highest stopband attenuation
* closest to nyquist frequency
* best waveform regeneration
* lowest frequency drift
* lowest harmonic distortion
* lowest noise
* low current consumption
* and so on..

Thus there are many solutions. Ask 100 engineers and get 100 different answers.

My answer is: It depends on your requirements. Thus nobody can give a general best solution for an AAF.

Tell us what you want to do with your ADC board, what is important for you and what is not important for you.
And tell us about your input signals.

Klaus
 

Hi,
Thanks for youق clarification for the word "best"
I'm asking about the best lowest noise & least pass band ripple
My input signal is a frequency it's about 1k Hz
Firstly i'm converting the signal from Frequency to Volt using LM331 after that conversion circuit my signal goes to + or -10 volts & i need to take that signal to an ADC Data acquisition card but between the DAQ AND my signal i need to make an AAF just to make sure that my signal is pure and stable

Thanks in advance
 
Last edited:

I'm asking about the best lowest noise & least pass band ripple
My input signal is a frequency it's about 1k Hz
Firstly i'm converting the signal from Frequency to Volt using LM331 after that conversion circuit my signal goes to + or -10 volts & i need to take that signal to an ADC Data acquisition card but between the DAQ AND my signal i need to make an AAF just to make sure that my signal is pure and stable
Doesn't make much sense yet. The output signal of a F/V converter isn't a frequency. It's more or less smoothed stair case waveform.

Useful filter specification depends on the input signal spectrum and actual ADC data processing rate, not theoretical maximal sampling rate. What's the LM331 input frequency range, what's the expected ADC readout rate?

I guess that a LM331 F/V converter with post filter according to datasheet figure 19 can be used without dedicated anti-aliasing filter.
 

Hi,

Almost scary - for me is:
My input signal is a frequency it's about 1k Hz
Firstly i'm converting the signal from Frequency to Volt using LM331
Here I expect the major of errors. All noise, distortion, drift errors caused by the AAF are negligible....in my opinion.

Why not simply ADconvert the original 1kHz signal?
You don't loose information, but avoid any additional errors.
Any simple microcontroller will work more accurate and precise than the LM331.

Btw: You say you convert f-->V ... then you are not interested in amplitude and waveform. Why an ADC then?

It's more simple to:
1kHz input signal --> simple signal conditioning --> zero cross comparator --> microcontroller_capture_periferal.
And using suitable software (which is more flexible) will result in much more precise values than the ADC solution.
Cost, hardware effort, ammount of data....will be greatly reduced...while still getting better results.

I really recommend to specify your needs ... then find a solution
...not the other way round.

*****
i need to make an AAF just to make sure that my signal is pure and stable
This requirement rather sounds like for a "sample and hold" circuit, rather than for an AAF.

The AAF will (should) only have influence on input frequencies above f_sample/2 which is 125kHz in your case.
Because alias frequencies happen only at input frequencies higher than f_sample/2.

Klaus
 
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